Hierarchy of I/O Control Devices8155

I/O + Timer

8255I/O

8253/54Timer

2 Port (A,B), No BidirectionalHS mode (C)4 mode timer

2 Port (A,B)A is BidirectionalHS mode (C)Extra controls

6 mode timer

8259 Interrupt controller

8237DMA controller

8251Serial I/O USART

controller

Outline• Parallel Vs Serial Communication • Characteristics of serial communication – Synchronous/A-synchronous, Simplex/Duplex,

Baud rate and Error Correction• Introduction to 8251 USART controller

Data Comm: Serial Vs Parallel

• Serial – Cheaper – Slower

• Parallel – Faster– Data skew– Limited to small distances

Data Transmission

Parallel Serial

Synchronous ASynchronous

Serial Communication: How ?Two basic modes of data transmission

Sender

11001001

Receiver

11001001

10010011Sender

11001001

Receiver

11001001

Parallel to serial Conversion

Serial to parallelConversion

Serial Transmission Parallel Transmission

Type of Serial Communication• Synchronous – Sender and receiver must synchronize • Done in hardware using phase locked loops (PLLs)

– Block of data can be sent – More efficient : Less overhead than asynchronous

transmission – Expensive

• Asynchronous – Each byte is encoded for transmission • Start and stop bits

– No need for sender and receiver synchronization

Type of Serial Communication

Sender

Sender

Receiver

ReceiverData Data Data Data Data

Data Data Dataa

Transmission Gaps

Asynchronous transmission

Synchronous transmission

CLK

Framing in Asynchronous

• Character oriented • Each character carried start bit and stop bits • When No data are being transmitted – Receiver stay at logic 1 called mark, logic 0 is Space

• Framing: – Transmission begins with one start bit (low/0)– Followed by DATA (8bit) and – Stop bits (1 or 2 bits of logic high)

Type of Serial CommunicationAsynchronous transmission

8 bit Data

Start Bit Start Bits

1 0 0 0 1 1 1 0 LSB MSB

Time

1 startbit

1 or 2 StopbitSource data

Simplex and Duplex Transmission

• Simplex– Data are transmitted in one directions– Example: CPU to printer

• Duplex– Data flow in both direction – Half Duplex (Transmission goes on way at a time)– Full Duplex (Both ways simultaneously)

Rate of transmission

• Rate at which bits are transmitted (BAUD)• Number of signal changes per second• Bit time: how long the Bit stay On or Off• Printer, Terminal Baud Adjustable (50-9600)• 1200Baud means: Bit stay for 1/1200=0.83ms

Error Check• Parity Check– Even parity: When odd numbers of 1 make D7=1

• Send Even number of 1

– Odd parity: When even number of 1 make D7=1• Send Odd number of 1

• Check Sum– Used for block of data– Sum of all Bytes without carry and 2’s complements – Total Sum Result should be Zero

• Cyclic Redundancy Code (CRC)– Synchronous Communication – Stream of Data can be represented by Cyclic polynomial that

divided by a constant polynomial – Reminder to set Bits and Send out as check for error

Steps to be followed : Transmitting

• Inform RX the start bit, end bits and parity check

• Convert parallel word into stream of bits• Create a transmit word by adding start, end

and proper parity bit .• Transmit one bit at a time with appropriate

time delay using one data line– Time delay is determined by the speed of

transmission

Steps to be followed : Receiving

• Recognize bit of transmission• Receive serial bits, one bit at a time• Dismantle the start bits, end bit, parity bit,

Data bits• Check the error and recognize the end of

transmission• Convert serial data bit in to parallel word

Software control Asynchronous I/0using Microprocessor

• 8 bit Data to be send • Steps:– Output a start bit – Convert the character to be sent in a stream of

serial bits with appropriate delay– Add a parity information if needed– Output one or two stop bit

Serial Transmission in Software

8 0 8 5

D7D6D5D4D3D2D1D0

Output Port

Decode CSb

WRb

Program/Flow chat to Transmit data serially

Start

•Set up Character bit counter•Send start bit

Wait bit time

Get Char in ACC

Output Bit using D0

Wait bit time

•Rotate Next bit in D0

•Decrement the bit counter

Is Last bit ?

•Add parity if necessary •Send two stop bit

Return

Serial Reception in Software

8 0 8 5

D7D6D5D4D3D2D1D0

Input Port

Decode CSb

DRb

Flow chart to receive data serially

Start

Read out put port

Wait half bit time

•Set up Bit counter•Clear register to Save bits

•Wait bit time •Read input port •Save bit

•Get ready to next bit •Decrement bit counter

Is Last bit ?

•Check parity if necessary •Wait for two stop bit

Return

Is it bit Start?

NO

Is bit Still low ?

NO

8085 Serial I/O lines

• Serial Input Data (SID)• Serial Output Data (SOD)– Instruction SIM is necessary to output data– Interpretations (ACC contents)

D7 D6 D5 D4 D3 D2 D1 D0

SOD SDE (0/1Dis/Ena SOD)

X For interrupts

MVI A, 80 ; Set D7 in the ACC=1RAR ;Set D6 =1 and bring carry into

D7

SIM ; output D7

Data transmission Program on SOD

• Transmit an ASCII Char stored in Register B

MVI B ASCIIDatabyte ; get data byte in BMVI C,0BH ; set up counter for 11 bitsXRA A ; reset carry to 0

NXTbit: MVI A,80H ;set D7=1 in ACCRAR ;bring Carry in D7 and set D6=1SIM ;output D7CALL DELAYBittime ;wait for fixed time (BWT)STC ;set Carry 1MOV A,B ;Place ASIII car in accRAR ; place ASCII D0 in Carry

;and shift 1 in D7MOV B,A ;Save BDCR CJNZ NXTbitRET

Hardware control Serial I/O

• Programmable chip 8251• Requirement of HW control serial I/O– An input/output port are required for interfacing– Converts data bits in to Parallel to serial & vice

versa– Data transfer to be synchronized between I/O– USART (Universal Synchronous Asynchronous

Receiver and Transmitter )

UART/USART• Writing a program compatible with all different serial

communication protocols is difficult and it is an inefficient use of microprocessor.

• UART: Universal Asynchronous Receiver/Transmitter chip.

• USART: Universal Synchronous/Asynchronous Receiver/Transmitter chip.

• The microprocessor sends/receives the data to the UART in parallel, while with I/O, the UART transmits/receive data serially.

• 8251 functions are integrated into standard PC interface chip.

UART / CPU interface

CPU8251

status(8 bit)

data(8 bit)

serialport

xmit/rcv

• UART/USART • 8251 USART• 8250/16450 UART is a newer version of 8251.• 16550 is the latest version UART.

Thanks