Embedded Systems with 8051 µC

Embedded Embedded Systems with Systems with 8051 8051 µCµC

Part 1Part 1

8051 8051 µCµC

INTRODUCTIONINTRODUCTION

µPs Vs µCsµPs Vs µCs

On the left is a block diagram of a general purpose microprocessor On the left is a block diagram of a general purpose microprocessor based computer like the PCs and the laptops all of us are familiar with based computer like the PCs and the laptops all of us are familiar with [minus the output device (f.e. CRT display), the input device (f.e. [minus the output device (f.e. CRT display), the input device (f.e. keyboard) and the secondary storage (f.e. hard disk)]keyboard) and the secondary storage (f.e. hard disk)]

To the right is the block diagram of the same computer, implemented To the right is the block diagram of the same computer, implemented on a single chip of silicon. on a single chip of silicon.

We call this one chip implementation of a computer, or We call this one chip implementation of a computer, or

computer-on-a-single-chip, a MICROCONTROLLERcomputer-on-a-single-chip, a MICROCONTROLLER

µCsµCs in Embedded Systems in Embedded Systems

The Intel 8051 µCThe Intel 8051 µC In 1981, Intel came out with an 8-bit uC called In 1981, Intel came out with an 8-bit uC called

80518051

This uC had 128 bytes of RAM, 4K (4096) bytes This uC had 128 bytes of RAM, 4K (4096) bytes of on-chip ROM, 2 timers, 1 serial port, and 4 8-of on-chip ROM, 2 timers, 1 serial port, and 4 8-bit wide I/O ports all on a single chipbit wide I/O ports all on a single chip

The 8051 is an 8-bit processor, meaning that the The 8051 is an 8-bit processor, meaning that the CPU can work on only 8 bits of data at a time. CPU can work on only 8 bits of data at a time.

Data larger than 8 bits has to be broken into 8-Data larger than 8 bits has to be broken into 8-bit pieces to be processed by the CPU. bit pieces to be processed by the CPU.

Although the 8051 can have a maximum of 64K Although the 8051 can have a maximum of 64K bytes of on-chip ROM, many manufacturers bytes of on-chip ROM, many manufacturers have put only 4K bytes or 4096 bytes on the have put only 4K bytes or 4096 bytes on the chip.chip.

The 8051 became widely popular after Intel The 8051 became widely popular after Intel allowed other manufacturers to make and allowed other manufacturers to make and market any flavors of the 8051 they please with market any flavors of the 8051 they please with the condition that they remain code-compatible the condition that they remain code-compatible with the 8051. with the 8051.

This has led to many versions of the 8051 with This has led to many versions of the 8051 with different speeds and amounts of on-chip ROM different speeds and amounts of on-chip ROM marketed by more than half a dozen marketed by more than half a dozen manufacturers. manufacturers.

Block Diagram of 8051 µCBlock Diagram of 8051 µC

Features of Intel 8051 µCFeatures of Intel 8051 µC

8052 µC8052 µC 8052 is an advanced 8051.8052 is an advanced 8051. The 8052 an extra 128 bytes of RAM and The 8052 an extra 128 bytes of RAM and

an extra timer. an extra timer.

Therefore, 8052 has 256 bytes of RAM Therefore, 8052 has 256 bytes of RAM and 3 timers. and 3 timers.

It has 8K bytes of on-chip program ROM It has 8K bytes of on-chip program ROM instead of 4K bytes. instead of 4K bytes.

AT89S8253 AT89S8253 µCµC from Atmel Corporation from Atmel Corporation in Dual In-Line (DIP) Packagein Dual In-Line (DIP) Package

Same AT89S8253 Same AT89S8253 µCµC from Atmel from Atmel Corporation in Quad PackageCorporation in Quad Package

AT89S8253 from Atmel AT89S8253 from Atmel CorporationCorporation

The most widely used registers of the The most widely used registers of the 8051 uC are the A, B, R0, R1, R2, R3, 8051 uC are the A, B, R0, R1, R2, R3, R4, R5, R6, R7, DPTR and PCR4, R5, R6, R7, DPTR and PC

All these registers are 8-bits, except All these registers are 8-bits, except DPTR and the program counter which DPTR and the program counter which are 16 bitare 16 bit

COMMON REGISTERS OF THE COMMON REGISTERS OF THE 8051 uC8051 uC

80518051 8-bit Registers or Byte Registers8-bit Registers or Byte Registers

16-bit Registers16-bit Registers

THE A-REGISTER OR THE THE A-REGISTER OR THE ACCUMULATOR REGISTERACCUMULATOR REGISTER

REGISTER A IS CALLED ACCUMULATORREGISTER A IS CALLED ACCUMULATOR

THE ACCUMULATOR IS AN 8-BITS WIDE THE ACCUMULATOR IS AN 8-BITS WIDE REGISTERREGISTER

ACCUMULATOR IS USED FOR ALL ACCUMULATOR IS USED FOR ALL ARITHMETIC AND LOGIC INSTRUCTIONSARITHMETIC AND LOGIC INSTRUCTIONS

MOV instructionMOV instruction MOV destination, source MOV destination, source copies source to destinationcopies source to destination

MOV A,#55HMOV A,#55H ;load value 55H into reg A ;load value 55H into reg A

MOV R0,AMOV R0,A ;copy contents of A into R0 ;copy contents of A into R0 ;(A=R0=55H);(A=R0=55H)

MOV R1,AMOV R1,A ;copy contents of A into R1 ;copy contents of A into R1 ;(A=R0=R1=55H);(A=R0=R1=55H)

MOV R2,AMOV R2,A ;copy contents of A into R2 ;copy contents of A into R2 ;(A=R0=R1=R2=55H);(A=R0=R1=R2=55H)

MOV R3,#95HMOV R3,#95H ;load value 95H into R3 (R3=95H);load value 95H into R3 (R3=95H)

MOV A,R3MOV A,R3 ;copy contents of R3 into A (A=R3=95H);copy contents of R3 into A (A=R3=95H)

Simple InstructionsSimple Instructions

ADD instructionADD instruction ADD A, source ADD A, source ADD the source operand to the ADD the source operand to the

accumulator registeraccumulator register

MOV A,#25HMOV A,#25H ;load 25H into A;load 25H into A

MOV R2,#34HMOV R2,#34H ;load 34H into R2;load 34H into R2

ADD A,R2ADD A,R2 ;add R2 to accumulator;add R2 to accumulator

Executing the program above results Executing the program above results

in A = 59H in A = 59H

ASSEMBLY PROGRAMMING ASSEMBLY PROGRAMMING

Structure of Assembly Language CodeStructure of Assembly Language Code

ORG 0HORG 0H ;start (origin) at 0;start (origin) at 0MOV R5,#25HMOV R5,#25H ;load 25H into R5;load 25H into R5MOV R7,#34HMOV R7,#34H ;load 34H into R7;load 34H into R7MOV A,#0MOV A,#0 ;load 0 into A;load 0 into AADD A,R5ADD A,R5 ;add contents of R5 to A ;add contents of R5 to A

;now A = A + R5;now A = A + R5ADD A,R7ADD A,R7 ;add contents of R7 to A ;add contents of R7 to A

;now A = A + R7;now A = A + R7ADD A, #12HADD A, #12H ;add to A value 12H ;add to A value 12H

;now A = A + 12H ;now A = A + 12H HERE: SJMP HEREHERE: SJMP HERE ;stay in this loop ;stay in this loop ENDEND ;end of asm source file;end of asm source file

Sample Assembly Language ProgramSample Assembly Language Program

ASSEMBLING AND RUNNING ASSEMBLING AND RUNNING AN 8051 PROGRAMAN 8051 PROGRAM

An Assembly language instruction An Assembly language instruction

consists of four fields:consists of four fields:

[[label :label : ] ]mnemonicmnemonic [[operandsoperands] ] [;[;commentcomment] ]

ASSEMBLING AND RUNNING ASSEMBLING AND RUNNING AN 8051 PROGRAMAN 8051 PROGRAM

Steps to Create a Program

PC & ROM SPACE IN 8051PC & ROM SPACE IN 8051

Program counter in the 8051Program counter in the 8051 16 bits wide16 bits wide can access program addresses 0000 to can access program addresses 0000 to

FFFFHFFFFH total of 64K bytes of codetotal of 64K bytes of code

PC & ROM IN 8051PC & ROM IN 8051

Where the 8051 wakes up when it is Where the 8051 wakes up when it is powered up:powered up: wakes up at memory address 0000 when it wakes up at memory address 0000 when it

is powered upis powered up first opcode must be stored at ROM first opcode must be stored at ROM

address 0000Haddress 0000H


Placing code in program ROMPlacing code in program ROM the opcode and operand are placed in the opcode and operand are placed in

ROM locations starting at memory 0000ROM locations starting at memory 0000

ROM memory map in the 8051 familyROM memory map in the 8051 family

8051 On-Chip ROM Address Range


8051 DATA TYPES AND 8051 DATA TYPES AND DIRECTIVESDIRECTIVES

8051 data type and directives8051 data type and directives

DB (define byte)DB (define byte) ORG (origin)ORG (origin) EQU (equate)EQU (equate) END directiveEND directive

8051 DATA TYPES AND 8051 DATA TYPES AND DIRECTIVESDIRECTIVES

Rules for labels in Assembly languageRules for labels in Assembly language each label name must be uniqueeach label name must be unique first character must be alphabeticfirst character must be alphabetic reserved words must not be used as labels reserved words must not be used as labels

