Maarten Hofman wrote:
>> /* Delay of exactly 2 Tcy */
>> #define delay_2tcy() asm("nop"); asm("nop")
>
> As Chen Xiao Fan indicated, this could be better done using a "goto
> $+1" operation: it would take only one word instead of two.

Even better is to use a macro that you simply tell the number of cycles to
waste and it figures out when to use GOTO $+1, BRA $+2, and NOP.  That's
what my WAITCY macro in STD.INS.ASPIC at http://www.embedinc.com/pic does.

However in most cases of inserting NOPs, you are really trying to do timing.
Just iserting a fixed number of instruction cycles is a bad idea since that
makes your source code implicitly dependent on the processor clock speed.
WAITCY should only be used when you want to wait a specific number of
*instructions*, which is rare.

An even better answer is to specify the amount of *time* to waste.  Let the
assembler calculate how many instruction cycles that is, then calclulate the
least number of instructions to take that many cycles.  But there's another
wrinkle.  Often you need two instructions to be a fixed minimum time apart,
but you have other things you can do during those instructions although they
don't guarantee taking that minimum amount of time.  So you want a minimum
guaranteed delay in time, but are going to spend some of that with a fixed
number of instruction cycles.  Therefore what you really need is a way of
saying, for example, "Take 2uS, but I've already used 5 instruction cycles
out of that 2uS.".  This shows the part you know about and can control in
the source code without any implicit reliance on the processor clock speed.

This is exactly what my WAITNS and WAITUS macros in STD.INS.ASPIC do.

> Note that at some point it will become better making a small loop
> using decfsz and goto $-1.

As long as the side effect on the additional state and the status flags are
clearly documented.


******************************************************************
Embed Inc, Littleton Massachusetts, (978) 742-9014.  #1 PIC
consultant in 2004 program year.  http://www.embedinc.com/products
-- 
http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist