Post on 17-Mar-2020
PROGRAMMABLE PERIPHERAL
INTERFACE (PPI) -8255
• 8255 is a general purpose programmable device usedfor data transfer between processor and I/O devices.
• It has 3 programmable I/O ports (PA,PB &PC) andport operation (IN/OUT Port) is defined by controlword in the control word register.
• Ports are operated in two modes:
• i) I/O modes: Mode 0, Mode 1,& Mode 2
• Ii) BSR (Bit set/Reset) mode
About 8255
• PPI has 40 pins and it has three distinct modes ofoperation.
• Port A (PA7-PA0) :8 pins
• Port B (PB7-PB0) :8 pins
• Port C (Pc: Upper: PC7-PC4) : 4 pins
• Port C (Pc: Lower: PC3-PC)) : 4 pins
• Data Bus (D7-D0) : 8 pins
• Control signals : 6 pins
• VCC and Gnd : 2 pins
Pin Diagram
Pin names and functionPin name No.of
pinsI/O functions
PA0-PA7 8 i/o Tristate
Port can be configured either input or output by
software
Port has output latch buffer and input buffer
PA can be programmed by mode 0 , mode 1,
mode 2 .
PB can be programmed by mode 0 and mode 1.
PC can be programmed by bit set/reset
operation.
Port C can be divided into two 4 bit ports namely
PC7-PC4 & PC3-PCO and used for control
signals to PA and PB
PB0-PB7 8 i/o Tristate
PC0-PC7 8 i/o Tristate
D0-D7 8 i/o Tristate
Used for data transfer with MPU
Transfer of control words to PPI
Read status information from PPI
8255 Block Diagram
Group A and Group B control:
Group A and B get the Control Signal from CPU and send the command to the
individual control blocks.
Group A send the control signal to port A and Port C (Upper) PC7-PC4.
Group B send the control signal to port B and Port C (Lower) PC3-PC0.
• FOR I/O MODE:
The control word mode format for I/O as shown in figure
D7 D6 D5 D4 D3 D2 D1 D0
Group A
Port C Upper
1=Input
0=Output
Port A
1=Input
0=Output
Mode selection
00=mode 0
01=mode 1
1x=mode 2
Group B
Port C Lower
1=Input
0=Output
Port B
1=Input
0=Output
Mode selection
0=mode 0
1=mode 1
Mode set
1: i/o MODE
0: BSR mode
operation modes: i) I/O modes (M0,M1,&M2) ii) BSR (Bit set/Reset) mode
BIT SET/RESET MODE:The PORT C can be Set or Reset by sending OUT instruction to the CONTROL registers.
Mode 1:Handshake interrupt i/p port
When i/p device has data to send it checks if IBF (input buffer full) signal is 0.
If 0, it sends data on PA/PB7-0 and activates STB* (Strobe) signal. (STB* is active low. )
When STB* goes high, the data enters the port and IBF gets activated.
If the Port interrupt is enabled, INT is activated. This interrupts the processor.
Processor reads the port during the ISS. Then IBF and INT get deactivated.
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82C55: Mode 1 Strobed Input
INTE A Controlled by bit set / reset of PC4. INTE B Controlled by bit set / reset of PC2.
Handshake interrupt o/p portWhen o/p device wants to receive data it checks if OBF* (output buffer full) signal is 0.
If 0, it receives data on PB7-0 and activates ACK* (Acknowledge) signal. ACK* is active low.
When ACK* goes high, the data goes out of the port and OBF* is set to 1.
If the Port interrupt is enabled, INT is activated. This interrupts the processor.
Processor sends another byte to the port during the ISS. Then OBF* and INT are reset to 0.
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Mode 1 o/p mode
INTE A Controlled by bit set/reset of PC6. INTE B Controlled by bit set/reset of PC2.
82C55: Mode 2 Bi-directional Operation:
82C55: Mode 2 Bi-directional Operation
• INTR : Interrupt request is an output that requests an interrupt.
• ~OBF : Output Buffer Full is an output indicating that that output buffer contains data for the bi-directional bus.
• ~ACK : Acknowledge is an input that enables tri-state buffers which are otherwise in their high-impedance state.
• ~STB : The strobe input loads data into the port A latch.
82C55: Mode 2 Bi-directional Operation
• IBF : Input buffer full is an output indicating that the input latch contains information for the external bi-directional bus.
• INTE : Interrupt enable are internal bits that enable the INTR pin. BIT PC6(INTE1) and PC4(INTE2).
• PC2,PC1,PC0 : These port C pins are general-purpose I/O pins that are available for any purpose.
FOR BIT SET/RESET MODE (Port C only)
• This is bit set/reset control word format.
