LECTURE 16 8250 UART : UNIVERSAL ASYNCHRONOUS RECEIVER TRANSMITTER

8250 UART : UNIVERSAL ASYNCHRONOUS RECEIVER TRANSMITTER Serial communications is sufficiently complex that special chips have been designed to do the work of forming and timing the stri

8250 UART : UNIVERSAL ASYNCHRONOUS RECEIVER TRANSMITTER

Serial communications is sufficiently complex that special chips have been designed to do the work of forming and timing the strings of bits that comprise serial data Such a chip is called a universal

asynchronous receiver transmitter or UART Without UARTs, communications programming would

be very complicated The IBM family of microcomputers use the Intel 8250 UART

The Intel 8250 is a programmable Universal Asynchronous Receiver/Transmitter packaged in a

40-pin integrated circuit

• The UART is used to convert parallel data to serial format on the transmit side (data going to a modem for example) and to convert serial data to a parallel format on the receive side (coming

from a modem)

• The serial format, in order of transmission and reception, is a start bit, followed by five to eight data bits, a parity bit (if programmed) and one or two stop bits The maximum recommended data rate is 54K bits per second

• It can transmit characters at the same time as it is receiving characters (full duplex)

• The transmission speed of the UART is also programmable

• Newer UARTs are now used in serial ports in PCs : 16450 and 16550 capable of speeds up to 115,000 bps They are more efficient than the 8250 but are compatible with it from a programming

point of view

Internal registers enable the user to program various types of interrupts, modem controls, and character

formats The user can read the status of the UART at any time, monitoring transmission conditions,

interrupts and modem status

FEATURES

* Automatically generates and strips the Serial Async Control Bits (start, stop, parity)

* Full double buffering allows precise synchronization

* Independently controlled Transmit, Receive, Line Status, and data set Interrupts

* Modem interface capabilities

* Fully programmable Serial-Interface characteristics:

- 5-, 6-, 7-, or 8-Bit Characters

- Even, Odd, or No-parity Bit Generation and Detection

- 1-, 1 1/2-, or 2-Stop Bit Generation

- Baud Rate Generation (DC to 56K Baud)

* False Start Bit detector

* Complete status reporting capabilities

* Internal Diagnostic capabilities

- Loopback Controls for Communications Link Fault Isolation

- Break, Parity, Overrun, and Framing Error Simulation

DOS supports at least two communications ports, and hence requires at least two UARTS Their base addresses are kept at 0040:0000 for COM1 and 0040:0002 for COM2 A base address is the lowest two-byte port address of the group of port addresses by which the UART is accessed COM1 starts at 3F8H and COM2 at 2F8H For convenience, the discussion here refers to the registers numbered 3FxH, but the same specifications apply to the registers at 2FxH

The 8250 has ten programmable one-byte registers by which to control and monitor the serial port Most are devoted to initializing the port, a process that can be rather complicated The ten registers are accessed through seven port addresses, numbers 3F8H-3FEH (or 2F8H-2FEH) Detailed knowledge of these registers is necessary for programming the UART at the register level We will use a simpler method called BIOS INT14

UART Internal Registers of Importance:

Transmitter Holding Register holds the byte of data about to be sent to the modem

Receiver Data Register keeps the most recent byte of data received from the modem

Line Control Register is used to initialize the UART for a particular mode of operation

Line Status Register monitors the status of the serial line indicating when characters have been

received, or fully sent and when errors occur

The Modem Control and Modem Status Registers are used only for modem communications to

control and monitor the control lines in the RS-232 circuits

• Polling Mode: Simple communications routines constantly monitor the line status register, waiting

for an incoming character, or waiting until the register indicates that it is all right to transmit another

byte of data Serial data transmission rates are very slow compared to a microprocessors speed A

program using polling mode spends most of its time waiting for characters to be transmitted, bit by bit, or received bit by bit

• Interrupt Mode: Because the CPU operates very quickly relative to the 1200 or 2400

bit-per-second rate at which serial data typically moves, this method can be wasteful of CPU time that might otherwise be devoted to processing the incoming/outgoing data

• For this reason the 8250 may be set up to bring about an interrupt whenever a character arrives, an error occurs, etc

• The interrupt momentarily brings into action a procedure in your program that would, say, output the next character from a communications buffer