8051 FLAG BITS AND THE 8051 FLAG BITS AND THE PSW REGISTERPSW REGISTER

PSW (program status word) registerPSW (program status word) register

Bits of the PSW Register

8051 FLAG BITS AND THE 8051 FLAG BITS AND THE PSW REGISTERPSW REGISTER

Instructions That Affect Flag Bits

8051 REGISTER BANKS AND 8051 REGISTER BANKS AND STACKSTACK

RAM memory space allocation in the RAM memory space allocation in the 80518051

RAM Allocation in the 8051


Register banks in the 8051Register banks in the 8051

8051 Register Banks and their RAM Addresses


How to switch register banksHow to switch register banks

PSW Bits Bank Selection


Stack in the 8051Stack in the 8051 section of RAM used to store information section of RAM used to store information

temporarilytemporarily could be data or an addresscould be data or an address CPU needs this storage area since there CPU needs this storage area since there

are only a limited number of registersare only a limited number of registers

Part 2Part 2

8051 8051 µCµC DATA DATA PROCESSING PROCESSING INSTRUCTIONS – IINSTRUCTIONS – I

ARITHMETIC ARITHMETIC INSTRUCTIONSINSTRUCTIONS

Arithmetic InstructionsArithmetic Instructions

AddAdd SubtractSubtract IncrementIncrement DecrementDecrement MultiplyMultiply Divide Divide Decimal adjustDecimal adjust

Arithmetic InstructionsArithmetic Instructions

MnemonicMnemonic DescriptionDescription

ADD A, byteADD A, byte add A to byte, put result in Aadd A to byte, put result in A

ADDC A, byteADDC A, byte add with carryadd with carry

SUBB A, byteSUBB A, byte subtract with borrowsubtract with borrow

INC AINC A increment Aincrement A

INC byteINC byte increment byte in memoryincrement byte in memory

INC DPTRINC DPTR increment data pointerincrement data pointer

DEC ADEC A decrement accumulatordecrement accumulator

DEC byteDEC byte decrement bytedecrement byte

MUL ABMUL AB multiply accumulator by b registermultiply accumulator by b register

DIV ABDIV AB divide accumulator by b registerdivide accumulator by b register

DA ADA A decimal adjust the accumulatordecimal adjust the accumulator

ADD InstructionsADD Instructionsadd a, byteadd a, byte ; a ; a a + byte a + byte

addc a, byteaddc a, byte ; a ; a a + byte + C a + byte + C

These instructions affect 3 bits in PSW:These instructions affect 3 bits in PSW:

C = 1 if result of add is greater than FFC = 1 if result of add is greater than FF

AC = 1 if there is a carry out of bit 3AC = 1 if there is a carry out of bit 3

OV = 1 if there is a carry out of bit 7, but not from bit 6, OV = 1 if there is a carry out of bit 7, but not from bit 6, or visa versa.or visa versa.

Instructions that Affect Instructions that Affect PSW bitsPSW bits

ADD ExamplesADD Examples

mov a, #0x3Fmov a, #0x3F

add a, #0xD3add a, #0xD3

What is the value of What is the value of the C, AC, OV flags the C, AC, OV flags after the second after the second instruction is instruction is executed?executed?

0011 1111

1101 0011

0001 0010C = 1

AC = 1

OV = 0

Signed Addition and Signed Addition and OverflowOverflow

0111 1111 (positive 127)

0111 0011 (positive 115)

1111 0010 (overflow cannot represent 242 in 8 bits 2’s complement)

2’s complement:

0000 0000 00 0

…

0111 1111 7F 127

1000 0000 80 -128

…

1111 1111 FF -1

1000 1111 (negative 113)

1101 0011 (negative 45)

0110 0010 (overflow)

0011 1111 (positive)

1101 0011 (negative)

0001 0010 (never overflows)

Addition Example for the Addition Example for the Assembler Assembler

; Computes Z = X + Y; Adds values at locations 0x78 and 0x79 and puts them in location 0x7A; Computes Z = X + Y; Adds values at locations 0x78 and 0x79 and puts them in location 0x7A$INCLUDE (C8051F020.inc)$INCLUDE (C8051F020.inc); EQUATES; EQUATES;-----------------------------------------------------------------------------;-----------------------------------------------------------------------------XX equ equ 0x780x78YY equ equ 0x790x79ZZ equ equ 0x7A0x7A; RESET and INTERRUPT VECTORS; RESET and INTERRUPT VECTORS;-----------------------------------------------------------------------------;----------------------------------------------------------------------------- cseg at 0cseg at 0

ljmp Mainljmp Main; CODE SEGMENT; CODE SEGMENT;-----------------------------------------------------------------------------;-----------------------------------------------------------------------------

cseg at 100hcseg at 100hMain: Main: mov 0xFF, #0DEhmov 0xFF, #0DEh ; Disable watchdog timer ; Disable watchdog timer mov 0xFF, #0ADhmov 0xFF, #0ADh

mov a, Xmov a, Xadd a, Yadd a, Ymov Z, amov Z, anopnopendend

Subtract Subtract

SUBB A, byteSUBB A, byte subtract with borrowsubtract with borrow

Example:

SUBB A, #0x4F ; A A – 4F – C

Notice that there is no subtraction WITHOUT borrow. Therefore, ifa subtraction without borrow is desired, it is necessary to clear the Cflag.

Increment and Increment and DecrementDecrement

The increment and decrement instructions do The increment and decrement instructions do NOT affect the C flag.NOT affect the C flag.

Notice we can only INCREMENT the data Notice we can only INCREMENT the data pointer, not decrement.pointer, not decrement.

INC AINC A increment Aincrement A

INC byteINC byte increment byte in memoryincrement byte in memory

INC DPTRINC DPTR increment data pointerincrement data pointer

DEC ADEC A decrement accumulatordecrement accumulator

DEC byteDEC byte decrement bytedecrement byte

Example: Increment 16-Example: Increment 16-bit Wordbit Word Clearly we have to take the word Clearly we have to take the word

in 2 8-bit long registersin 2 8-bit long registers Assume 16-bit word in R3:R2Assume 16-bit word in R3:R2

mov a, r2 mov a, r2 add a, #1 add a, #1 ; use add rather than increment to ; use add rather than increment to

affect C affect C mov r2, a mov r2, a mov a, r3 mov a, r3 addc a, #0 addc a, #0 ; add C to most significant byte; add C to most significant bytemov r3, a mov r3, a

MultiplyMultiply

When multiplying two 8-bit numbers, the When multiplying two 8-bit numbers, the size of the maximum product is 16-bitssize of the maximum product is 16-bits

FF x FF = FE01FF x FF = FE01

(255 x 255 = 65025)(255 x 255 = 65025)

MUL AB ; BA A * B

Note: B gets the HIGH byte, A gets the LOW byte

DivisionDivision

Integer DivisionInteger Division

DIV ABDIV AB ; divide A by B; divide A by B

A A Quotient(A/B), B Quotient(A/B), B Remainder(A/B) Remainder(A/B)

OV - used to indicate a divide by zero condition.OV - used to indicate a divide by zero condition.C – set to zeroC – set to zero

Decimal AdjustDecimal Adjust

DA aDA a ; decimal adjust a; decimal adjust a

Used to facilitate BCD addition. Adds “6” to either high or Used to facilitate BCD addition. Adds “6” to either high or low nibble after an addition to create a valid BCD low nibble after an addition to create a valid BCD number.number.

Example:Example:mov a, #0x23mov a, #0x23mov b, #0x29mov b, #0x29add a, badd a, b ; a ; a 23 + 29 = 4C (wanted 23 + 29 = 4C (wanted

52)52)DA aDA a ; a ; a a + 6 = 52 a + 6 = 52

Note: This instruction does NOT convert binary to BCD!Note: This instruction does NOT convert binary to BCD!

Part 3Part 3

8051 8051 µCµC DATA DATA PROCESSING PROCESSING INSTRUCTIONS – IIINSTRUCTIONS – II

LOGIC INSTRUCTIONSLOGIC INSTRUCTIONS

Logic InstructionsLogic InstructionsBitwise logic operations (AND, OR, XOR, NOT)Bitwise logic operations (AND, OR, XOR, NOT)

ClearClear

RotateRotate

SwapSwap

Logic instructions do NOT affect the flags in PSWLogic instructions do NOT affect the flags in PSW

Bitwise LogicBitwise Logic

ANL – AND ANL – AND

ORL – OR ORL – OR

XRL – eXclusive ORXRL – eXclusive OR

CPL – ComplementCPL – Complement

Examples:0000111110101100ANL

0000111110101100ORL

0000111110101100XRL

10101100CPL

00001100

10101111

10100011

01010011

Address Modes with Address Modes with LogicLogic

a, bytea, bytedirect, reg. indirect, reg, direct, reg. indirect, reg, immediateimmediate

byte, abyte, adirectdirect

byte, #constantbyte, #constant

a ex: cpl aa ex: cpl a

ANL – AND

ORL – OR

XRL – eXclusive oR

CPL – Complement

Uses of Logic InstructionsUses of Logic Instructions

Force individual bits low, without affecting other bits.Force individual bits low, without affecting other bits.anl PSW, #0xE7anl PSW, #0xE7 ;PSW AND 11100111;PSW AND 11100111anl PSW, #11100111banl PSW, #11100111b ; can use “binary”; can use “binary”

Force individual bits high.Force individual bits high.orl PSW, #0x18orl PSW, #0x18 ;PSW OR 00011000;PSW OR 00011000

Complement individual bitsComplement individual bitsxrl P1, #0x40xrl P1, #0x40 ;P1 XRL 01000000;P1 XRL 01000000

Other Logic InstructionsOther Logic Instructions

CLR - clearCLR - clear RL – rotate leftRL – rotate left RLC – rotate left through CarryRLC – rotate left through Carry RR – rotate rightRR – rotate right RRC – rotate right through CarryRRC – rotate right through Carry SWAP – swap accumulator nibblesSWAP – swap accumulator nibbles

CLR – Set all bits to 0CLR – Set all bits to 0

CLR ACLR A

CLR byteCLR byte (direct mode)(direct mode)

CLR RiCLR Ri (register mode)(register mode)

CLR @RiCLR @Ri (register indirect mode)(register indirect mode)

RotateRotate

Rotate instructions operate only on Rotate instructions operate only on aa

rl arl a

mov a, #0xF0mov a, #0xF0 ; a; a 11110000 11110000

rl arl a ; a; a 11100001 11100001

Rotate through CarryRotate through Carry

rrc arrc a

mov a, #0A9hmov a, #0A9h ; a ; a A9 A9

add a, #14hadd a, #14h ; a ; a BD (10111101), C BD (10111101), C00

rrc arrc a ; a ; a 01011110, C01011110, C11

C

SwapSwap

swap aswap a

mov a, #72hmov a, #72h

swap aswap a ; a ; a 27h 27h

Bit Logic OperationsBit Logic Operations

Some logic operations can be used with single bit Some logic operations can be used with single bit operandsoperands

ANL C, bitANL C, bit ANL C, /bit ANL C, /bitORL C, bitORL C, bit ORL C, /bit ORL C, /bitCLR CCLR CCLR bitCLR bitCPL CCPL CCPL bitCPL bitSETB CSETB CSETB bitSETB bit

“bit” can be any of the bit-addressable RAMlocations or SFRs.

