The 8051 Microcontroller

Prepared By,R-THANDAIAH PRABU M.E.,

Lecturer - ECEthandaiah@gmail.com

Serial Communication

8051 and PC• The 8051 module connects to PC by using RS232.

• RS232 is a protocol which supports half-duplex, synchronous/asynchronous, serial communication.

• We discuss these terms in following sections.

PC 8051

COM 1 port

RS232

MAX232 UART

RS232 pins

Data communication classification

Simplex vs. Duplex Transmission

• Simplex transmission: the data can sent in one direction.– Example: the computer only sends data to

the printer.

• Duplex transmission: the data can be transmitted and receive

Transmitter Receiver

Transmitter

ReceiverReceiver

Transmitter

Half vs. Full Duplex

• Half duplex: if the data is transmitted one way at a time.

• Full duplex: if the data can go both ways at the same time.– Two wire conductors for the data lines.

Transmitter

Receiver

Receiver

Transmitter

Transmitter

Receiver

Receiver

Transmitter

Serial vs Parallel Data Transfer

Sender Receiver Sender Receiver

Serial Transfer Parallel Transfer

D0-D7D0

Other control lines

Other control lines

Serial Communication• How to transfer data?

– Sender:• The byte of data must be converted to serial bits using a

parallel-in-serial-out shift register.• The bit is transmitted over a single data line.

– Receiver• The receiver must be a serial-in-parallel-out shift register to

receive the serial data and pack them into a byte.

11101000001011

‘A’

register8-bit character

register8 1

parallel-in serial-out

serial-in parallel-out

Asynchronous vs. Synchronous• Serial communication uses two methods:

– In synchronous communication, data is sent in blocks of bytes.

– In asynchronous communication, data is sent in bytes.

byte byte byte byte 01011111

preamble 01010101

sender receiver

byte

sender receiverstart bitstop bit

bytebyte

UART & USART• It is possible to write software to use both methods,

but the programs can be tedious and long.

• Special IC chips are made for serial communication:– USART (universal synchronous-asynchronous receiver-

transmitter)– UART (universal asynchronous receiver-transmitter)

• The 8051 chip has a built-in UART.– Half-duplex– Asynchronous mode only

Framing (1/3)• How to detect that a character is sent via the

line in the asynchronous mode?– Answer: Data framing!

• Each character is placed in between start and stop bits. This is called framing.

stopbit

startbit

mark0 0 0 0 0 01 1

D7 D0goes out last goes out first

Time (D0 first)

mark

Framing (2/3)

• The LSB is sent out first.

• The start bit is 0 (low) and always one bit.

• The stop bits is 1 (high).

• The stop bit can be one (if 8 bits used in ASCII) or two bits (if 7 bits used in ASCII).

– In asynchronous serial communication, peripheral chips and modems can be programmed for data that is 7 or 8 bits.

• When there is no transfer, the signal is 1 (high), which is referred to as mask.

Framing (3/3)• We have a total of 10 bits for each character:

– 8-bits for the ASCII code– 2-bits for the start and stop bits

• In some systems in order to maintain data integrity, the parity bit is included in the data frame.– In an odd-parity bit system the total number of bits,

including the parity bit, is odd.– UART chips allow programming of the parity bit for

odd-, even-, and no-parity options.

TxD and RxD pins in the 8051

• In 8051, the data is received from or transmitted to– RxD: received data (Pin 10, P3.0)– TxD: transmitted data (Pin 11, P3.1)

• TxD and RxD of the 8051 are TTL compatible.

• The 8051 requires a line driver to make them RS232 compatible.

Baud Rates in the 8051

• The 8051 transfers and receives data serially at many different baud rates by using UART.

• UART divides the machine cycle frequency by 32 and sends it to Timer 1 to set the baud rate.