X X X
Don’t care
Bit select for Port C (Pc0-Pc7)
B0
B1
B2
D7 D6 D5 D4 D3 D2 D1 D0
0 1 2 3 4 5 6 7
0 1 0 1 0 1 0 1
0 0 1 1 0 0 1 1
0 0 0 0 1 1 1 1
BIT SET/RESET1=SET0=RESET
BIT SET/RESET FLAG=0 Active
• The control word for both mode is same.
• Bit D7 is used for specifying whether word loaded in to Bit set/reset mode or Mode definition word.
• D7=1=Mode definition mode.
• D7=0=Bit set/Reset mode.
• PC0-PC7 is set or reset as per the status of D0.
• A BSR word is written for each bit
• Example:
• PC3 is Set then control register will be 0XXX0111.
• PC4 is Reset then control register will be 0XXX01000.
• X is a don’t care.
8259A PROGRAMMABLE
INTERRUPT CONTROLLER
8259A PIC FEATURES• Manage 8 interrupts according to the instructions
written into the control registers
• Vector location can be assigned anywhere in the
memory map. However all the 8 interrupts are
spaced at an interval of four to eight locations.
• Resolve 8 levels of interrupt priorities in variety of
modes.
• Be expanded to 64 priority levels by cascading
additional 8259As.
• Compatible with 8-bit as well as 16-bit processors.
8259A PIC- PIN DIGRAM
8259A PIC- BLOCK DIAGRAM
8259A PIC- CASCADE BUFFER/ COMPARATOR
Slave Program/ Enable Buffer:
• Used to specify whether 8259 is to act as a
master or a slave
High Master
Low Slave
• In Non-Buffered Mode, this pin is used to specify
whether 8259 is to act as a master or a slave.
• In Buffered mode this pin is used as an output to
enable the data bus buffer of the system.
8259A- Priority ModesFULLY NESTED MODE:
• General purpose mode.
• All IRs are arranged from highest to lowest.
• IR0 Highest IR7Lowest
• In addition any IR can be assigned the HP in this mode; the priority sequence will then begin at that IR
IR0 IR1 IR2 IR3 IR4 IR5 IR6 IR7
4 5 6 7
(LP)
0
(HP)
1 2 3
AUTOMATIC ROTATION MODE:
• In this mode, a device after being serviced, receivesthe lowest priority.
• Assuming that the IR2 has just been serviced, it willreceive the 7th priority
SPECIFIC ROTATION MODE:
• Similar to automatic rotation mode, except that the user can select any IR for the lowest priority, thus fixing all other priorities.
IR0 IR1 IR2 IR3 IR4 IR5 IR6 IR7
5 6 7 0 1 2 3 4
End of Interrupt (EOI)
• After the completion of interrupt service, the corresponding
ISR bit needs to be reset to update the information in the ISR.
This is called EOI command. It can be issued in three formats.
• Non Specific EOI: When this command send to the 8259 PIC,
it resets the highest priority ISR bit.
• Specific EOI: This command specifies which ISR bit to reset
• Automatic EOI: In this mode no command is necessary.
During the third INTA* the ISR bit is reset.
Programming of 8259A
88259 can be initialized with
four ICW and two OCW.
ICW1 & ICW2 are Compulsory
command Words in the
initialization sequence.
ICW3 & ICW4 are Optional.
ICW3 is read only when more
than one 8259 used in the
system ( SNGL bit in
ICW1 is 0).
For 8086 Don’t Care
ADI=1 for 8086 based system
p
8253/8254 Programmable counter / timer
• The Intel 8253 and 8254 are Programmable Interval
Timers (PITs), which perform timing and counting
functions using three 16-bit counters.
• Compatible with 8085/86 processor.
• The Intel 82c54 variant handles up to 10 MHz clock
signals.
• The timer interrupt is usually assigned to IRQ-0 (highest
priority hardware interrupt) because of the critical
function it performs and because so many devices
depend on it.
Intel 8253/54 : Programmable counter / timer
chip
3 counters ;Counter #0, #1, #2
• Each counter is identical, and each consists of a 16-bit,pre-settable, down counter.
• Each is fully independent and can be easily read by theCPU.
• Each counter is operated simultaneously but in differentmode condition (M0,M1,M2,M3,M4, & M5)
• When the counter is read, the data within the counter willnot be disturbed.
• This allows the system or your own program to monitorthe counter's value at any time, without disrupting theoverall function of the 8253.
Data Bus: This tri-state, bi-directional, 8-bit buffer is
used to interface the 8253/54 to the system data bus.
The Data bus buffer has three basic functions.
• 1. Programming the modes of 8253/54.