Rotate and Rotate and Multiplication/DivisionMultiplication/Division

Note that a shift left is the same as Note that a shift left is the same as multiplying by 2, shift right is divide by 2multiplying by 2, shift right is divide by 2

mov a, #3mov a, #3 ; A; A 00000011 (3) 00000011 (3)

clr Cclr C ; C; C 0 0

rlc arlc a ; A; A 00000110 (6) 00000110 (6)

rlc arlc a ; A; A 00001100 (12) 00001100 (12)

rrc arrc a ; A; A 00000110 (6) 00000110 (6)

Shift/Multiply ExampleShift/Multiply Example

Program segment to multiply by 2 and Program segment to multiply by 2 and add 1 add 1

clr cclr c

rl arl a ;multiply by 2;multiply by 2

inc ainc a ;and add one;and add one

Part 4Part 4

JUMP AND JUMP AND LOOP LOOP INSTRUCTIONSINSTRUCTIONS

LOOP AND JUMP LOOP AND JUMP INSTRUCTIONSINSTRUCTIONS

Repeating a sequence of instructions a certain number Repeating a sequence of instructions a certain number of times is called a loop.of times is called a loop.

The loop action is performed by the instruction The loop action is performed by the instruction DJNZ reg, labelDJNZ reg, label..

In this instruction, the register is decremented; if it is In this instruction, the register is decremented; if it is not zero, it jumps to the target address referred to by not zero, it jumps to the target address referred to by the label. the label.

Prior to the start of the loop the register is loaded with Prior to the start of the loop the register is loaded with the counter for the number the counter for the number of of repetitions. repetitions.

In this instruction both the register decrement and the In this instruction both the register decrement and the decision to jump arc combined into a single instruction.decision to jump arc combined into a single instruction.

The registers can be any of R0 - R7. The counter can The registers can be any of R0 - R7. The counter can also be a RAM locationalso be a RAM location


Looping in the 8051Looping in the 8051 Problem 1Problem 1

In the 8051, looping action with the instruction In the 8051, looping action with the instruction "DJNZ Rx, rel address" is limited to _______ "DJNZ Rx, rel address" is limited to _______ iterations.iterations.

Answer : Answer : 256256 Problem 2Problem 2

If a conditional jump is not taken, what is the If a conditional jump is not taken, what is the next instruction to be executed?next instruction to be executed?

Answer : Answer : the instruction following the jumpthe instruction following the jump


SJMP to itself using $ signSJMP to itself using $ sign Problem 3Problem 3

In calculating the target address for a jump, a In calculating the target address for a jump, a displacement is added to the contents of displacement is added to the contents of register ___________ .register ___________ .

Answer: Answer: PCPC Problem 3Problem 3

The mnemonic SJMP stands for __________ The mnemonic SJMP stands for __________ and it is a ____ - byte instruction.and it is a ____ - byte instruction.

Answer: Answer: short jump , 2short jump , 2

Loop inside a loopLoop inside a loop

What happens if we want to repeat an action more times than What happens if we want to repeat an action more times than

256?256?

Write a program to (a) load the accumulator with the value 55H, Write a program to (a) load the accumulator with the value 55H,

and (b) complement the ACC 700 times.and (b) complement the ACC 700 times.

Since 700 is larger than 255 (the maximum capacity of any register), Since 700 is larger than 255 (the maximum capacity of any register),

we use two registers to hold the count. The following code shows how we use two registers to hold the count. The following code shows how

to use R2 and R3 for the count.to use R2 and R3 for the count.

MOV A,#55H MOV A,#55H ;A=55H;A=55H

NEXT:NEXT: MOV R3,#10MOV R3,#10 ;R3=10, the outer loop count;R3=10, the outer loop count

AGAIN:AGAIN: MOV R2,#70MOV R2,#70 ;R2=70, the inner loop count;R2=70, the inner loop count

CPL ACPL A ;complement;complement


Other conditional jumpsOther conditional jumps

Other conditional jumps Other conditional jumps

JZ (jump if A = 0) JZ (jump if A = 0) In this instruction the content of register A is In this instruction the content of register A is

checked. If it is zero, it jumps to the target checked. If it is zero, it jumps to the target address.address.

JZ instruction can be used only for register A. JZ instruction can be used only for register A. It can only check to see whether the It can only check to see whether the

accumulator is zero, and it does not apply to accumulator is zero, and it does not apply to any other register. any other register.

Don't have to perform an arithmetic instruction Don't have to perform an arithmetic instruction such as decrement to use the JZ instruction. such as decrement to use the JZ instruction.


JNZ (jump if A JNZ (jump if A 0) 0) In this instruction the content of register A is In this instruction the content of register A is

checked. If it is not zero, it jumps to the checked. If it is not zero, it jumps to the target address.target address.


JNC Jump JNC Jump if if no no carry, jumps carry, jumps if CY = 0)if CY = 0) carry flag bit in the flag (PSW) register is carry flag bit in the flag (PSW) register is

used to make the decision whether to jumpused to make the decision whether to jump "JNC label", the processor looks at the carry "JNC label", the processor looks at the carry

flag to see if it is raised (CY = 1). flag to see if it is raised (CY = 1). if it is not, the CPU starts to fetch and if it is not, the CPU starts to fetch and

execute instructions from the address of the execute instructions from the address of the label. label.

if CY = 1, it will not jump but will execute the if CY = 1, it will not jump but will execute the next instruction below JNC.next instruction below JNC.


JC Jump JC Jump if if carry, jumps carry, jumps if CY = 1)if CY = 1) if CY = l it jumps to the target addressif CY = l it jumps to the target address

JB (jump if bit is high)JB (jump if bit is high) JNB (jump if bit is low)JNB (jump if bit is low)


All conditional jumps are short jumpsAll conditional jumps are short jumps

Table 3–1 8051 Conditional Jump Instructions

Unconditional jump Unconditional jump instructionsinstructions

There are two unconditional jumps: There are two unconditional jumps: LJMP LJMP (long jump) and (long jump) and SJMP SJMP (short jump).(short jump).

LJMP is 3-byte instruction in which the first LJMP is 3-byte instruction in which the first byte is the op-code, and the second and third byte is the op-code, and the second and third bytes represent the 16-bit address of the target bytes represent the 16-bit address of the target location.location.

The 2-byte target address allows a jump to any The 2-byte target address allows a jump to any memory loca tion from 0000 to FFFFH. memory loca tion from 0000 to FFFFH.

Unconditional jump Unconditional jump instructionsinstructions

SJMP (short jump)SJMP (short jump) 2-byte instruction, the first byte is the op-code and the 2-byte instruction, the first byte is the op-code and the

second byte is the relative address of the target second byte is the relative address of the target location. location.

The relative address range of 00 – FFH is divided into The relative address range of 00 – FFH is divided into forward and backward jumps; that is, within -128 to forward and backward jumps; that is, within -128 to +127 bytes of memory relative to the address of the +127 bytes of memory relative to the address of the current PC (program counter). current PC (program counter).

If the jump is forward, the target address can be If the jump is forward, the target address can be within a space of 127 bytes from the current PC. within a space of 127 bytes from the current PC.

If the target address is backward, the target address If the target address is backward, the target address can be within -128 bytes from the current PC.can be within -128 bytes from the current PC.

Calculating the short jump Calculating the short jump addressaddress

Using the following list tile, verify the jump forward address calculation.Using the following list tile, verify the jump forward address calculation.

Line Line PCPC Op-codeOp-code Mnemonic Mnemonic OperandOperand11 00000000 ORG 0000ORG 00002 2 00000000 78007800 MOV R0, MOV R0, #003 #003 33 00020002 74557455 MOV A,MOV A, #55H0#55H04 4 0004 0004 60036003 JZ NEXTJZ NEXT5 5 00060006 0808 INC R0INC R06 6 00070007 0404 AGAIN:AGAIN: INC AINC A7 7 00080008 0404 INC AINC A8 8 00090009 24772477 NEXT:NEXT: ADD A,ADD A, #77h#77h9 9 000B000B 50055005 JNC OVERJNC OVER1010 000D 000D E4E4 CLR ACLR A11 11 000E000E F8F8 MOV R0, AMOV R0, A12 12 000F000F F9F9 MOV R1, AMOV R1, A13 13 00100010 FAFA MOV R2, AMOV R2, A14 14 00110011 FBFB MOV R3, AMOV R3, A15 15 00120012 2B2B OVER:OVER: ADD A, R3ADD A, R316 16 00130013 50F250F2 JNC AGAINJNC AGAIN1717 00150015 80FE80FE HERE:HERE: SJMP HERESJMP HERE

18 18 00170017 ENDEND

Calculating the short jump Calculating the short jump addressaddress

Part 5Part 5

CALL CALL INSTRUCTIONSINSTRUCTIONS

CALL INSTRUCTIONSCALL INSTRUCTIONS

CALL is used to call a subroutine. CALL is used to call a subroutine. Subroutines are often used to perform tasks Subroutines are often used to perform tasks

that need to be performed frequently. that need to be performed frequently. This makes a program more structured in This makes a program more structured in

addition to saving memory space. addition to saving memory space. There are two instructions : LCALL (long call) There are two instructions : LCALL (long call)

and ACALL (absolute call). and ACALL (absolute call). Deciding which one to use depends on the Deciding which one to use depends on the

target address. target address.