• Signal change for each roll over of timer 1

XTAL oscillator

÷ 12÷ 32

By UART

Machine cycle frequency

28800 Hz

To timer 1 To set the Baud rate

921.6 kHz

11.0592 MHz

Timer 1

Instructions Set

• 255 instructions, mnemonic code

• 1-3 byte instructions

• Opcode (operation code) +1,2 bytes address, data

• 1-byte instructions – 139

2-byte instructions – 92

3-byte instructions – 24

• Coding format– Opcode (operation code) + operands

• Operands – addressing modes– Registers– Direct : address– Indirect– Immediate : constant– Relative : jump, branch, call (change PC)– Long : call– Indexed : array (base+offset)

8051 Addressing modes. (a) Register addressing (b) Direct addressing (c Indirect addressing (d) Immediate addressing (e) Relative addressing (f) Absolute addressing (g) Long addressing (h) Indexed addressing

Coding format- Addressing modes

8051 RegistersD7 D6 D5 D4 D3 D2 D1 D0

DPTR

PC PC (Program counter)

DPH DPL

8 bit Registers of the 8051

8051 16 bit Registers

8 bit Registers

R6

R5

R4

R3

R2

R1

R0

B

A

R7

Five Addressing Modes

• Immediate

• Register

• Direct

• Register indirect

• Indexed

Immediate Addressing Mode

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

MOV R4,#62 ;load the decimal value 62 into R4

MOV B,#40H ;load 40H into B

MOV DPTR,#4521H ;DPTR=4521H

MOV DPTR,#2550H ;is the same as:

MOV DPL,#50H

MOV DPH,#25H

Register Addressing Mode

MOV A,R0 ;copy the contents of R0 into A

MOV R2,A ;copy the contents of A into R2

ADD A,R5 ;add the contents of R5 to contents of A

ADD A,R7 ;add the contents of R7 to contents of A

MOV R6,A ;save accumulator in R6

MOV DPTR,#25F5H

MOV R7,DPL

MOV R6,DPH

Direct Addressing Mode

• RAM addresses 00 to 7FH

MOV R0,40H ;save content of RAM location 40H in R0

MOV 56H,A ;save content of A in RAM location 56H

MOV R4,7FH ;move contents of RAM location 7FH to R4

MOV A,4 ;is same as

MOV A,R4 ;which means copy R4 into A

MOV A,7 ;is same as

MOV A,R7 ;which means copy R7 into A

MOV A,2 ;is the same as

MOV A,R2 ;which means copy R2 into A

MOV A,0 ;is the same as

MOV A,R0 ;which means copy R0 into A

MOV R2,#5 ;R2=05

MOV A,2 ;copy R2 to A (A=R2=05)

MOV B,2 ;copy R2 to B (B=R2=05)

MOV 7,2 ;copy R2 to R7

;since “MOV R7,R2” is invalid

SFR Registers & Their Addresses

MOV 0E0H,#55H ;is the same as

MOV A,#55H ;which means load 55H into A (A=55H)

MOV 0F0H,#25H ;is the same as

MOV B,#25H ;which means load 25H into B (B=25H)

MOV 0E0H,R2 ;is the same as

MOV A,R2 ;which means copy R2 into A

MOV 0F0H,R0 ;is the same as

MOV B,R0 ;which means copy R0 into B

SFR Addresses ( 1 of 2 )

SFR Addresses ( 2 of 2 )

Example

Stack and Direct Addressing Mode

• Only direct addressing is allowed for stack

Register Indirect Addressing Mode

• Only R0 & R1 can be used

MOV A,@R0 ;move contents of RAM location whose

;address is held by R0 into A

MOV @R1,B ;move contents of B into RAM location

;whose address is held by R1

Advantage of Register Indirect Addressing

• Looping not possible in direct addressing

Index Addressing Mode & On-chip ROM Access

• Limitation of register indirect addressing:

• 8-bit addresses (internal RAM)

• DPTR: 16 bits

• MOVC A, @A+DPTR ; “C” means

program (code) space ROM

• Instructions Registers– Arithmetic– Logic– Data transfer– Boolean– Program branch

Quick reference chart

Arithmetic/Logic operations

Carryborrow

MOV Instruction

• MOV destination, source ; copy source to dest.• MOV A,#55H ;load value 55H into reg. A

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

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

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

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

MOV A,R3 ;copy contents of R3 into A ;now A=R3=95H

Notes on Programming

• Value (proceeded with #) can be loaded directly to registers A, B, or R0 – R7– MOV R5, #0F9H

• If values 0 to F moved into an 8-bit register, the rest assumed all zeros– MOV A, #5

• A too large value causes an error– MOV A, #7F2H

ADD Instruction

• ADD A, source ;ADD the source operand

;to the accumulator

• MOV A, #25H ;load 25H into AMOV R2,#34H ;load 34H into R2ADD A,R2 ;add R2 to accumulator

;(A = A + R2)

Structure of Assembly Language

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

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

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

;now A = A + 12HHERE: SJMP HERE ;stay in this loop

END ;end of asm source file

Sample of an Assembly Language Program

Interrupt :

Interrupt Enable Register :

• EA : Global enable/disable.• --- : Undefined.