We will not use Interrupt Mode but will rather Poll the status of the UART before sending or receiving

characters

INITIALIZING THE SERIAL PORT

When a communications port is initialized ("opened"), all of the parameters by which it operates are

set These parameters include the word length, the number of stop bits, the parity setting, and the baud

rate The word length is the number of bits that form the basic data unit While we are accustomed to

working in eight bits, seven bits are adequate for standard ASCII files (where all characters are below ASCII 128)

Initialization of the 8250 involves supplying the following information:

Baud rate (speed) 75, 100, 150, 300, 600, 1200, 4800, 2400, 4800, or 9600 bits per second

Parity given as a one-character code: O for ODD parity; E for EVEN parity (the

default); N for NONE (no parity); S for SPACE, where the parity bit is always 0; and M for MARK, where the parity bit is always 1 If eight data bits are used, specify N; if four bits are used, do not use N

Data bits 4, 5, 6, 7, or 8

Stop bits 1 or 2

PROGRAMMING THE SERIAL PORT

Two techniques for programming the serial port on a PC are:

1 BIOS INT 14H SERVICES

a) There are 4 "services" available to control the serial port

b) This method is not suitable for transmission speeds in excess of 1200 baud

The 8250 registers must be programmed directly for high performance applications It usually

requires the use of interrupt handlers We will not access the serial port addresses directly in this

course

USING BIOS INT 14H FOR SERIAL PORT ACCESS:

Service Number 0 (INT 14H) - Initialize the Serial Port

This service sets up the communications parameters for the serial port: baud rate (speed), parity, stop

bits and character length

An 8-bit number is loaded into the microprocessor’s AL Register according to the table below:

Character Length

1 0 = 7 bits

11 = 8 bits

Stop Bits

0 = 1 stop bit

1 = 2 stop bits

Parity

01 = Odd

11 = Even

00 = None

10 = None

010 = 300

011 = 600

100 = 1200

101 = 2400

110 = 4800

111 = 9600

Fig 1 Initializing The Serial Port

Sample Assembly Language Program:

Initialization of COM1 for 1200 baud, odd parity, 7 bits, 1 stop bit

(Note: BIOS COM1 is port number 0; COM2 is port number 1)

MOV AL,8AH ;SET UP BYTE 10001010 binary MOV AH,0 ;REQUEST SERVICE NUMBER 0

MOV DX,0 ;COMM PORT NUMBER 0 (COM1)

INT 14H ; BIOS SERVICE

Sample C Program:

A C Program to initialize the UART to the same value could use the int86 instruction to execute BIOS int14 See attached listing of a sample program appended at the end

Service Number 1 (INT 14H) - Transmit a Character

This function is used to transmit a character through the communications line The character is simply loaded into the microprocessor’s AL register The DX register, as in all BIOS INT 14H

services, is loaded with the number of the communications port and the AH register with the service number (01 in this case)

Example: Send a '$' sign through the communications line

MOV AL,'$' ;Char to send

MOV AH,1 ;Request service no 1

MOV DX,0 ; Comm port no 0 (COM1)

Upon return to the caller, bit 7 of the AH is set if the routine was unable to transmit a byte of data, indicating some error condition

Service Number 2 (INT 14H) - Receive a Character

This function receives one character from the communications port The character received is

returned in the AL register Bit 7 of the AH register is set if the operation failed because of the data

set ready signal, indicating an error; for example, there was no data there to be received yet

Service Number 3 (INT 14H) - Read the Serial Port Status

This service returns the status of the serial port The AH register reports the status of the

communications line and the AL register contains the modem status

Meaning of bits set by FUNCTION 3, INT 14H

Bit 7= Time-out error Received line signal detect

Bit 6= Trans shift register empty Ring Indicator

Bit 5= Trans Holding Register Empty Data Set Ready

Bit 4= Break Detect Clear to Send

Bit 3= Framing Error Delta Rx Line signal detect

Bit 2= Parity error Trailing edge Ring detector

Bit 1= Overrun error Delta Data Set Ready

The (THRE) Transmit Holding Register Empty Bit

0 ==> a character is still in the process of being sent Transmitter is still Busy so a new character

cannot be sent until this bit becomes a 1

1 ==> Transmit Holding Register is empty so a new character can now be transmitted

Data Ready Bit

0 ==> Data has not been received yet

1 ==> A character has arrived in the Data Received Register and must be read in as soon as

possible

Simple Program to Endlessly Transmit a character using a UART:

UART Transmit Program

Initialize the UART

Transmit Character

Has Last Char been sent yet?