LCALL (long call)LCALL (long call) 3-byte instruction, the first byte is the op-code and the second and 3-byte instruction, the first byte is the op-code and the second and

third bytes are used for the address of the target subroutine.third bytes are used for the address of the target subroutine. LCALL can be used to call subroutines located anywhere within LCALL can be used to call subroutines located anywhere within

the 64K-byte address space of the 8051. the 64K-byte address space of the 8051. To make sure that after execution of the called subroutine the To make sure that after execution of the called subroutine the

8051 knows where to come back to, the processor automatically 8051 knows where to come back to, the processor automatically saves on the stack the address of the instruction immediately saves on the stack the address of the instruction immediately below the LCALL. below the LCALL.

When a subroutine is called, control is transferred to that When a subroutine is called, control is transferred to that subroutine, and the processor saves the PC (program counter) on subroutine, and the processor saves the PC (program counter) on the stack and begins to fetch instructions from the new location.the stack and begins to fetch instructions from the new location.

After finishing execution of the subroutine, the instruction RET After finishing execution of the subroutine, the instruction RET (return) transfers control back to the caller. Every subroutine (return) transfers control back to the caller. Every subroutine needs RET as the last instruction.needs RET as the last instruction.


LCALL (long call)LCALL (long call)

Figure 3–1 8051 Assembly Main Program That Calls Subroutines

Part 6Part 6

TIMING TIMING CALCULATIONSCALCULATIONS

TIME DELAY FOR TIME DELAY FOR VARIOUS 8051 CHIPSVARIOUS 8051 CHIPS

Machine cycle for the 8051Machine cycle for the 8051 The MCU takes a certain number of clock cycles to The MCU takes a certain number of clock cycles to

execute an instruction. execute an instruction. These clock cycles are referred to as These clock cycles are referred to as machine cycles. machine cycles. In the 8051 family, the length of the machine cycle In the 8051 family, the length of the machine cycle

depends on the frequency of the crystal oscillator.depends on the frequency of the crystal oscillator. The frequency of the crystal connected to the 8051 The frequency of the crystal connected to the 8051

family can vary from 4 MHz to 30 MHz.family can vary from 4 MHz to 30 MHz. In the original 8051, one machine cycle lasts 12 In the original 8051, one machine cycle lasts 12

oscillator periods. oscillator periods. Therefore, to calculate the machine cycle for the 8051, Therefore, to calculate the machine cycle for the 8051,

we take 1/12 of the crystal frequency, then take its we take 1/12 of the crystal frequency, then take its inverse.inverse.


Machine cycle for the 8051Machine cycle for the 8051

Clocks per Machine Cycle (MC) for Various 8051 Versions


Delay calculation for other versions of Delay calculation for other versions of 80518051

Table 3–3 Comparison of 8051 and DS89C4x0 Machine Cycles

Part 7Part 7

8051 I/O PORT 8051 I/O PORT PROGRAMMINGPROGRAMMING

WHAT ARE I/O PORTS?WHAT ARE I/O PORTS? I/O Ports are basically the interfaces which I/O Ports are basically the interfaces which

connect the 8051 uC to external devicesconnect the 8051 uC to external devices

After basic connection, that is, after setting up After basic connection, that is, after setting up the hardware connection, I/O ports can read, the hardware connection, I/O ports can read, that is, receive data from those devices and can that is, receive data from those devices and can also write, that is, send data to themalso write, that is, send data to them

Inputs can be read/received from ports and Inputs can be read/received from ports and outputs can be written/sent to the I/O ports by outputs can be written/sent to the I/O ports by writing suitable programs for the 8051 uC.writing suitable programs for the 8051 uC.

This is a very useful function because the This is a very useful function because the 8051 uC can then be used for monitoring 8051 uC can then be used for monitoring real world processes as we see in digital real world processes as we see in digital instruments where different parameters instruments where different parameters like Pressure, Temperature, Speed of like Pressure, Temperature, Speed of Rotation, etc can be acquired and Rotation, etc can be acquired and displayeddisplayed

This is also very useful because This is also very useful because microcontrollers have data processing microcontrollers have data processing capability (though limited) which can be capability (though limited) which can be used for processing these signals through used for processing these signals through programs (which we write), and based on programs (which we write), and based on the result perform some control function by the result perform some control function by sending data to the device through the sending data to the device through the output ports or pins of the microcontroller.output ports or pins of the microcontroller.

So for example, we can use the input port/pin of So for example, we can use the input port/pin of the 8051 uC to keep reading the temperature of the 8051 uC to keep reading the temperature of an oven, and when the temperature exceeds a an oven, and when the temperature exceeds a certain value, we can use the output port/pin of certain value, we can use the output port/pin of the 8051 uC to either turn-off the switch of the the 8051 uC to either turn-off the switch of the oven automatically or to activate a sound alarm oven automatically or to activate a sound alarm (for manual intervention) and to once again turn (for manual intervention) and to once again turn it on when the temperature falls below a it on when the temperature falls below a specified value or to activate a sound alarm.specified value or to activate a sound alarm.

ANALOGY OF PORTSANALOGY OF PORTS We have got sea ports, air ports, bus ports (what are We have got sea ports, air ports, bus ports (what are

called bus terminals) and rail ports (railway stations) in a called bus terminals) and rail ports (railway stations) in a city.city.

These ports are used for connecting the city with the rest These ports are used for connecting the city with the rest of the world for bringing people and cargo inside the city of the world for bringing people and cargo inside the city (Input Port)(Input Port)

They are also used for sending people and cargo outside They are also used for sending people and cargo outside the city (Output Port)the city (Output Port)

Generally we do not see separate physical ports in cities Generally we do not see separate physical ports in cities for Input and Output Functionsfor Input and Output Functions

Now this analogy pretty much applies for the 8051 uC as Now this analogy pretty much applies for the 8051 uC as wellwell

4 I/O Ports in the 8051 are like the 4 4 I/O Ports in the 8051 are like the 4 railway stations of a big cityrailway stations of a big city

Individual Pins are like Platforms of the Individual Pins are like Platforms of the

Railway StationRailway Station

You got 8 platforms (pins) per I/O port in You got 8 platforms (pins) per I/O port in 8051!8051!

8051 Pin Diagram

8051 I/O PORT 8051 I/O PORT PROGRAMMINGPROGRAMMING You have got 4 I/O ports in the 8051 uC. They are You have got 4 I/O ports in the 8051 uC. They are

named as P0, P1, P2, and P3.named as P0, P1, P2, and P3.

You have to always keep in mind that each of these 4 You have to always keep in mind that each of these 4 ports have 8 pins each. ports have 8 pins each.

Individual pins of these ports are named as P0.1, Individual pins of these ports are named as P0.1, P0.2, P0.3, and so on (Port 0 Platform No.1, Port 0 P0.2, P0.3, and so on (Port 0 Platform No.1, Port 0 Platform No.2, and so on)Platform No.2, and so on)

When you When you RESETRESET the 8051 uC then all the ports of the 8051 uC then all the ports of the 8051 are configured as input ports, and are ready the 8051 are configured as input ports, and are ready to read data, that is, take data inside, that is, all to read data, that is, take data inside, that is, all stations and all platforms of the stations are ready stations and all platforms of the stations are ready for receiving incoming trains!for receiving incoming trains!


And how do you reset the 8051 uC? And how do you reset the 8051 uC?

This is done by holding the RST pin (pin no. 9 This is done by holding the RST pin (pin no. 9 in the 8051 pin diagram) at Logic 1 for 2 in the 8051 pin diagram) at Logic 1 for 2 consecutive machine cycles.consecutive machine cycles.

In practice, it means that you apply the voltage In practice, it means that you apply the voltage level corresponding to Logic 1 (+5V) to the level corresponding to Logic 1 (+5V) to the RST pin of the 8051 uC for a very short time RST pin of the 8051 uC for a very short time interval corresponding to 2 machine cycles of interval corresponding to 2 machine cycles of the 8051 uC.the 8051 uC.


When you write the first 0 to a port, then that port When you write the first 0 to a port, then that port becomes an output port and can be used for writing becomes an output port and can be used for writing data or sending data outside.data or sending data outside.

For reconfiguring (phir se configuring ya dobara se For reconfiguring (phir se configuring ya dobara se configuring) the port (which you have made output configuring) the port (which you have made output port) as an input port, you have to move 1 to that port) as an input port, you have to move 1 to that port.port.

For using any of these 4 uC ports for data input or For using any of these 4 uC ports for data input or output, you have to write programs in Assembly or C output, you have to write programs in Assembly or C language.language.

Port 0Port 0 You can use this port as either an input port or an You can use this port as either an input port or an

output port.output port.

Before using port 0 as an input or an output port, Before using port 0 as an input or an output port, you have to connect you have to connect each pineach pin of port 0 to 10 Kilo of port 0 to 10 Kilo ohm resistors as shown in the diagram on next slide.ohm resistors as shown in the diagram on next slide.

This is because the port P0 has no internal resistors. This is because the port P0 has no internal resistors.