• 2. Loading the count registers.
• 3. Reading the count values
A1 A0 Operation
0 0 Counter 0
0 1 Counter 1
1 0 Counter 2
1 1 Control word register
Counter operationTo operate a counter, a desired 16-bit count is loaded in its
register and, on command, it begins to decrement the count until
it reaches 0. At the end of the count, it generates a pulse that can
be used to interrupt the CPU.
Control Word Register (CWR)
• This internal register is used to write information to, prior to using
the device.
• This register is addressed when A0 and A1 inputs are logical 1's.
• The data in the register controls the operation mode and the
selection of either binary or BCD ( binary coded decimal )
counting format.
• The register can only be written to. You can't read information
from the register.
Programming of 8253 (CWR)
Read operation (performed by CPU)In event counters, it is necessary to read the value of the count in process.
This is done by three methods
• Simple read operation (Rw1: Rw2)
• Counter Latch Command (RW1/Rw2:0/0;
• Read Back command ( Available in 8254)
CWR for read back command
Counter status format
Modes of opertaion
• Mode 0 Interrupt on terminal count
• Mode 1 H/W retriggerable one shot
• Mode 2 Rate generator
• Mode 3 Square wave generator
• Mode 4 S/W triggered strobe
• Mode 5 H/W triggered strobe
`The OUT pin is set low after the Control Word is written, and counting
starts one clock cycle after the COUNT programmed. OUT remains low
until the counter reaches 0, at which point OUT will be set high until the
counter is reloaded or the Control Word is written.
The Gate signal should remain active high for normal counting. If Gate
goes low counting gets terminated and current count is latched till Gate
pulse goes high again.
In this mode 8253 can be used as Monostable Multivibrator. GATE
input is used as trigger input.
OUT will be initially high. OUT will go low on the Clock pulse
following a trigger to begin the one-shot pulse, and will remain low
until the Counter reaches zero. OUT will then go high and remain
high until the CLK pulse after the next trigger.
In this mode, the device acts as a divide-by-n counter, which is commonly
used to generate a real-time clock interrupt.
Like other modes, counting process will start the next clock cycle after COUNT
is sent. OUT will then remain high until the counter reaches 1, and will go low
for one clock pulse. OUT will then go high again, and the whole process
repeats itself.
8237DMA CONTROLLER
Introduction:
Direct memory access (DMA) is a method that allows an
input/output (I/O) device to send or receive data directly to
or from the main memory, bypassing the CPU to speed up
memory operations. The process is managed by a chip known
as a DMA controller(DMAC).
Basic DMA Operation:
Two control signals are used to request and acknowledge a
direct memory access (DMA) transfer in the microprocessor-
based system.
The HOLD signal as an input(to the processor) is used to
request a DMA action.
The HLDA signal as an output that acknowledges the DMA
action.
When the processor recognizes the hold, it stops its execution and
enters hold cycles.
HLDA becomes active to indicate that the processor has placed its
buses at high-impedance state.
Basic DMA Definitions:
Direct memory accesses normally occur between an I/Odevice and memory without the use of the microprocessor.
A DMA read transfers data from the memoryto the I/O device.
A DMA write transfers data from an I/O deviceto memory.
The system contains separate memory and I/O controlsignals. Hence the Memory & the I/O are controlledsimultaneously
The DMA controller provides memory with its address, andthe controller signal selects the I/O device during thetransfer.
Data transfer speed is determined by speed of the memorydevice or a DMA controller.
The 8237 DMA Controller
8237 is a four-channel device compatible with
8086/8088, adequate for small systems.
Each channel is capable of addressing a full
64K-byte section of memory.
Expandable to any number of DMA channel inputs
8237 is capable of DMA transfers at rates up to 1.6MB
per second.
CPU having the control over the bus: When DMA operates:
Programmable DMA controller. (a) Block diagram and (b) pin-out.
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8237 Internal Registers
CAR
The current address register holds a 16-bit memory
address used for the DMA transfer.
Each channel has its own current address register for this
purpose.
When a byte of data is transferred during a DMA operation,
CAR is either incremented or decremented depending on
how it is programmed.
CWCR
The current word count register programs a channel for
the number of bytes to transferred during a DMA action.
CR
The command register programs the operation of the
8237 DMA controller.
The register uses bit position 0 to select the memory-to-
memory DMA transfer mode.
Memory-to-memory DMA transfers use DMA channel 0 to
hold the source address
DMA channel 1 holds the destination address
command register.
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BA and BWC
The base address (BA) and base word count (BWC)
registers are used when auto-initialization is selected for a
channel.
In auto-initialization mode, these registers are used to reload
the CAR and CWCR after the DMA action is completed.
