;********************************************************************
; Listing 1 -- RS-232 TRANSMISSION-RATE DETECTION ROUTINE
; 
; "uC detects transmission rate of RS-232 interface,"
; EDN, Sept 30, 1999, pg 114.
; http://www.ednmag.com/ednmag/reg/1999/093099/designideas.htm#20di2
;
;********************************************************************

Autobaud		clrf	AUTOBAUD_LOW	; reset register
			clrf	AUTOBAUD_HIGH	; reset register
			clrf	AUTOHALF_LOW	; reset register
			clrf	AUTOHALF_HIGH	; reset register
			clrf	AUTOB_STATUS	; reset autobaud status 
; register
TestStartBit	btfsc	PORTA, RX			; check for start-bit
			goto	TestStartBit		; Start-bit not found
		
TestBitHigh		btfsc	PORTA, RX		; Test for end of bit 
; stream
goto	Calculate		; End of bit stream, now 
; calculate
							; bit time for one bit	
			incfsz	AUTOBAUD_LOW, f		; increment Autobaud 
;low register
			goto	TestBitHigh			; test for high bit
			incfsz	AUTOBAUD_HIGH, f		; increment high byte 
; of autobaud 
; register
			goto	TestBitHigh	; test for end of bit stream
			goto	Signal2Slow	; High byte got an overflow. 
; Transmitted
						; signal is too slow for clock 
; speed of the uC

		; divide by measure time by 6 (6 one's where transmitted)
Calculate	movlw	0x03			; Initialize count register
		movwf	COUNTER		; Counter for number for rotates 
; = 3
Divide	bcf	STATUS, C			; clear carry bit
		rrf	AUTOBAUD_HIGH,f	; rotate autobaud high register
		rrf	AUTOBAUD_LOW, f 	; rotate autobaud low register
		decfsz	COUNTER, f	; decrement counter
		goto	Divide		; divide

	
		; Calculate half the bit time
CalcHalfBit	bcf	STATUS, C		; clear carry bit
		rrf	AUTOBAUD_HIGH,w	; rotate autobaud high register
		movwf	AUTOHALF_HIGH	; copy result into AUTOHALF_HIGH 
; register
		rrf	AUTOBAUD_LOW, w 	; rotate autobaud high register
		movwf	AUTOHALF_LOW	; copy result into AUTOHALF_LOW 
; register

AdjustLowByte	movlw	0x3		; 18-19 instruction cycles 
;overhead from
						; transmit and receive routine. 
		subwf	AUTOBAUD_LOW, f	; Adjust low byte from Autobaud 
;counter
		btfss	STATUS, C		; Is result negative? (equal=0 
; will be checked
						; at ErrorCheck). C=0 result is 
; negative
		goto	Signal2Fast		; Signal is too fast for receive 
;and transmit routine
		movlw	0x02			; substract 2 from low byte of 
; half the bit time
subwf AUTOHALF_LOW, f	; substract from low byte of half ; the bit time
		btfss	STATUS, C		; Is result negative? (equal=0 
; will be checked
						; at ErrorCheck). C=0 result is 
; negative
		goto	Signal2Fast		; Signal is too fast for receive 
; and transmit routine	

		; check if AUTOBAUD_HIGH and AUTOBAUD_LOW are zero. This
		; means the transmission time for one byte is too high
ErrorCheck	movf	AUTOBAUD_HIGH,w 	; copy high byte of autobaud 
;counter register into
						; w-register
		xorwf	AUTOBAUD_LOW, w	; AUTOBAUD_HIGH = AUTOBAUD_LOW?
		btfss	STATUS, Z		; is result zero?
goto	ErrorCheckHalf	; Result is not zero, therefore
; finish autobaud routine 
		goto	Signal2Fast		; Signal is too fast for routine
ErrorCheckHalf	movf	AUTOHALF_HIGH,w   ; copy high byte of 
; autobaud 
;counter register into
							; w-register
			xorwf	AUTOHALF_LOW, w	; AUTOBAUD_HIGH = 
; AUTOBAUD_LOW?
			btfss	STATUS, Z		;is result zero?
			goto	EndAutoBaud		; Result is not zero, 
; therefore finish autobaud 
				; routine 

		; Error: delay for half the bit time is zero, therefore a
		; delay cannot be generated with the delay routines. 
; Incoming signal is too fast for clock speed.
Signal2Fast	bsf	AUTOB_STATUS, SIGNAL_FAST	; set error flag
retlw	0x00			; return to OS

Signal2Slow		bsf	AUTOB_STATUS, SIGNAL_SLOW	; set error 
;flag		


EndAutoBaud		retlw	0x00		; Return to operating system


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