The other ports P1, P2, and P3 have internal or built-The other ports P1, P2, and P3 have internal or built-in resistors on the 8051 chip itself.in resistors on the 8051 chip itself.

So external resistors should not be connected to the So external resistors should not be connected to the pins of P1, P2, and P3 ports.pins of P1, P2, and P3 ports.

8051 I/O PROGRAMMING8051 I/O PROGRAMMING

Port Port 00

Port 0 with External Resistors

Port 0 as inputPort 0 as input After connecting 10Kohm external After connecting 10Kohm external

resistors to all pins of port 0resistors to all pins of port 0, , in the in the manner shown earlier, in order manner shown earlier, in order to make to make port 0 an input portport 0 an input port, the port must be , the port must be programmed by programmed by writing 1 to all its bitswriting 1 to all its bits. .

Dual role of port 0 Dual role of port 0

Port Port 0 0 is also designated as AD0 - is also designated as AD0 - AD7, allowing it to be used for both AD7, allowing it to be used for both address and data.address and data.

When connecting an 8051/31 to an When connecting an 8051/31 to an external memory, port 0 provides both external memory, port 0 provides both address and data. address and data.

The 8051 multiplexes address and The 8051 multiplexes address and data through port 0 to save pins.data through port 0 to save pins.

8051 I/O PROGRAMMING8051 I/O PROGRAMMING

Port 1Port 1 It can be used as input or output. It can be used as input or output. This port does not need any resistors This port does not need any resistors

since it already has resistors since it already has resistors internally. internally.

Upon reset, port I is configured as an Upon reset, port I is configured as an input port. input port.

Port 1 as input Port 1 as input

If port 1 has been configured as an If port 1 has been configured as an output port, to make it an input port output port, to make it an input port again, it must programmed as such by again, it must programmed as such by writing 1 to all its bits. writing 1 to all its bits.

Port 1 as inputPort 1 as input

In the following code, port 1 is configured first as In the following code, port 1 is configured first as an input port by writing 1 s to it, then data is an input port by writing 1 s to it, then data is received from that port and saved in R7, R6, and received from that port and saved in R7, R6, and

R5.R5.

DELAY SUBROUTINEDELAY SUBROUTINE

..

..MOV R0, #30HMOV R0, #30H....ORG 300HORG 300HDELAY: NOPDELAY: NOPAGAIN: DJNZ R0, AGAINAGAIN: DJNZ R0, AGAINRETRETENDEND

Port Port 22

Port 2 occupies a total of 8 pins (pins 21 Port 2 occupies a total of 8 pins (pins 21 through 28). through 28).

It can be used as input or output. It can be used as input or output.

Port Port 2 2 does not need any external resistors does not need any external resistors since it already has external resistors since it already has external resistors internally. internally.

Upon reset, port 2 is configured as an input Upon reset, port 2 is configured as an input port. port.

Port 2 as inputPort 2 as input

To make port 2 an input, it must programmed as To make port 2 an input, it must programmed as such by writing 1 to all its bits. such by writing 1 to all its bits.

In the following code, port 2 is configured first as In the following code, port 2 is configured first as an input port by writing is to it. an input port by writing is to it.

Dual role of port 2Dual role of port 2

In 8031-based systems, port 2 must be used along with In 8031-based systems, port 2 must be used along with P0 to provide the 16-bit address for external memory. P0 to provide the 16-bit address for external memory.

Port 2 is also designated as A8 - A15, indicating its dual Port 2 is also designated as A8 - A15, indicating its dual function. function.

Since an 8051/31 is capable of accessing 64K bytes of Since an 8051/31 is capable of accessing 64K bytes of external memory, it needs a path for the 16 bits of the external memory, it needs a path for the 16 bits of the address. address.

P0 provides the lower 8 bits via AD0 - AD7P0 provides the lower 8 bits via AD0 - AD7 P2 provides bits A8 - A 15 of the address. P2 provides bits A8 - A 15 of the address. When the 8051 /31 is connected to external memory, P2 When the 8051 /31 is connected to external memory, P2

is used for the upper 8 bits of the 16-bit address, and it is used for the upper 8 bits of the 16-bit address, and it cannot be used for I/O. cannot be used for I/O.

Port 3Port 3

Port 3 can be used as input or output. Port 3 can be used as input or output. P3 does not need any external resistors. P3 does not need any external resistors. Port 3 is configured as an input port upon Port 3 is configured as an input port upon

reset. reset. Port 3 has the additional function of Port 3 has the additional function of

providing some extremely important providing some extremely important signals such as interrupts, serial I/O, signals such as interrupts, serial I/O, timer/counter and read/write control for timer/counter and read/write control for external memory. external memory.

Port 3Port 3

Port 3 Alternate Functions

Different ways of toggling the entire 8 Different ways of toggling the entire 8 bitsbits

Different ways of toggling Different ways of toggling the entire 8 bitsthe entire 8 bits

Ports status upon resetPorts status upon reset

Reset Value of Some 8051 Ports

PROGRAMMING INDIVIDUAL PROGRAMMING INDIVIDUAL I/O PINS BY BIT I/O PINS BY BIT MANIPULATIONMANIPULATION

A powerful feature of 80511/0 ports is their A powerful feature of 80511/0 ports is their capability to access individual bits of the port capability to access individual bits of the port without altering the rest of the bits in that port. without altering the rest of the bits in that port.

Of the four 8051 ports, we can access either the Of the four 8051 ports, we can access either the entire 8 bits or any single bit without altering the entire 8 bits or any single bit without altering the rest. rest.

"SETB X. Y" where X is the port number 0, 1, 2, or "SETB X. Y" where X is the port number 0, 1, 2, or 3, and Y is the desired bit number from 0 to 7 for 3, and Y is the desired bit number from 0 to 7 for data bits DO to D7. data bits DO to D7.

"SETB P1.5" sets bit 5 of port 1 high."SETB P1.5" sets bit 5 of port 1 high.

The following code toggles bit P1.2 The following code toggles bit P1.2 continuously. continuously.

I/O ports and bit-addressabilityI/O ports and bit-addressability

Single-Bit Addressability of Ports

I/O BIT MANIPULATION I/O BIT MANIPULATION PROGRAMMINGPROGRAMMING

I/O ports and bit-addressabilityI/O ports and bit-addressability

Table 4–4 Single-Bit Instructions

I/O BIT MANIPULATION I/O BIT MANIPULATION PROGRAMMINGPROGRAMMING

Checking an input bitChecking an input bit

Instructions For Reading an Input Port

Reading a single bit into the Reading a single bit into the carry flagcarry flag

Reading input pins vs. port Reading input pins vs. port latchlatch

Some instructions read the contents of a Some instructions read the contents of a port, modify its value, and write it back. port, modify its value, and write it back. (READ-MODIFY-WRITE)(READ-MODIFY-WRITE)

F.E. ANL P1, AF.E. ANL P1, A Reads internal port latch & brings it to Reads internal port latch & brings it to

CPUCPU Data is ANDed with contents of AData is ANDed with contents of A Result is rewritten to port latchResult is rewritten to port latch Port Pin Data = Port LatchPort Pin Data = Port Latch

Instructions for reading Instructions for reading input portsinput ports

To make any bit of any 8051 port an To make any bit of any 8051 port an input port, we must write 1 (logic high) input port, we must write 1 (logic high) to that bit. to that bit.

After we configure the port bits as After we configure the port bits as input, we can use only certain input, we can use only certain instructions in order to get the instructions in order to get the external data present at the pins into external data present at the pins into the CPU.the CPU.

Pin & LatchPin & Latch

Port PinsPort Pins Current state of Current state of portport pin pin Reading from Port: Data is read directly from Reading from Port: Data is read directly from portport

pins in case the port is configured as input.pins in case the port is configured as input. Writing to Port: Writes to an internal register (called Writing to Port: Writes to an internal register (called

latch reglatch reg) which is reflected to pin if ) which is reflected to pin if portport is is configured as output.configured as output.

LatchLatch To write data on To write data on portport Data will be reflected/moved to Data will be reflected/moved to portport pin if configured pin if configured

as o/p, and will have no effect incase of i/p.as o/p, and will have no effect incase of i/p.

Reading latch for output portReading latch for output port

Instructions Reading a Latch (Read-Modify-Write)

Read-modify-write featureRead-modify-write feature

The ports in the 8051 can be accessed The ports in the 8051 can be accessed by the read-modify-write technique. by the read-modify-write technique. (1) reading the port (1) reading the port (2) modifying its value (2) modifying its value (3) writing to the port (3) writing to the port

Sample ProgramsSample Programs Toggle all bits of P0, P1, and P2 every ¼ sec Toggle all bits of P0, P1, and P2 every ¼ sec

assuming crystal frequency of 11.0592 MHzassuming crystal frequency of 11.0592 MHz

Main thing here is the delay loopMain thing here is the delay loop 50% & 66% duty cycle square waves on P1.050% & 66% duty cycle square waves on P1.0 Monitor P0.1 till it is highMonitor P0.1 till it is high

When high, read data from port 1When high, read data from port 1 Low to High Pulse on P0.2 when this happensLow to High Pulse on P0.2 when this happens

P2.3 represents oven conditionP2.3 represents oven condition If high, oven is hot If high, oven is hot high to low pulse on P1.5 to activate buzzerhigh to low pulse on P1.5 to activate buzzer

Sample ProgramsSample Programs

60% duty cycle sq waveform60% duty cycle sq waveform

SETB P1.3SETB P1.3

LCALL DELAYLCALL DELAY


CLR P1.3CLR P1.3


SJMP BACKSJMP BACK

OvenOven

HERE JNB 2.3 HEREHERE JNB 2.3 HERE

SETB P1.5SETB P1.5

CLR P1.5CLR P1.5

SJMP HERESJMP HERE

SWITCH STATUSSWITCH STATUS

SETB P1.7SETB P1.7

AGAIN: JB P1.2 OVERAGAIN: JB P1.2 OVER

MOV P2, #’N’MOV P2, #’N’