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The mode registerprograms the mode of operation for a channel.
Each channel has its own mode register as selected by bit positions 1 and 0.
Remaining bits of the mode register select operation, auto-initialization, increment/decrement, and mode for the channel
MR
15
The bus request register is used to request
a DMA transfer via software.
very useful in memory-to-memory transfers, where an external signal is
not available to begin the DMA transfer
BR
16
The mask register set/reset sets or clears the channel mask.
if the mask is set, the channel is disabled
the RESET signal sets all channel masks
to disable them
MRSR
17
The mask register clears or sets all of
the masks with one command instead of individual channels, as with
the MRSR.
MSR
18
The status register shows status
of each DMA channel. The TC bits
indicate if the channel has reached
its terminal count (transferred all
its bytes).
When the terminal count is
reached, the DMA transfer is
terminated for most modes
of operation.
The request bits indicate whether
the DREQ input for a given channel
is active.
SR
Master clear
Acts exactly the same as the RESET signal to the 8237.
As with the RESET signal, this command disables all channels
Clear mask register
Enables all four DMA channels.
Clear the first/last flip-flop
Clears the first/last (F/L) flip-flop within 8237.
The F/L flip-flop selects which byte (low or high order) is
read/written in the current address and current count registers.
if F/L = 0, the low-order byte is selected
if F/L = 1, the high-order byte is selected
Any read or write to the address or count register automatically
toggles the F/L flip-flop.
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Memory-to-memory transfer is much more powerful
than the automatically repeated MOVSB instruction.
most modern chip sets do not support the memory-to-
memory feature
8237 requires only 2.0 µs per byte, which is over twice as
fast the existing data transfer.
8251 USART (Universal
Synchronous Asynchronous
Receiver Transmitter)The 8251 is a USART (Universal Synchronous Asynchronous
Receiver Transmitter) for serial data communication. As a
peripheral device of a microcomputer system, the 8251
receives parallel data from the CPU and transmits serial data
after conversion. This device also receives serial data from
the outside and transmits parallel data to the CPU after
conversion.
Serial data transmission is
classified as
• Simplex: the data are transmitted in only
one direction. Ex. Transmission from
computer to printer
• Half Duplex: Data are transmitted in both
direction but not simultaneously. Ex .
Walky talky
• Full Duplex: Data are transmitted in both
direction simultaneously. ex. Telephone
Syn and asycn transmission
a) Synch format b) asynch format
Serial bit format
• Baud: number of signal changes per second; bits/second.
• Baud Rate: Bits transmitted per second
• ASCII character I (49H) to be transmitted at 1200 baud;
• 11 bits includes 1 start, 8 data and 2 stop bits.
• start bit +7 bits for ASCII + parity bit + 2 stop bits
• Transmission time for one bit = 1/1200 = 0.83 ms
• Time for transmitting one ASCII = 9.13 ms
H/W controlled serial I/O
• SW control has following requirements:
– An input port and an output port are required for interfacing.
– In transmission, MPU converts parallel data into serial bits.
– In reception, MPU converts bits from serial to parallel.
– Trans and receiver must match the time delay.
• In HW control has serial IO, all these features are incorporated in
one chip, like 8251A (USART).
8251 BLOCK DIAGRAM
The transmitter sectionThe transmitter section accepts parallel data from CPU and converts them into
serial data.
The transmitter section is double buffered, i.e., it has a buffer register to hold
an 8-bit parallel data and another register called output register to convert the
parallel data into serial bits.
When output register is empty, the data is transferred from buffer to output
register. Now the processor can again load another data in buffer register.
If buffer register is empty, then TxRDY is goes to high.
If output register is empty then TxEMPTY goes to high.
The clock signal, TxC (low) controls the rate at which the bits are transmitted
by the USART.
The clock frequency can be 1,16 or 64 times the baud rate.
Receiver section• The receiver section accepts serial data and convert them into parallel
data
• The receiver section is double buffered, i.e., it has an input register to
receive serial data and convert to parallel, and a buffer register to hold the
parallel data.
• The CPU reads the parallel data from the buffer register.
• RxD: bits are received serially on this line
• RxC: controls the rate at which bits are received by USART. In asych
mode, it can be 1, 16 or 64 times the baud.
• RxRDY : When the input register loads a parallel data to buffer register, the
RxRDY line goes high. RxRDY Can be used either to indicate the status or
to interrupt MPU.
• During asynchronous mode, the signal SYNDET/BRKDET will indicate the
break in the data transmission.
• During synchronous mode, the signal SYNDET/BRKDET will indicate the
reception of synchronous character.
