That has got to be one of the finest, most detailed, accurate and complete
answers to an open ended newbie question that I have ever seen.

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> -----Original Message-----
> From: piclist-bounces@mit.edu 
> [mailto:piclist-bounces@mit.edu] On Behalf Of Maarten Hofman
> Sent: 2005 Nov 28, Mon 12:08
> To: Microcontroller discussion list - Public.
> Subject: Re: [PIC] newbie question on timing
> 
> > Using a basic compiled program using a for-next loop,  a 
> flash PICchip.
> > can I expect toget a pulse every 55 ms. or about 18pulses 
> per second?
> >
> > I know a lot of it will depend on what speed I am running 
> the chip at.
> 
> Yes, and on the BASIC compiler.
> 
> Let me see, hypothetically (non working code, just counting a bit):
> 
> FOR A = 0 TO 255
> PORTC = PORTC & 254
> PORTC = PORTC | 1
> NEXT A
> 
> ; FOR A = 0 TO 255
>   movlw 0
>   movwf A
>   goto NEXT1002
> NEXT1001
>   movlw 255
>   xorwf A,w
>   btfsc STATUS,Z
>   goto NEXT1003
>   incf A,f
> NEXT1002
> ; PORTC = PORTC & 254
>   movlw 254
>   andwf PORTC,f
> ; PORTC = PORTC | 1
>   movlw 1
>   iorwf PORTC,f
> ; NEXT A
>   goto NEXT1001
> NEXT1003
> 
> Which indicates there are 4 cycles of setup and 11 cycles per loop.
> Given a not unreasonable clock frequency of 4 MHz, that means 
> you get a pulse every 11us, so about 90909 pulses per second.
> 
> As said, this highly depends on the clock frequency, but also 
> on the BASIC compiler (the code could be further optimized 
> using incfsz/decfsz, which would make it faster. Or the BASIC 
> compiler outputs more generic code, allowing values greater 
> than 255, which would make things slower... Also if it would 
> allow full expressions in the FOR loop this might add 
> superfluous dummy variables, slowing things down).
> 
> But 90909 pulses per second seems a lot closer to reality 
> than 18. If you DO want 18 pulses per second, you might need 
> to add a suitable PAUSE command, of known duration.
> 
> Greetings,
> Maarten Hofman.
> 
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