SJMP AGAINSJMP AGAIN

OVER: MOV P2, #’Y’OVER: MOV P2, #’Y’


SWITCH TO P0.1SWITCH TO P0.1CHECK SWITCHCHECK SWITCHSWITCH =1 THEN HIGH TO LOW TO SWITCH =1 THEN HIGH TO LOW TO ACTIVATE SIREN ON P1.7ACTIVATE SIREN ON P1.7

SETB P0.1SETB P0.1AGAIN: MOV C, P0.1AGAIN: MOV C, P0.1JNC AGAINJNC AGAINSETB P1.7SETB P1.7CLR P1.7CLR P1.7SJMP AGAINSJMP AGAIN

SWITCH TO P1.0SWITCH TO P1.0

LED TO P2.7LED TO P2.7

SWITCH STATUS SEND TO LEDSWITCH STATUS SEND TO LED

SETB P1.7SETB P1.7

AGAIN: MOV C, P1.0AGAIN: MOV C, P1.0

MOV P2.7, CMOV P2.7, C


Instructions vs. Instructions vs. DirectivesDirectives

Assembler DirectivesAssembler Directives Instructions for the ASSEMBLERInstructions for the ASSEMBLER NOT 8051 instructionsNOT 8051 instructions

Examples:Examples:;;cseg stands for “code segment”cseg stands for “code segment”

cseg atcseg at 1000h 1000h ;address of next ;address of next instruction instruction is 1000his 1000h

GREEN_LED GREEN_LED equequ P1.6 P1.6 ;symbol for Port 1, bit 6;symbol for Port 1, bit 6

Assembler DirectivesAssembler Directives DATADATA

Used to define a name for memory locationsUsed to define a name for memory locationsSP DATA 0x81 ;special function registersSP DATA 0x81 ;special function registersMY_VAL DATA 0x44 ;RAM locationMY_VAL DATA 0x44 ;RAM location

EQUEQU Used to create symbols that can be used to Used to create symbols that can be used to

represent registers, numbers, and addressesrepresent registers, numbers, and addressesLIMIT EQU 2000LIMIT EQU 2000VALUE EQU LIMIT – 200 + 'A'VALUE EQU LIMIT – 200 + 'A'SERIAL EQU SBUFSERIAL EQU SBUFCOUNT EQU R5COUNT EQU R5MY_VAL EQU 0x44MY_VAL EQU 0x44

Address

Registers, numbers, addresses

Data Transfer Data Transfer InstructionsInstructionsMOV dest, sourceMOV dest, source dest dest source source

6 basic types:6 basic types:MOV a, byteMOV a, byte ;move byte to accumulator ;move byte to accumulator

MOV byte, aMOV byte, a ;move accumulator to byte ;move accumulator to byte

MOV Rn, byteMOV Rn, byte ;move byte to register of ;move byte to register of

;current bank;current bank

MOV direct, byte ;move byte to internal RAMMOV direct, byte ;move byte to internal RAM

MOV @Rn, byteMOV @Rn, byte ;move byte to internal RAM ;move byte to internal RAM ;with address contained ;with address contained

in Rnin Rn

MOV DPTR, data16 ;move 16-bit data into data MOV DPTR, data16 ;move 16-bit data into data

;pointer;pointer

Other Data Transfer Other Data Transfer InstructionsInstructions

Stack instructionsStack instructionsPUSH bytePUSH byte ;increment stack pointer, ;increment stack pointer,

;move byte on stack;move byte on stackPOP bytePOP byte ;move from stack to byte, ;move from stack to byte,

;decrement stack ;decrement stack pointerpointer

Exchange instructionsExchange instructionsXCH a, byteXCH a, byte;exchange accumulator and ;exchange accumulator and

;byte;byteXCHD a, byteXCHD a, byte ;exchange low nibbles ;exchange low nibbles of of ;accumulator and ;accumulator and bytebyte

Addressing Modes Addressing Modes

Immediate Mode Immediate Mode – specify data by its value– specify data by its value

mov a, #0mov a, #0 ;put 0 in the accumulator;put 0 in the accumulator

a = 00000000a = 00000000

mov a, #0x11mov a, #0x11 ; put 11hex in the accumulator; put 11hex in the accumulator

a = 00010001a = 00010001

mov a, #11mov a, #11 ; put 11 decimal in accumulator; put 11 decimal in accumulator

a = 00001011a = 00001011

mov a, #77hmov a, #77h ; put 77 hex in accumulator; put 77 hex in accumulator

a = 01110111a = 01110111

Addressing ModesAddressing ModesDirect ModeDirect Mode – specify data by its 8-bit address – specify data by its 8-bit address

mov a, 0x70 mov a, 0x70 ; copy contents of RAM at 70h to a; copy contents of RAM at 70h to a

mov 0xD0, amov 0xD0, a ; put contents of a into PSW; put contents of a into PSW

Addressing ModesAddressing Modes

Register AddressingRegister Addressing – either source or – either source or destination is one of R0-R7 destination is one of R0-R7

mov R0, amov R0, a

mov a, R0mov a, R0

Addressing ModesAddressing ModesRegister IndirectRegister Indirect – the address of the source or – the address of the source or

destination is specified in registersdestination is specified in registers

Uses registers R0 or R1 for 8-bit address:Uses registers R0 or R1 for 8-bit address:mov 0xD0, #0mov 0xD0, #0 ; use register bank 0; use register bank 0

mov r0, #0x3Cmov r0, #0x3C

mov @r0, #3mov @r0, #3 ; memory at 3C gets #3; memory at 3C gets #3

; M[3C] ; M[3C] 3 3

Uses DPTR register for 16-bit addresses:Uses DPTR register for 16-bit addresses:mov dptr, #0x9000mov dptr, #0x9000 ; dptr ; dptr 9000h 9000h

mov a, @dptrmov a, @dptr ; a ; a M[9000] M[9000]

Note that 9000 is an address in external memoryNote that 9000 is an address in external memory

Exercise: Use Exercise: Use Register Indirect to Register Indirect to access upper RAM access upper RAM blockblock


Register Indexed ModeRegister Indexed Mode – source or – source or destination address is the sum of the destination address is the sum of the base addressbase address and the and the accumulatoraccumulator..

Base address can be Base address can be DPTRDPTR or PC or PCmov dptr, #4000hmov dptr, #4000h

mov a, #5mov a, #5

movc a, @a + dptr ;a movc a, @a + dptr ;a M[4005] M[4005]


Register Indexed ModeRegister Indexed Mode Base address can be DPTR or Base address can be DPTR or PCPC

AddrAddr cseg at 0x1000hcseg at 0x1000h

1000 1000 mov a, #5mov a, #5

10021002 movc a, @a + movc a, @a + PCPC ;a ;a M[1008]M[1008]

10031003 nopnop

PC

A and B RegistersA and B Registers

A and B are “accumulators” for arithmetic A and B are “accumulators” for arithmetic instructionsinstructions

They can be accessed by They can be accessed by direct modedirect mode as as special function registers:special function registers:

B – address 0F0hB – address 0F0h

A – address 0E0h - use “ACC” for direct modeA – address 0E0h - use “ACC” for direct mode

Address ModesAddress ModesStack-oriented data transfer – another form of Stack-oriented data transfer – another form of

register direct addressing, but using SPregister direct addressing, but using SP

mov sp, #0x40mov sp, #0x40 ; Initialize SP; Initialize SPpush 0x55push 0x55 ; SP ; SP SP+1, M[SP] SP+1, M[SP] M[55] M[55]

; M[41] ; M[41] M[55] M[55]pop bpop b ; b ; b M[55] M[55]

Note: can only specify RAM or SFRs (direct mode) to push or Note: can only specify RAM or SFRs (direct mode) to push or pop. Therefore, to push/pop the accumulator, must use acc, pop. Therefore, to push/pop the accumulator, must use acc, not a:not a:

push accpush accpush apush a

StacksStacks

pushpop

stack

stack pointer

Go do the stack exercise…..

Address ModesAddress Modes

Exchange Instructions – two way data Exchange Instructions – two way data transfertransferXCH a, 0x30XCH a, 0x30 ; a ; a M[30] M[30]

XCH a, R0XCH a, R0 ; a ; a R0 R0

XCH a, @R0XCH a, @R0 ; a ; a M[R0] M[R0]

XCHD a, R0XCHD a, R0 ; exchange ; exchange “digit”“digit” R0[7..4] R0[3..0]a[7..4] a[3..0]

Only 4 bits exchanged

Address ModesAddress Modes Bit-Oriented Data TransferBit-Oriented Data Transfer – transfers between individual bits. – transfers between individual bits. SFRs with addresses ending in 0 or 8 are bit-addressable. (80, 88, 90, 98, SFRs with addresses ending in 0 or 8 are bit-addressable. (80, 88, 90, 98,

etc)etc) Carry flag (C) (bit 7 in the PSW) is used as a single-bit accumulatorCarry flag (C) (bit 7 in the PSW) is used as a single-bit accumulator RAM bits in addresses 20-2F are bit addressableRAM bits in addresses 20-2F are bit addressable

Examples of bit transfers of special function register bits:Examples of bit transfers of special function register bits:mov C, P0.0mov C, P0.0 ; C ; C bit 0 of P0 bit 0 of P0

Bit Addressable MemoryBit Addressable Memory20h – 2Fh (16 locations X 8-bits = 128 bits)

7F7F 7878

1A1A

1010

0F0F 0808

0707 0606 0505 0404 0303 0202 0101 0000

27

26

25

24

23

22

21

20

2F

2E

2D

2C

2B

2A

29

28

Bit addressing:mov C, 1Ahormov C, 23h.2

SFRs that are Bit SFRs that are Bit AddressableAddressableSFRs with addresses SFRs with addresses

of multiples of 0 and of multiples of 0 and 8 are bit 8 are bit addressable.addressable.