•
Control logic and registers
CS’ C/D’ RD’ WR’ Function
0 1 1 0 MPU writes in the control register
0 1 0 1 MPU reads status register
0 0 1 0 MPU outputs data to data buffer
0 0 0 1 MPU reads data from data buffer
1 X X X USART is not selected
Data comm over telephone: MODEM control
The MODEM control unit allows to interface a MODEM to 8251A and to
establish data communication through MODEM over telephone lines.
This unit takes care of handshake signals for MODEM interface.
DSR (Data Set Ready : i/p)
This is an input port for MODEM interface. The input status of the terminal can
be recognized by the CPU reading status words.
DTR (Data Terminal : o/p)
This is an output port for MODEM interface. It is possible to set the status of
DTR by a command.
CTS (clear to send : i/p)
This is an input terminal for MODEM interface which is used for controlling a
transmit circuit.
RTS (Request to send: o/p)
This is an output port for MODEM interface. It is possible to set the status RTS
by a command.
Data Carrier Detect (DCD)
MODEM signals
Serial I/O standards
• Standard is used to interface between hostsystem (DTE: Data terminal equipment) andperipherals system(DCE:Data CommunicationEquipment)
• RS 232 (Recommended standard) is serial I/O cable
RS 232C
• Speed 20Kbaud.
• Distance 50 ft.
• data signal;
• Logic zero: +3v to +15V
• Logic one: -3v to -15V
• Other signals are
TTL.(timing and control
signals and ground
signal)
• 25 pins
Interfacing RS232 terminal using 8251A
Minimum interface with RS232C
• The null modem cable is frequently called a
crossover cable. It is used to allow two serial
Data Terminal Equipment (DTE) devices to
communicate with each other without using
a modem or a Data Communications Equipment
(DCE) device in between.
Other standard
Specs RS232C RS422A RS423A
Speed 20kbd 10Mbd 100kbd
Distance 50ft 4000ft 4000ft
Logic 0 3 to 15 B>A 4 to 6V
Logic 1 -3 to -15 B<A -4 to -6
Rcvr
input volt
±15V ±7 ±12
RS 232C
Initializing 8251A
• Mode, baud, stop bits, parity, etc.
• Control word: a) mode word b) command word
• After a reset operation, a mode word must be written in
the control register (16 bit register) followed by a
command word. Command word can be changed at any
time during operation, but mode can only be changed
only after a reset operation. It can be reset using internal
reset bit (D6) in the command word.
Control Words
There are two types of control word.
1. Mode instruction (setting of function)
2. Command (setting of operation)
1) Mode Instruction
Mode instruction is used for setting the function of the 8251. Items set by mode
instruction are as follows:
• Synchronous/asynchronous mode
• Stop bit length (asynchronous mode)
• Character length
• Parity bit
• Baud rate factor (asynchronous mode)
• Internal/external synchronization (synchronous mode)
• Number of synchronous characters (Synchronous mode)
The bit configuration of mode instruction is shown in Figures 2 and 3. In the case of
synchronous mode, it is necessary to write one-or two byte sync characters. If sync
characters were written, a function will be set because the writing of sync characters
constitutes part of mode instruction.
2) Command
Command is used for setting the operation of the 8251. It is possible to
write a command whenever necessary after writing a mode instruction
and sync characters.
Items to be set by command are as follows:
• Transmit Enable/Disable
• Receive Enable/Disable
• DTR, RTS Output of data.
• Resetting of error flag.
• Sending to break characters
• Internal resetting
• Hunt mode (synchronous mode)
Pin description of 8251
D 0 to D 7 (l/O terminal)
This is bidirectional data bus which receive control words and transmits data
from the CPU and sends status words and received data to CPU.
RESET (Input terminal)
A "High" on this input forces the 8251 into "reset status."
The device waits for the writing of "mode instruction." The min. reset width is six
clock inputs during the operating status of CLK.
CLK (Input terminal)
CLK signal is used to generate internal device timing.
CLK signal is independent of RXC or TXC.
However, the frequency of CLK must be greater than 30 times the RXC and
TXC at Synchronous mode and Asynchronous "x1" mode, and must be greater
than 5 times at Asynchronous "x16" and "x64" mode.
WR (Input terminal)
This is the "active low" input terminal which receives a signal for writing
transmit data and control words from the CPU into the 8251.
RD (Input terminal)
This is the "active low" input terminal which receives a signal for reading
receive data and status words from the 8251.
C/D (Input terminal)
This is an input terminal which receives a signal for selecting data or command
words and status words when the 8251 is accessed by the CPU.
If C/D = low, data will be accessed. If C/D = high, command word or status word
will be accessed.