• ET2 :Enable Timer 2 interrupt.• ES :Enable Serial port interrupt.• ET1 :Enable Timer 1 interrupt.• EX1 :Enable External 1 interrupt.• ET0 : Enable Timer 0 interrupt. • EX0 : Enable External 0 interrupt.

SJCET

• Six interrupts in the 8051

– 1 reset interrupt, when the reset pin is activated, the 8051 jumps to address location 0000

– 2 timer interrupts

– 2 external hardware interrupts

– pin 12 (P3.2) and 13 (P3.3) in port 3 are for the external hardware interrupts

– 1 serial communication interrupt that belongs to both receive and transmit

– a limited number of bytes is set aside for each interrupt

SJCET

• Enabling and disabling an interrupt

– upon reset all interrupts are disabled

– interrupts must be enabled by software

– IE register (interrupt enable) is responsible for enabling and disabling the interrupts

– IE is a bit-addressable register

SJCET

• Steps in enabling an interrupt

1. EA must be set to 1

2. set the relevant bits in IE register to high

– EA = 0, no interrupt will be responded to, even if the relevant bit in the IE register is high

SJCET

• Interrupt priority upon reset

SJCET

INTERFACING TO EXTERNAL MEMORY

SEMICONDUCTOR MEMORY

SJCET

• Memory capacity

• The number of bits that a semiconductor memory chip can store is called chip capacity.

• It can be in units of Kbits (kilobits), Mbits (megabits), and so on.

SJCET

Memory organization

• Memory chips are organized into a number of locations within the IC.

• Each location can hold 1 bit, 4 bits, 8 bits, or even 16 bits, depending on how it is designed internally.

Speed

SJCET

• The speed of the memory chip is commonly referred to as its access time.

• The access time of memory chips varies from a few nanoseconds to hundreds of nanoseconds, depending on the IC technology used in the design and process.

ROM (read-only memory)

SJCET

• ROM is a type of memory that does not lose its contents when the power is turned off.

• For this reason, ROM is also called nonvolatile memory.

PROM (programmable ROM)

SJCET

• PROM is programmed by blowing the fuses.

• If the information burned into PROM is wrong, that PROM must be discarded since its internal fuses are blown permanently.

Flash memory EPROM

SJCET

• flash memory can be programmed while it is in its socket on the system board, it is widely used to upgrade the BIOS ROM of the PC.

• flash memory is semiconductor memory with access time in the range of 100 ns compared with disk access time in the range of tens of milliseconds.

Mask ROM

SJCET

• Mask ROM refers to a kind of ROM in which the contents are programmed by the IC manufacturer.

• Mask ROM is used when the needed volume is high (hundreds of thousands) and it is absolutely certain that the contents will not change.

RAM (random access memory)

SJCET

• RAM memory is called volatile memory since cutting off the power to the IC results in the loss of data.

SRAM (static RAM)

SJCET

• Storage cells in static RAM memory are made of flip-flops and therefore do not require refreshing in order to keep their data. This is in contrast to DRAM.

DRAM (dynamic RAM)

SJCET

• uses a capacitor to store each bit

• requires constant refreshing due to

leakage

SJCET

The 8051 is designed for small systems, in order to reduce the number of pins used, time multiplexing of signals is used. For example, port 0 and p2 are used as input/output ports or memory interface pins. related hardware pins are as follows:

(1)Address and data bus time-multiplexing for external memory devices

(2) External Program memory read: Hardware pins involved: port 0 and port 2 (P0.1~P0.7, P2.1~P2.7 = 16 bits), and /ALE, /PSEN

(3) External Data memory read/write: Hardware pins involved: port 0 and port 2 (P0.1~P0.7, P2.1~P2.7 = 16 bits), and /ALE, /PSEN, /RD, /WR