(Transmit Register EmptyYet ?)

Read the UART Status Register

Yes

No

The C program to transmit character 'a' continuously:

/* view_a.c , C Program for transmitting the character 'a' continuously */

/* A single character is transmitted repeatedly on COM1 for viewing on an oscilloscope */

#include <dos.h> /* Essential for int86 function */

#include <stdio.h>

#include <conio.h>

union REGS inregs, outregs; /* Provides Global access to microprocessors internal registers

The REGS union is declared in dos.h header file*/

/* int14( ) function prototype */

unsigned int int14(unsigned char, unsigned char, unsigned int);

/* int14(initializer, service number, comm port) is a user written function which simulates the BIOS INT14 operation */

int main(void)

{

unsigned int return_val;

int ready = 0; /* status flag indicating NOT ready when 0*/

char letter = 'a'; /* character to be transmitted */

unsigned char initializer; /* 8-bit value used to initialize the UART */

char from_kb;

int keypress;

/* To Initialize the COM1 port for 2400 bps, 7 bit data, even parity, 1 stop bit */ initializer = 0xBA; /* binary 1011 1010 or, 101 11 0 10 */

11 = Even parity

0 = One stop bit

/* Initialize the port */

int14(initializer, 0, 0);

while(1) /* Endless loop: ESC key is pressed to stop transmission */

{

keypress = kbhit();

if (keypress)

if(( from_kb = getch()) == 0x1B) /* ESC key pressed?*/

break;

/* Transmit a character */

int14(letter, 1, 0);

/* Wait for Character to be fully transmitted */ return_val =int14(0, 3, 0); /* before transmitting the next one */

ready = (return_val >> 13) & 1; /* Read UART Status */

/* Test Transmit Holding Register

Empty Bit in the Line Status Reg */

}while(!ready);

} /* end_while */

return (0);

}

/* ***************INT14 (equivalent) function ******************/

/* To Use BIOS INT 14h for COM1 control:

Put initializer to be used in AL register

Put INT14 service # in AH register

Put 0 in DX register for COM1 (or, 1 for COM2) */

unsigned int int14( unsigned char initializer,

unsigned char service_number,

unsigned int com_port)

{

inregs.h.al = initializer; /* value passed for initialization or sending a char */ inregs.h.ah = service_number; /* Service # to be executed for BIOS INT14 */

inregs.x.dx = com_port; /* Serial Port to be used: COM1 =0, COM2=1 */

int86(0x14, &inregs, &outregs); /* int86 instr'n enables us to execute BIOS

software interrupts from C */

return (outregs.x.ax); /* return the value in the microprocessor's AX */

Special Note:

The Intel microprocessors AX register is a 16 bit register which can also be accessed as two separate 8-bit registers AL (lower 8 8-bits) and as AH (upper 8 8-bits) Int86 instruction returns the 16 8-bit value in

AX

For example:

AH (8 bits) = FFh AL (8 bits) = 72h

AX (16 bits) = FF72h

UART or Universal Asynchronous Receiver Transmitter:

UART or Universal Asynchronous Receiver Transmitter is an integrated circuit used in PC serial ports

to send and receive data serially It must perform parallel to serial conversion of the data so data can

be sent to the UART from the microprocessor in parallel on the system bus and then transmitted out the serial port serially Common UART chips in serial ports are known as 8250, 16450 and 16550

Example Transmission of an ASCII character on a UART’s Transmit Data pin:

Sending ASCII character ‘a’ at a speed of 1200 bits per second, using 7 data bits, Odd parity and 1 Stop bit

‘a’ === Hexadecimal 61 or 110 0001 as a 7-bit number

Note: The Output of a UART is in a “1” state (also called the MARK state) before and after

transmission of a character

1 bit ===> +5 volts (TTL logic) Mark state

0 bit ===> 0 volts Space state

Bit time = 1 / (bit rate) = 1/2400 = 0.417 m sec = 417 μs

mark (1)

+5 v

start

0

LSB

1

odd parity 0

stop

1

space (0)

0 v

0 0 0 0 1 1

+5v

+12

-12

0v

RS232 Signaling TTL Logic