CS (Input terminal)
This is the "active low" input terminal which selects the 8251 at low level when the
CPU accesses.
Note: The device won’t be in "standby status"; only setting CS = High.
TXD (output terminal)
This is an output terminal for transmitting data from which serial-converted data is
sent out. The device is in "mark status" (high level) after resetting or during a status
when transmit is disabled. It is also possible to set the device in "break status" (low
level) by a command.
TXRDY (output terminal)
This is an output terminal which indicates that the 8251is ready to accept a
transmitted data character. But the terminal is always at low level if CTS = high or
the device was set in "TX disable status" by a command.
Note: TXRDY status word indicates that transmit data character is receivable,
regardless of CTS or command. If the CPU writes a data character, TXRDY will be
reset by the leading edge or WR signal.
TXEMPTY (Output terminal)
This is an output terminal which indicates that the 8251 has transmitted all the
characters and had no data character.
In "synchronous mode," the terminal is at high level, if transmit data characters
are no longer remaining and sync characters are automatically transmitted. If the
CPU writes a data character, TXEMPTY will be reset by the leading edge of WR
signal.
Note : As the transmitter is disabled by setting CTS "High" or command, data
written before disable will be sent out. Then TXD and TXEMPTY will be "High".
Even if a data is written after disable, that data is not sent out and TXE will be
"High". After the transmitter is enabled, it sent out. (Refer to Timing Chart of
Transmitter Control and Flag Timing)
TXC (Input terminal)
This is a clock input signal which determines the transfer speed of transmitted
data.
In "synchronous mode," the baud rate will be the same as the frequency of TXC.
In "asynchronous mode", it is possible to select the baud rate factor by mode
instruction. It can be 1, 1/16 or 1/64 the TXC. The falling edge of TXC sifts the
serial data out of the 8251.
RXD (input terminal)
This is a terminal which receives serial data.
RXRDY (Output terminal)
This is a terminal which indicates that the 8251 contains a character that is ready to
READ. If the CPU reads a data character, RXRDY will be reset by the leading edge
of RD signal. Unless the CPU reads a data character before the next one is received
completely, the preceding data will be lost. In such a case, an overrun error flag
status word will be set.
RXC (Input terminal)
This is a clock input signal which determines the transfer speed of received data.
In "synchronous mode," the baud rate is the same as the frequency of RXC. In
"asynchronous mode," it is possible to select the baud rate factor by mode
instruction. It can be 1, 1/16, 1/64 the RXC.
SYNDET/BD (Input or output terminal)
This is a terminal whose function changes according to mode.
In "internal synchronous mode." this terminal is at high level, if sync characters are
received and synchronized. If a status word is read, the terminal will be reset.
In "external synchronous mode, "this is an input terminal. A "High" on this input
forces the 8251 to start receiving data characters.
In "asynchronous mode," this is an output terminal which generates "high level“
output upon the detection of a "break" character if receiver data contains a "low-
level" space between the stop bits of two continuous characters. The terminal will be
reset, if RXD is at high level. After Reset is active, the terminal will be output at low
level.
• Transmitter section
– TxD: serial bits are transmitted on this line.
– TxC: controls bit trans rate. Clk freq can be 1,16,64 times the baud.
– TxRDY: o/p signal,high indicates the trans buffer is empty and USRT ready to accept a byte. Signal is reset when data is loaded in the buffer.
– TxE: o/p signal High indicates that the O/P register is empty. Reset when a byte is transferred from buffer to o/p register.
Read/Write control logic:The Read/Write Control logic interfaces the 8251A with CPU, determines
the functions of the 8251A according to the control word written into its
control register.
It monitors the data flow.
This section has three registers and they are control register, status
register and data buffer.
The active low signals RD, WR, CS and C/D(Low) are used for read/write
operations with these three registers.
When C/D(low) is high, the control register is selected for writing control
word or reading status word.
When C/D(low) is low, the data buffer is selected for read/write operation.
When the reset is high, it forces 8251A into the idle mode.
The clock input is necessary for 8251A for communication with CPU and
this clock does not control either the serial transmission or the reception
rate.
Interfacing RS232 terminal using 8251A
• TxC is 153.6 kHz.
• Asycn mode with 9600 baud
• Character length = 7 bits, two stop
bits
• No parity chck.
