PIC pulse train (assembly)

[riccardo]

Hi,

I'm trying to generate a pulse train on PORTB of a 18F2455. I had a simple Idea of running a loop and incrementing the port number, but this gives me an 'argument out of range message. Does anyone know how to do this properly?

I've just listed "channel" in a cblock and set it to 0 before this loop

Step
	BSF PORTB, channel			; Set output channel high
	CALL Wait					; Delay
	BCF PORTB, channel			; Clear output channel
	CALL Wait					; Delay
	INCF channel
	; If channel > 7 make it 0
	MOVF channel, W				; Copy channel value to W for testing
	SUBWF 8, W					; Subtract W from 8
	BTFSC STATUS, Z				; Check if result is Zero (channel needs to be reset)
		CLRF channel			; Reset to zero
	GOTO Step

 

[Nigel Goodwin]

You can't use a variable for the BCF or BSF instructions, just a 3 bit fixed value that's hardcoded in the line.

If you're wanting to run a bit along the port, check my tutorials for a number of ways of doing it - including multiple patterns stored in tables.

 

[riccardo]

Could you point me to the tutorial that covers this? I can't find what I need.

Can I just use...?

RLNCF PORTB

 

[Nigel Goodwin]

Could you point me to the tutorial that covers this? I can't find what I need.

Can I just use...?

RLNCF PORTB

I don't even recognise that 'instruction', my tutorials 1.5 - 1.9 show various ways of sequencing LED's.

 

[riccardo]

That instruction is "Rotate Left f (no carry)". I expect I will be able to make use of this.

 

[Nigel Goodwin]

That instruction is "Rotate Left f (no carry)". I expect I will be able to make use of this.

Sorry, hadn't noticed you were using an 18F.

 

[riccardo]

Here's the code I used for anyone who might need it..

"maxChannel" is an 8-bit value representing how many channels should be pulsed before the loop restarts. 10000000 being 8 channels, while 00000001 is just one.

"channel" is the currently selected channel using the same logic as above.

Start
	CALL ResetChannels		; Reset to channel 1

Step
	MOVF channel, W		; Copy channel selection
	MOVWF PORTB			; SET outputs to selected channel combo
	CALL Wait				; Delay
	CLRF PORTB			; Clear outputs
	CALL Wait				; Delay
	RLNCF channel			; Increment Channel


	; If channel > maxChannels reset to 1
	MOVF channel, W			; Copy channel value to W for testing
	MOVWF L4
	RRNCF L4					; Go back one because of previous increment
	MOVF L4, W
	SUBWF maxChannel, W		; Subtract W from maxChannel
	BTFSC STATUS, Z			; Check if result is Zero (channel needs to be reset)
		CALL ResetChannels		; Reset to channel 1
	GOTO Step				; ELSE do next channel

ResetChannels	
	MOVLW b'00000001'			; Start with Channel 1
	MOVWF channel
	RETURN