Notice that all 4 Notice that all 4 parallel I/O ports are parallel I/O ports are bit addressable.bit addressable.

SFRsSFRs

Pink are Pink are implemented in implemented in

enhanced enhanced C8051F020C8051F020

AddressAddress RegisterRegister

0xF80xF8 SPI0CNSPI0CN

0xF00xF0 BB

0xE80xE8 ADC0CNADC0CN

0xE00xE0 ACCACC

0xD80xD8 PCA0CNPCA0CN

0xD00xD0 PSWPSW

0xC80xC8 T2CONT2CON

0xC00xC0 SMB0CNSMB0CN

0xB80xB8 IPIP

0xB00xB0 P3P3

0xA80xA8 IEIE

0xA00xA0 P2P2

0x980x98 SCONSCON

0x900x90 P1P1

0x880x88 TCONTCON

0x800x80 P0P0

8051 SERIAL PORT 8051 SERIAL PORT PROGRAMMINGPROGRAMMING

BASICS OF SERIAL BASICS OF SERIAL COMMUNICATIONCOMMUNICATION

Figure 1 Serial versus Parallel Data Transfer


serial communication uses single data line making it much serial communication uses single data line making it much cheapercheaper

enables two computers in different cities to communicate enables two computers in different cities to communicate over the telephoneover the telephone

byte of data must be converted to serial bits using a parallel-byte of data must be converted to serial bits using a parallel-in-serial-out register and transmitted over a single data linein-serial-out register and transmitted over a single data line

receiving end there must be a serial-in-parallel-out shift receiving end there must be a serial-in-parallel-out shift registerregister

if transferred on the telephone line, it must be converted to if transferred on the telephone line, it must be converted to audio tones by audio tones by modemmodem

for short distance the signal can be transferred using wirefor short distance the signal can be transferred using wire how PC keyboards transfer data to the motherboard how PC keyboards transfer data to the motherboard


2 methods, asynchronous and synchronous2 methods, asynchronous and synchronous synchronous synchronous method transfers a block of data method transfers a block of data

(characters) at a time(characters) at a time asynchronous asynchronous method transfers a single byte method transfers a single byte

at a timeat a time Uses special IC chips called UART (universal Uses special IC chips called UART (universal

asynchronous receiver-transmitter) and asynchronous receiver-transmitter) and USART (universal synchronous asynchronous USART (universal synchronous asynchronous receiver-transmitter)receiver-transmitter)

8051 chip has a built-in UART 8051 chip has a built-in UART


Figure 2 Simplex, Half-, and Full-Duplex Transfers


Half- and full-duplex transmissionHalf- and full-duplex transmission if the data can be transmitted and received, it is a if the data can be transmitted and received, it is a

duplex duplex transmissiontransmission simplex simplex transmissions the computer only sends datatransmissions the computer only sends data duplex transmissions can be half or full duplexduplex transmissions can be half or full duplex depends on whether or not the data transfer can be depends on whether or not the data transfer can be

simultaneoussimultaneous If one way at a time, it is If one way at a time, it is half duplexhalf duplex If can go both ways at the same time, it is full duplexIf can go both ways at the same time, it is full duplex full duplex requires two wire conductors for the data full duplex requires two wire conductors for the data

lines (in addition to the signal ground)lines (in addition to the signal ground)


Asynchronous serial communication Asynchronous serial communication and data framingand data framing data coming in 0s and 1sdata coming in 0s and 1s to make sense of the data sender and to make sense of the data sender and

receiver agree on a set of rulesreceiver agree on a set of rules ProtocolProtocol

how the data is packedhow the data is packed how many bits/characterhow many bits/character when the data begins and endswhen the data begins and ends


Start and stop bitsStart and stop bits asynchronous method, each character is asynchronous method, each character is

placed between start and stop bitsplaced between start and stop bits called called framingframing start bit is always one bitstart bit is always one bit stop bit can be one or two bitsstop bit can be one or two bits start bit is always a 0 (low)start bit is always a 0 (low) stop bit(s) is 1 (high)stop bit(s) is 1 (high) LSB is sent out firstLSB is sent out first

SECTION 10.1: BASICS OF SECTION 10.1: BASICS OF SERIAL COMMUNICATIONSERIAL COMMUNICATION

Figure 3 Framing ASCII “A” (41H)


in modern PCs one stop bit is standardin modern PCs one stop bit is standard when transferring a text file of ASCII when transferring a text file of ASCII

characters using 1 stop bit there is total characters using 1 stop bit there is total of 10 bits for each characterof 10 bits for each character

8 bits for the ASCII code (1 parity bit), 1 8 bits for the ASCII code (1 parity bit), 1 bit each for the start and stop bitsbit each for the start and stop bits

for each 8-bit character there are an extra for each 8-bit character there are an extra 2 bits, which gives 20% overhead2 bits, which gives 20% overhead


Data transfer rateData transfer rate rate of data transfer rate of data transfer bps bps (bits per second)(bits per second) widely used terminology for bps is widely used terminology for bps is baud baud

raterate baud and bps rates are not necessarily baud and bps rates are not necessarily

equalequal baud rate is defined as the number of baud rate is defined as the number of

signal changes per secondsignal changes per second


RS232 standardsRS232 standards most widely used serial I/O interfacing standardmost widely used serial I/O interfacing standard input and output voltage levels are not TTL compatibleinput and output voltage levels are not TTL compatible 1 bit is represented by -3 to -25 V1 bit is represented by -3 to -25 V 0 bit is +3 to +25 V0 bit is +3 to +25 V -3 to +3 is undefined-3 to +3 is undefined to connect RS232 to a microcontroller system must use to connect RS232 to a microcontroller system must use

voltage converters such as MAX232 to convert the TTL voltage converters such as MAX232 to convert the TTL logic levels to the RS232 voltage levels, and vice versalogic levels to the RS232 voltage levels, and vice versa

MAX232 IC chips are commonly referred to as line MAX232 IC chips are commonly referred to as line driversdrivers


Table 2 IBM PC DB-9 Signals

Figure 5 DB-9 9-Pin Connector


Data communication classificationData communication classification DTE (data terminal equipment) DTE (data terminal equipment) DCE (data communication equipment)DCE (data communication equipment) DTE - terminals and computers that send and DTE - terminals and computers that send and

receive datareceive data DCE - communication equipment responsible for DCE - communication equipment responsible for

transferring the datatransferring the data simplest connection between a PC and simplest connection between a PC and

microcontroller requires a minimum of three microcontroller requires a minimum of three pins, TxD, RxD, and groundpins, TxD, RxD, and ground


Figure 6 Null Modem Connection


Examining RS232 hand shaking Examining RS232 hand shaking signalssignals many of the pins of the RS-232 connector many of the pins of the RS-232 connector

are used for handshaking signalsare used for handshaking signals they are not supported by the 8051 UART they are not supported by the 8051 UART

chipchip


PC/compatible COM portsPC/compatible COM ports PC/compatible computers (Pentium) PC/compatible computers (Pentium)

microprocessors normally have two COM microprocessors normally have two COM portsports

both ports have RS232-type connectorsboth ports have RS232-type connectors COM ports are designated as COM 1 and COM ports are designated as COM 1 and

COM 2COM 2 can connect the 8051 serial port to the can connect the 8051 serial port to the

COM 2 portCOM 2 port

8051 CONNECTION TO 8051 CONNECTION TO RS232RS232

RxD and TxD pins in the 8051RxD and TxD pins in the 8051 8051 has two pins used for transferring and 8051 has two pins used for transferring and

receiving data seriallyreceiving data serially TxD and RxD are part of the port 3 group TxD and RxD are part of the port 3 group pin 11 (P3.1) is assigned to TxDpin 11 (P3.1) is assigned to TxD pin 10 (P3.0) is designated as RxDpin 10 (P3.0) is designated as RxD these pins are TTL compatiblethese pins are TTL compatible require a line driver to make them RS232 require a line driver to make them RS232

compatiblecompatible driver is the MAX232 chipdriver is the MAX232 chip


MAX232MAX232 converts from RS232 voltage levels to TTL converts from RS232 voltage levels to TTL

voltage levelsvoltage levels uses a +5 V power sourceuses a +5 V power source MAX232 has two sets of line drivers for MAX232 has two sets of line drivers for

transferring and receiving datatransferring and receiving data line drivers used for TxD are called T1 and T2line drivers used for TxD are called T1 and T2 line drivers for RxD are designated as R1 and R2line drivers for RxD are designated as R1 and R2 T1 and R1 are used together for TxD and RxD of T1 and R1 are used together for TxD and RxD of

the 8051the 8051 second set is left unusedsecond set is left unused

SECTION 10.2: 8051 SECTION 10.2: 8051 CONNECTION TO RS232CONNECTION TO RS232

Figure 7 (a) Inside MAX232(b) its Connection to the 8051 (Null Modem)

SECTION 10.2: 8051 SECTION 10.2: 8051 CONNECTION TO RS232CONNECTION TO RS232

MAX233MAX233 MAX233 performs the same job as the MAX233 performs the same job as the

MAX232MAX232 eliminates the need for capacitorseliminates the need for capacitors much more expensive than the MAX232much more expensive than the MAX232


Figure 8 (a) Inside MAX233(b) Its Connection to the 8051 (Null Modem)

8051 SERIAL PORT 8051 SERIAL PORT PROGRAMMING IN PROGRAMMING IN ASSEMBLYASSEMBLY

Baud rate in the 8051Baud rate in the 8051 serial communications of the 8051 with serial communications of the 8051 with

the COM port of the PCthe COM port of the PC must make sure that the baud rate of the must make sure that the baud rate of the