• Port add
– Data register: FEh
– Control/status register: FFh
D7 D6 D5 D4 D3 D2 D1 D0
Stop bitsNo parity
7 bits chr
1 0 0 1 0 1 01
Baud= TxC/16=153.6k/16 = 9600
•Mode word:
•Command word (asynch mode):
0 X 1 X 0 X 1
D7 D6 D5 D4 D3 D2 D1 D0
X
Tr EnblRcv DisblErr RstPrvnts
Intrnal
reset
X X X X X X 1
D7 D6 D5 D4 D3 D2 D1 D0
X
•Status word:
Tr rdy
=01h
=11h
=CAh
Initialization intruction:
SETUP: MVI A, CAh ; load mode word
OUT FFh ; write mode word to control rgstr
MVI A, 11h ; load command word
OUT FFh ; enable trnsmitter
STATUS: IN FFh ; read stats word
ANI 01h ; mask all bits except D0
JZ STATUS ; if D0 = 0, Tr buffer is full, go back and wait
transmit
Init bit cntr
Snd strt bit
Wait bit time
Get chr into A
o/p bit using D0
Wait bit time
Rotate nxt bit to D0.
Dcr bit cntr
Last bit?
Add parity
Snd stop bits
return
rcv
Rd o/p port
Wait ½ bit time
Set bit cntr
Clr data rgstr
Wait bit time
Rd i/p
Save bit
Redy to rcv nxt bit
Dcr bit cntr
Last bit?
Chk parity
Wait for stop bits
return
Strt bit?
Bit still low?
N
N
N
N
Y
Y
Y
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The keyboard display controller chip 8279 provides
A set of four scan lines and eight return lines for interfacing keyboard
A set of eight output lines for interfacing display.
Scan line are used to drive multiplexed 7 segment display
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WHY 8279???
8255 can be used in interfacing keyboards and displays.The disadvantages of this method of interfacing keyboardand display is that the processor has to refresh the displayand check the status of the keyboard periodically usingpolling technique.
Thus a considerable amount of CPU time is wasted,reducing the system operating speed.
Intel’s 8279 is a general purpose keyboard displaycontroller that simultaneously drives the display of a systemand interfaces a keyboard with the CPU, leaving it free for itsroutine task.
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8279
Keyboard segment
i)Scans the keyboard
ii) detects key if any key is pressed
iii) Key code is stored in 8x8 FIFO RAM
iv) data in FIFO RAM sends Interrupt signal to CPU
vi) CPU reads the key code stored in FIFO RAM
Display segment
vii) Then CPU writes the key code in 16x8 display RAM
viii)Display devices display the data in the display RAM
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BLOCK
DIA
8279
KEYBOARD
i) Scanned Keyboard ( 2 Key lock out /N key roll over)
ii)strobed input mode
iii)scanned sensor matrix mode
SCAN
i) Encoded
ii) Decoded
MUX. DISPLAY (8 digit or 16 digit)
i)Left Entry
ii) Right Entry
MPU INTERFACE
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2 Key lock out/N key roll overKEY DEBOUNCEWhen a key is pressed, a debounce logic comes into operation. ReturnBuffers and Keyboard De-bounce and Control section scans for a key closurerow wise. If a key closer is detected, the keyboard debounce unit debouncesthe key entry (i.e. wait for 10 ms).
When a key is pressed, a debounce logic comes into operation. After thedebounce period (i.e. wait for 10 ms). , if the key continues to be detected,The code of key is directly transferred to the sensor RAM along with SHIFTand CONTROL key status.2 key lock out: If two keys are pressed simultaneously within a debouncecycle, no key is recognized and no key code is stored in FIFO RAM till one ofthem remains closed and the other is released.N – key roll overAny number of keys can be pressed simultaneously and recognized in theorder, the keyboard scan recorded them. All the codes of such keys areentered into FIFO.In this mode, the first pressed key need not be released before the second ispressed.
• CNTL/STB i/p mode:, control lines that enters data in FIFORAM . Shift: The status of shift is stored along with key code inFIFO RAM .
• In Scanned Sensor Matrix mode, a sensor array can beinterfaced with 8279 using either encoded or decoded scansto scan the key matrix and refresh the display.
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Output (Display) Modes : 8279 provides two output modes for selecting the display options.
Display Scan :
• In this mode 8279 provides 8 or 16 character multiplexed displays those can be organized as dual 4- bit or single 8-bit display units.
Display Entry
( right entry or left entry mode )
• 8279 allows options for data entryon the displays.
• The display data is entered fordisplay either from the right side orfrom the left side.
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Control and Timing Register and Timing Control • These registers store the keyboard and display modes and
other operating conditions programmed by CPU.
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The registers are written with A0=1 and WR=0.The Timing and control unit controls the basic timings forthe operation of the circuit.
All the command words or status words are written orread with A0 = 1 and CS = 0 to or from 8279.
a) Keyboard Display Mode Set : The format of the command word to select different modes of operation of 8279 is given below with its bit definitions.
