1. Hierarchy of I/O Control Devices
2 Port (A,B),
No Bidirectional
HS mode (C)
4 mode timer
8155
I/O + Timer
8253/54
Timer
8255
I/O
2 Port (A,B)
A is Bidirectional
HS mode (C)
Extra controls
6 mode timer
8259
Interrupt controller
8237
DMA controller
8251
Serial I/O USART controller

2. Outline Parallel Vs Serial Communication
Characteristics of serial communication
Synchronous/A-synchronous, Simplex/Duplex, Baud rate and Error Correction
Introduction to 8251 USART controller

3. Data Comm: Serial Vs Parallel
Data Transmission
Parallel
Serial
Synchronous
ASynchronous
Serial
Cheaper
Slower
Parallel
Faster
Data skew
Limited to small distances

4. Serial Communication: How ?
Two basic modes of data transmission
Parallel to serial
Conversion
Serial to parallel
Conversion
Sender 1
001001 1
001001
Receiver
Sender 1
001001 1
001001
Receiver
Serial Transmission
Parallel Transmission

5. 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

6. Type of Serial Communication
Transmission Gaps
Sender
Receiver a
Data
Data
Data
Asynchronous transmission
CLK
Sender
Receiver
Data
Data
Data
Data
Data
Synchronous transmission

7. 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)

8. Type of Serial Communication
Asynchronous transmission
1 start
bit
1 or 2 Stop
bit
Source data
LSB MSB
Time
Start Bit
Start Bits
8 bit Data

9. Simplex and Duplex Transmission
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)

10. 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 ( )
1200Baud means: Bit stay for 1/1200=0.83ms

11. Error Check Parity Check Check Sum Cyclic Redundancy Code (CRC)
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

12. 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

13. 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

14. Software control Asynchronous I/0 using 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

15. Serial Transmission in Software8 5
Decode
CSb
Output
Port D7 D6 D5
WRb D4 D3 D2 D1 D0

16. Program/Flow chat to Transmit data serially
Output Bit using D0
Start
Wait bit time
Set up Character bit counter
Send start bit
Rotate Next bit in D0
Decrement the bit counter
Wait bit time
Is Last bit ?
Get Char in ACC
Add parity if necessary
Send two stop bit
Return

17. Serial Reception in Software8 5
Input
Port D7 D6 D5
DRb D4 D3 D2 D1 D0
Decode
CSb

18. Flow chart to receive data serially
Set up Bit counter
Clear register to Save bits
Start
Wait bit time
Read input port
Save bit
Read out put port
Is it bit Start? NO
Get ready to next bit
Decrement bit counter
Is Last bit ?
Wait half bit time
Is bit Still low ? NO
Check parity if necessary
Wait for two stop bit
Return

19. 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 (0/1
Dis/Ena SOD) X
For interrupts
MVI A, 80 ; Set D7 in the ACC=1
RAR ;Set D6 =1 and bring carry into D7
SIM ; output D7

20. Data transmission Program on SOD
Transmit an ASCII Char stored in Register B
MVI B ASCIIDatabyte ; get data byte in B
MVI C,0BH ; set up counter for 11 bits
XRA A ; reset carry to 0
NXTbit: MVI A,80H ;set D7=1 in ACC
RAR ;bring Carry in D7 and set D6=1
SIM ;output D7
CALL DELAYBittime ;wait for fixed time (BWT)
STC ;set Carry 1
MOV A,B ;Place ASIII car in acc
RAR ; place ASCII D0 in Carry
;and shift 1 in D7
MOV B,A ;Save B
DCR C
JNZ NXTbit
RET

21. 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 )

22. 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.

23. UART / CPU interface UART/USART 8251 USART
status
(8 bit)
xmit/
rcv
data
(8 bit)
serial
port
UART/USART
8251 USART
8250/16450 UART is a newer version of 8251.
16550 is the latest version UART.

24. Thanks