8051 system matches the baud rate of the 8051 system matches the baud rate of the PC's COM portPC's COM port

can use Windows HyperTerminal can use Windows HyperTerminal programprogram


Table 3 PC Baud Rates


Baud rate in the 8051Baud rate in the 8051 baud rate in the 8051 is programmablebaud rate in the 8051 is programmable done with the help of Timer 1done with the help of Timer 1 relationship between the crystal frequency and the baud rate in relationship between the crystal frequency and the baud rate in

the 8051the 8051 8051 divides the crystal frequency by 12 to get the machine 8051 divides the crystal frequency by 12 to get the machine

cycle frequencycycle frequency XTAL = 11.0592 MHz, the machine cycle frequency is 921.6 kHzXTAL = 11.0592 MHz, the machine cycle frequency is 921.6 kHz 8051's UART divides the machine cycle frequency of 921.6 kHz 8051's UART divides the machine cycle frequency of 921.6 kHz

by 32 once more before it is used by Timer 1 to set the baud rateby 32 once more before it is used by Timer 1 to set the baud rate 921.6 kHz divided by 32 gives 28,800 Hz921.6 kHz divided by 32 gives 28,800 Hz Timer 1 must be programmed in mode 2, that is 8-bit, auto-reloadTimer 1 must be programmed in mode 2, that is 8-bit, auto-reload


Table 4 Timer 1 TH1 Register Values for Various Baud Rates

Example 1Example 1With XTAL = 11.0592 MHz, find the TH1 value With XTAL = 11.0592 MHz, find the TH1 value needed to have the following baud rates.needed to have the following baud rates.

(a) 9600(a) 9600 (b) 2400(b) 2400 (c) 1200(c) 1200 machine cycle frequencymachine cycle frequency

= 11.0592 MHz / 12 = 921.6 kHz= 11.0592 MHz / 12 = 921.6 kHz Timer 1 frequency provided by 8051 Timer 1 frequency provided by 8051

UARTUART= 921.6 kHz / 32 = 28,800 Hz = 921.6 kHz / 32 = 28,800 Hz

(a) 28,800 / 3 = 9600(a) 28,800 / 3 = 9600 where -3where -3 = FD (hex)= FD (hex)(b) 28,800 / 12 = 2400(b) 28,800 / 12 = 2400 where -12where -12 = F4 (hex)= F4 (hex)

(c) 28,800 / 24 = 1200(c) 28,800 / 24 = 1200 where -24where -24 = E8 (hex= E8 (hex))


11.0592 MHz11.0592 MHzXTAL XTAL

oscillatoroscillator


SBUF (serial buffer) registerSBUF (serial buffer) register a byte of data to be transferred via the TxD line must a byte of data to be transferred via the TxD line must

be placed in the SBUF registerbe placed in the SBUF register SBUF holds the byte of data when it is received by the SBUF holds the byte of data when it is received by the

RxD lineRxD line can be accessed like any other registercan be accessed like any other register

MOV SBUF,#'D'MOV SBUF,#'D' ;load SBUF=44H, ASCII for 'D‘;load SBUF=44H, ASCII for 'D‘MOV SBUF,AMOV SBUF,A ;copy accumulator into SBUF;copy accumulator into SBUFMOV A,SBUFMOV A,SBUF ;copy SBUF into accumulator;copy SBUF into accumulator

when a byte is written, it is framed with the start and when a byte is written, it is framed with the start and stop bits and transferred serially via the TxD pinstop bits and transferred serially via the TxD pin

when the bits are received serially via RxD, it is when the bits are received serially via RxD, it is deframe by eliminating the stop and start bits, making deframe by eliminating the stop and start bits, making a byte out of the data received, and then placing it in a byte out of the data received, and then placing it in the SBUFthe SBUF


SCON (serial control) registerSCON (serial control) register to program the start bit, stop bit, and data to program the start bit, stop bit, and data

bitsbits

Figure 9 SCON Serial Port Control Register (Bit-Addressable)


SM0 and SM1 determine the modeSM0 and SM1 determine the mode only mode 1 is importantonly mode 1 is important when mode 1 is chosen, the data framing is 8 when mode 1 is chosen, the data framing is 8

bits, 1 stop bit, and 1 start bitbits, 1 stop bit, and 1 start bit compatible with the COM port of PCscompatible with the COM port of PCs mode 1 allows the baud rate to be variable and mode 1 allows the baud rate to be variable and

is set by Timer 1 of the 8051is set by Timer 1 of the 8051 for each character a total of 10 bits are for each character a total of 10 bits are

transferred, where the first bit is the start bit, transferred, where the first bit is the start bit, followed by 8 bits of data, and finally 1 stop bit.followed by 8 bits of data, and finally 1 stop bit.


REN (receive enable) REN (receive enable) REN=1, allows 8051 to receive data on REN=1, allows 8051 to receive data on

the RxDthe RxD if 8051 is to both transfer and receive if 8051 is to both transfer and receive

data, REN must be set to 1data, REN must be set to 1 REN=0, the receiver is disabledREN=0, the receiver is disabled SETB SCON.4 and CLR SCON.4, SETB SCON.4 and CLR SCON.4,


TI (transmit interrupt)TI (transmit interrupt) when 8051 finishes the transfer of the 8-bit when 8051 finishes the transfer of the 8-bit

character, it raises the TI flag to indicate that it is character, it raises the TI flag to indicate that it is ready to transfer another byteready to transfer another byte

RI (receive interrupt)RI (receive interrupt) when the 8051 receives data serially via RxD, it when the 8051 receives data serially via RxD, it

places the byte in the SBUF registerplaces the byte in the SBUF register then raises the RI flag bit to indicate that a byte has then raises the RI flag bit to indicate that a byte has

been received and should be picked up before it is been received and should be picked up before it is lostlost


Program to transfer data seriallyProgram to transfer data serially1.1. TMOD register is loaded with the value 20HTMOD register is loaded with the value 20H2.2. TH1 is loaded with value to set the baud rateTH1 is loaded with value to set the baud rate3.3. SCON register is loaded with the value 50HSCON register is loaded with the value 50H4.4. TR1 is set to 1 to start Timer1TR1 is set to 1 to start Timer15.5. TI is cleared by the "CLR TI" instructionTI is cleared by the "CLR TI" instruction6.6. transmit character byte is written into the SBUF transmit character byte is written into the SBUF

registerregister7.7. TI flag bit is monitored to see if the character has TI flag bit is monitored to see if the character has

been transferred completelybeen transferred completely8.8. to transfer the next character, go to Step 5.to transfer the next character, go to Step 5.

Example 2Example 2Write a program to transfer letter "A" Write a program to transfer letter "A" serially at 4800 baud, continuously.serially at 4800 baud, continuously.

Example 3Example 3Write a program to transfer the message Write a program to transfer the message "YES" serially at 9600 baud, 8-bit data, 1 "YES" serially at 9600 baud, 8-bit data, 1 stop bit. Do this continuously. stop bit. Do this continuously.


Importance of the TI flagImportance of the TI flag check the TI flag bit, we know whether check the TI flag bit, we know whether

can transfer another bytecan transfer another byte TI flag bit is raised by the 8051TI flag bit is raised by the 8051 TI flag cleared by the programmerTI flag cleared by the programmer writing a byte into SBUF before the TI flag writing a byte into SBUF before the TI flag

bit is raised, may lead to loss of a portion bit is raised, may lead to loss of a portion of the byte being transferredof the byte being transferred


Program to receive data seriallyProgram to receive data serially1.1. TMOD register is loaded with the value 20HTMOD register is loaded with the value 20H2.2. TH1 is loaded with value set the baud rateTH1 is loaded with value set the baud rate3.3. SCON register is loaded with the value 50HSCON register is loaded with the value 50H4.4. TR1 is set to 1 to start Timer 1TR1 is set to 1 to start Timer 15.5. RI is cleared with the "CLR RI" instructionRI is cleared with the "CLR RI" instruction6.6. RI flag bit is monitored to see if an entire RI flag bit is monitored to see if an entire

character has been received yetcharacter has been received yet7.7. RI=1 SBUF has the byte, its contents are moved RI=1 SBUF has the byte, its contents are moved

into a safe placeinto a safe place8.8. to receive the next character, go to Step 5to receive the next character, go to Step 5

Example 4Example 4Program the 8051 to receive bytes of data Program the 8051 to receive bytes of data serially, and put them in P1. Set the baud serially, and put them in P1. Set the baud rate at 4800, 8-bit data, and 1 stop bit.rate at 4800, 8-bit data, and 1 stop bit.


Importance of the RI flag bitImportance of the RI flag bit1.1. it receives the start bit, next bit is the first bit of it receives the start bit, next bit is the first bit of

the characterthe character

2.2. when the last bit is received, a byte is formed when the last bit is received, a byte is formed and placed in SBUFand placed in SBUF

3.3. when stop bit is received, makes RI = 1when stop bit is received, makes RI = 1

4.4. when RI=1, received byte is in the SBUF when RI=1, received byte is in the SBUF register, copy SBUF contents to a safe placeregister, copy SBUF contents to a safe place

5.5. after the SBUF contents are copied the RI flag after the SBUF contents are copied the RI flag bit must be cleared to 0bit must be cleared to 0


Doubling the baud rate in the 8051Doubling the baud rate in the 8051 two ways to increase the baud ratetwo ways to increase the baud rate

1.1. Use a higher-frequency crystalUse a higher-frequency crystal

2.2. Change a bit in the PCON registerChange a bit in the PCON register

Table 5 Baud Rate Comparison for SMOD = 0 and SMOD = 1

Embedded Systems with 8051 µC

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