D7 D6 D5 D4 D3 D2 D1 D0
0 0 0 D D K K K
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SENSOR MATRIX
SENSOR MATRIX
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B) Programmable clock : The clock for operation of 8279 is obtained by dividingthe external clock input signal by a programmableconstant called pre scaler.
PPPPP is a 5-bit binary constant.
The input frequency is divided by a decimal constantranging from 2 to 31, decided by the bits of an internalprescaler, PPPPP.
D7 D6 D5 D4 D3 D2 D1 D0
0 0 1 P P P P P
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c) Read FIFO / Sensor RAM : The format of this command is given below.
AI – Auto Increment FlagAAA – Address pointer to 8 bit FIFO RAM
X- Don’t care
This word is written to set up 8279 for reading FIFO/ sensor RAM. In scanned keyboard mode, AI and AAA bits are of no use. The 8279 will automatically drive data bus for each subsequent read, in the same sequence, in which the data was entered.In sensor matrix mode, the bits AAA select one of the 8 rows of RAM. If AI flag is set, each successive read will be from the subsequent RAM location.
D7 D6 D5 D4 D3 D2 D1 D0
0 1 0 AI X A A A
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d) Read Display RAM : This command enables a programmer to read the display RAM data.
The CPU writes this command word to 8279 to prepare it for display RAM read operation. AI is auto increment flag and AAAA, the 4-bit address points to the 16-byte display RAM that is to be read.If AI=1, the address will be automatically, incremented after each read or write to the Display RAM. The same address counter is used for reading and writing.
D7 D6 D5 D4 D3 D2 D1 D0
0 1 1 AI A A A A
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d) Write Display RAM : This command enables a programmer to write the display RAM data.
AI – Auto increment Flag.AAAA – 4 bit address for 16-bit display RAM to be
written.e) Display Write Inhibit/Blanking :
D7 D6 D5 D4 D3 D2 D1 D0
1 0 0 AI A A A A
D7 D6 D5 D4 D3 D2 D1 D0
1 0 1 X IW IW BL BL
IW - inhibit write flag (Masking) BL - blank display bit flags (Blanking)
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g) Clear Display RAM :
CD: CLEAR DISPLAY ; CF: CLEAR FIFO RAM STATUS; CA: CLEAR ALL (both CD&CF)
D7 D6 D5 D4 D3 D2 D1 D0
1 1 0 CD2 CD1 CD0 CF CA
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h) End Interrupt / Error mode Set :
E- Error modeX- don’t care
For the sensor matrix mode, this command lowers the IRQ line and enables further writing into the RAM. Otherwise, if a change in sensor value is detected, IRQ goes high that inhibits writing in the sensor RAM. For N-Key roll over mode, if the E bit is programmed to be ‘1’, the 8279 operates in special Error mode
D7 D6 D5 D4 D3 D2 D1 D0
1 1 1 E X X X 1
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I/O Interface
FIFO status register
•Code given in text for reading keyboard.
•Data returned from 8279 contains raw data that need to be translated to ASCII:
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ADC 0809
ADC 0809• The ADC0809 is an 8-bit successive approximation type
ADC with inbuilt 8-channel multiplexer.
• The ADC0809 is suitable for interface with 8086
microprocessor.
• The ADC0809 is available as a 28 pin IC in DIP (Dual Inline
Package).
• The ADC0809 has a total unadjusted error of ±1 LSD (Least
Significant Digit).
PIN DESCRIPTION OF
ADC0809
SAR
Interfacing ADC with 8085 thro 8255
ADC interfacing with 8051
DAC• To convert the digital signal to analog signal a Digital-to-Analog Converter
(DAC) has to be employed. ( binary weighted and R/2R ladder. )
• The DAC will accept a digital (binary) input and convert to analog voltage
or current.
• Every DAC will have "n" input lines and an analog output.
• The DAC require a reference analog voltage (Vref) or current (Iref)
source.
• The smallest possible analog value that can be represented by the n-bit
binary code is called resolution.
• The resolution of DAC with n-bit binary input is 1/2nof reference analog
value.
DAC 0800
•
The DAC0800 is an 8-bit, high speed, current output DAC with a typical
settling time (conversion time) of 100 ns.
• It produces complementary current output, which can be converted to
voltage by using simple resistor load.
• The DAC0800 require a positive and a negative supply voltage in the range
of ± 5V to ±18V.
• It can be directly interfaced with TTL, CMOS, PMOS and other logic
families.
• For TTL input, the threshold pin should be tied to ground (VLC = 0V).
R-2R Ladder
pin configuration of DAC0800
DAC interfacing with 8085 thro 8255
DAC interfacing with 8051