RS232 Serial USART (For Small PIC Chips) - Joe's Cat Website

If you are looking for serial Transmit and Receive routines for PIC chips that don't already have a hardware USART built-in, this code can be useful. The code shown below is simple and does not call any other routines so it can be called by another calling routine (for the 2-level-call-stack PICs) and is based on example code seen in AN510, AN555 and a few other application notes you may have read.

If you did not notice, receiving a byte is called at RxByte, not at RxLoop.

This code also makes use of the Delay Macro described on another page.


You can download RS232 transmit/receive routines, or you can copy and paste the code shown below into your ASM code editor.

w	equ	0
f	equ	1
;portA bit definitions
DXbit	equ	0	;transmit pin (Bit0 of PortA)
DRbit	equ	1	;receive  pin (Bit1 of PortA)
;	   ____  ____
;	--|a2  \/  a1|<- !Rx
;	--|a3	   a0|-> !Tx
;	->|TOC	 osc1|<- XTAL
;!reset ->|MCLR	 osc2|-> XTAL
;   gnd ->|Vss	  Vdd|<- +5v
;	--|b0	   b7|--
;	--|b1	   b6|--
;	--|b2	   b5|--
;	--|b3	   b4|--
;	   ----------

;timing values
RSbitC	equ	4800000/1200/4	;cycles for RS232 bit @4.8MHz
RSbitC	equ	6144000/1200/4	;cycles for RS232 bit @6.144MHz
	org	08H	;register area
DlyCnt	res	2	;delay wait counter
RcvReg	res	1	;data received
XmtReg	res	1	;data to be transmitted
Count	res	1	;counter for #of Bits Transmit/Receive

;RS232 Receive Byte, enter thru RXbyte
;exit:	w=byte received
;ram:	RcvReg,Count,DlyCnt
;call:	none
RXloop	DelayM	RSbitC/2-3	;wait 0.5bit time

RXbyte	btfsc	PORTA,DRbit	;check for a Start Bit
	goto	RXloop		;delay for 0.5 bit time
	DelayM	RSbitC+RSbitC/4-7 ;delay for 1.25bit time
	movlw	8		;prepare to receive 8 data bits
	movwf	Count
RXbits	rrf	RcvReg,f	;shift RXed byte - left to right
	bcf	RcvReg,7	;set bit7 of RXed byte to 0
	btfsc	PORTA,DRbit	;check RXed bit
	bsf	RcvReg,7	;set to 1bit if logic 1
	DelayM	RSbitC-7	;wait until time for next bit
	decfsz	Count,f		;loop until all 8 bits done
	goto	RXbits
	movf	RcvReg,w	;return with w=RS232byte

;RS232 Transmit Byte
;enter:	w=RS232byte to send
;ram:	XmtReg,Count,DlyCnt
;call:	none
TXbyte	movwf	XmtReg		;this is the byte to transmit
	movlw	8		;prepare to send 8 data bits
	movwf	Count
	bcf	PORTA,DXbit	;send start bit
	DelayM	RSbitC-4
TXbits	rrf	XmtReg,f	;get next bit to send (1 or 0)
	bsf	PORTA,DXbit
	bcf	PORTA,DXbit
	bcf	XmtReg,7	;set XmtReg to 0 as we go...
	DelayM	RSbitC-9	;wait until time for next bit
	decfsz	Count,f		;loop until all 8 bits done
	goto	TXbits
	bsf	PORTA,DXbit	;send 2 stop bits
	DelayM	RSbitC*2
	retlw	0

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