8251 -USART

Serial I/O - Programmable Communication Interface

Data Communications
Data communications refers to the ability of one computer to exchange data with another computer or a peripheral Physically, the data comm. path may be a short, 5 to 10 feet ribbon cable connecting a microcomputer and parallel printer; or it might be a high speed telecommunications port connecting two computers thousands of miles apart. Standard data communication interfaces and standards are needed Centronics parallel printer interface RS-232 defines a serial communications standard We focus on serial I/O this week 8251 USART (Universal Synchronous/Asynchronous Receiver/Transmitter) is the key component for converting parallel data to serial form and vice versa Two types of serial data communications are widely used
Asynchronous communications Synchronous communications

Types of Transmission

Asynchronous Communications
Eliminates the need for a clock signal between two microprocessor based systems
Transmit data Receive data System 1 Signal common System 2

Asynchronous Communications
Data to be transmitted is sent out one character at a time and the receiver end of the communication line synchronization is performed by examining synchronization bits that are included at the beginning and at the end of each character

Examples
What is the data rate in bits/sec and character rate if the bit time is 3.33 ms
Bit rate = 1 / 3.33 ms = 300 bits/sec 11 x 3.33 ms = 36.63 ms required to transmit a character so character rate = 1/36.63 ms = 27.3 char/sec

Modems typically transmit data over the telephone network at 9600, 14400, 28800 or 56K bps. Ex: If 1 MByte file is to be transmitted to another computer using a modem calculate the transmission time
9600 bps: 1048576x10/ 9600 bits/sec = 1092 s = 18 minutes and 12 sec 28800 bps: 364 s = 6 minutes and 4 sec

Building a Serial I/O port Transmitter section

This program serializes data through software

Building a Serial I/O port Transmitter

Building a Serial I/O port Receiver- Flowchart

Synchronous Communications
Transmit data Receive data System 1 clk Signal common System 2

BISYNC: Each block of data has synch characters. The size of block data can be 100 or more bytes. BCC checks for errors.

Serial Data Link Control: Developed by IBM used for computer networking (Token Ring). After Flag byte the network address is sent. Control Byte stores information about sequence of data etc. Data is thousands of bits. 16 bit field is used for error checking.

USART
It is possible to use either of the two methods. There are special IC chips for serial data communication UART: universal asynchronous receiver transmitter USART: universal Synchronous/Asynchronous Receiver/Transmitter COM port in the original IBM PC uses 8250 UART INTEL has USART 8251 National Semiconductors improved version of 8250A is 16450. 16450, 16550, 16552 (dual 16550) Data Transmission
simplex half duplex full duplex

8251 receiver
The receiver section: whenever RxD line goes low, control assumes it is a start bit, waits for half bit time and samples again.
responsible for reading the serial bit stream of data atRxD(receive data) input and converting it into parallel form. RxRDY(receive ready) output switched to logic 1 level to tell the microprocessor that a char. is available and is sitting inside the USART and should be read from the receive buffer register. RxC Receiver Clock. Controls the rate which bits are received by the USART. In Asynchronous Mode, the clock can be set to 1,16 or 64 times the baud.

8251 transmitter
Transmitter section receives parallel data from the microprocessor over the data bus. The character is then automatically framed with the start bit, parity bit, correct number of stop bits, and put into the transmit data buffer register.
Finally, it is shifted out of this register to produce a bit serial output on the TxD line. TxRDY is switched to logic 1 when the transmit buffer register is empty. TxE transmitter Empty This is an output signal. Logic 1 on this line indicates the output register is empty. Reset when a byte is transferred from the buffer to output registers. TxC Transmitter Clock. Controls the rate which bits are transmitted by the USART. The clock can be set to 1,16 or 64 times the baud (using Mode Word next slide)

8251A serial com. interface

Initializing the 8251

To implement serial communication the MPU must inform the 8251 about the mode, baud, stop bits, parity etc. A set of control words must be loaded.
Mode Words
Specifies general characteristics of the operation.

Command Words
Enables the data transmission and/or reception

Status Word provides the information concerning register status and transmission errors.

Any control word written into the control register after a mode word is interpreted as a command word; that means a command word can be changed anytime, however 8251 should be reset prior to writing a Mode word. 8251 can be reset internally by using the Internal Reset Bit D6.

8251 A Serial Communication Interface

The 8251A internally interprets the C/D,RD and WR signals as follow:

Whether the mode, control or sync character register is selected depends on the accessing sequence.

A flowchart of the sequencing is given in Fig.

Format of the mode register

What value must be written into the mode control register with baud rate divided by 16, char. Size 16 bits, odd parity, one stop bit ? -----------------------------------01011110 b = 5Eh

Format of the control register

Example 1
A program sequence which initializes the mode register and gives a command to enable the transmitter and begin an asynchronous transmission of 7-bit characters followed by an even-parity bit and 2 stop bits is: MOV AL,11111010B OUT 51H,AL MOV AL,00110011B OUT 51H,AL

Example 2
This sequence assumes that the mode and control registers are at address 51H and the clock frequencies are to be 16 times the corresponding baud rates. The sequence: MOV AL,00111000B OUT 51H,AL MOV AL,16H OUT 51H,AL OUT 51H,AL MOV AL,10010100B OUT 51H,AL would cause the same 8251A to be put in synchronous mode and to begin searching for two successive ASCII sync characters

Format of the status register

Example 3
A typical program sequence which uses programmed I/O to input 80 characters from the 8251A, whose data buffer register's address is 0050, and put them in the memory buffer beginning at LINE.

Example 4
1000 000 0 : data register address: xx80h 1000 000 1: control or status register address: xx81h Mode word:
2 stop bits. no parity, 8 bit characters. Baud rate factor of 16 (1200 Kbps) 1110 1110 =EEh

Command Word: 0001 0101 = 15h ; enable TxRDY and RxRDY and reset all flags first

INIT8251:MOV AL,0EEh OUT 81h, AL MOV AL, 15h OUT 81h, AL CHKRX:IN AL,81h ROR AL,1 ROR AL,1 JNC CHKRX IN AL,80h NOT AL MOV BL,AL
CHKTX:IN AL,81h ROR AL,1 JNC CHKTX OUT 80h,AL JMP CHKRX

Initialize the Mode Word andCommand Word

Receive Ready?

If Ready get data

Send data if the T buffer register is available