Difference between revisions of "8051 Software UART Tutorial" From Rikipedia Embedded Wiki

Latest revision as of 19:48, 3 March 2015

To implement software UART successfully one must understand the basics of UART first and understand its protocol. As mentioned in our UART Basics tutorial. For 8bit, no parity and 1 stop bit UART Transmission, 10 bits are transmitted (startbit, 8-data bits and stop bit). There is a formula to calculate the delay time for 1 bit which is needed between bits to get correct baudrate.

1 bit time = (((crystal/baud)/12) - 5) / 2

Where crystal is the frequency of crystal in Hz baud is the required baudrate we are doing divide by 12 as 8051 architecture single machine cycle is crystal freq/12

Below is a software implemented UART, which can be used in C as well as Assembly programs. It is written for Keil software. But with a little modification you can use it in your programs.

?SU?PUTC SEGMENT CODE
?SU?GETC SEGMENT CODE

PUBLIC _putc
PUBLIC getc

txd_pin	EQU	P3.1		;Transmit on this pin
rxd_pin	EQU	P3.0		;Receive on this pin

;Formula to calculate the bit time delay constant
;This constant is calculated as: (((crystal/baud)/12) - 5) / 2
;crystal is the frequency of crystal in Hz
;baud is required baudrate
;Please try to keep baudrate below 9600
;to get best results :)

BITTIM	EQU	45;		(((11059200/9600)/12) - 5) / 2

;--------------------------------------------
;To send data serially
;For C programs
;Protype definition:
;		void putc(unsigned char);
;Usage:
;		putc(data);
;Return:
;		This function returns nothing
;
;For Assembly Programs:
;
;Usage:
;	data to be send has to be moved to R7
;	for example:
;		mov R7,#'a'
;		lcall _putc
;--------------------------------------------
RSEG ?SU?PUTC
_putc:
	push ACC
	Push PSW
	mov a,r7
	CLR txd_pin			;Drop line for start bit
	MOV R0,#BITTIM		;Wait full bit-time
	DJNZ R0,$			;For START bit
	MOV R1,#8			;Send 8 bits
putc1:
	RRC A				;Move next bit into carry
	MOV txd_pin,C		;Write next bit
	MOV R0,#BITTIM		;Wait full bit-time
	DJNZ R0,$			;For DATA bit
	DJNZ R1,putc1		;write 8 bits
	SETB txd_pin		;Set line high
	RRC A				;Restore ACC contents
	MOV R0,#BITTIM		;Wait full bit-time
	DJNZ R0,$			;For STOP bit
	POP PSW
	pop ACC
	RET

;--------------------------------------------
;To receive data Serially
;If you want to use this routine in your
;C program then define function prototype
; as:
;	unsigned char getc(void);
;
;	Usage:
;		data = getc();
;	Return value:
;		Returns data received
;
;
;If you are using it in assembly program
;	Usage:
;		lcall getc
;	Return:
;		data received is stored in R7
;--------------------------------------------

RSEG ?SU?GETC
getc:	
	Push ACC
	Push PSW
	JB rxd_pin,$		;Wait for start bit
	MOV R0,#BITTIM/2	;Wait 1/2 bit-time
	DJNZ R0,$			;To sample in middle
	JB rxd_pin,getc		;Insure valid
	MOV R1,#8			;Read 8 bits
getc1:
	MOV R0,#BITTIM		;Wait full bit-time
	DJNZ R0,$			;For DATA bit
	MOV C,rxd_pin		;Read bit
	RRC A				;Shift it into ACC
	DJNZ R1,getc1		;read 8 bits
	mov r7,a
	POP PSW
	pop ACC
	RET					;go home

Now you can have two serial ports or even 3 :) on a normal 8051. Have a happy programming.

See Also

• UART Communication Tutorial - Basics of UART
• 8051 UART/Serial Communication Tutorial
• AVR UART/Serial Communication Tutorial
• GPS Receiver Interfacing over UART Tutorial
• GSM Modem Interfacing over UART Tutorial

Help & Queries

If you have any queries, doubts or feedback on this tutorial please share in our discussion forum.