Post by thread:[PIC] Clever 3-cycle delay?

OK, I'm doing some very timing-sensitive waveform generation with a PIC, and I also want to conserve memory usage. For long delays I just call a subroutine which does the delay. One word and I'm done. For one and two cycle delays I use a NOP and a GOTO $+1, respectively. Again, each is one word. But for three cycle delays I don't have any tricks up my sleeve. I can do a NOP followed by a GOTO $+1, but that's two words. Any tricks out there? The 16F628 is what I'm using, but I'd be willing to hear tricks for other PIC lines. Tim. -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

On Sun, 30 Mar 2003 spam_OUTshoppa_piclistTakeThisOuTTRAILING-EDGE.COM wrote: > OK, I'm doing some very timing-sensitive waveform generation with > a PIC, and I also want to conserve memory usage. > > For long delays I just call a subroutine which does the delay. One > word and I'm done. > > For one and two cycle delays I use a NOP and a GOTO $+1, respectively. > Again, each is one word. > > But for three cycle delays I don't have any tricks up my sleeve. I > can do a NOP followed by a GOTO $+1, but that's two words. Any > tricks out there? The 16F628 is what I'm using, but I'd be willing > to hear tricks for other PIC lines. It's not possible in just one instruction. On the 18F devices you could do this: here: GOTO $+2 That's a 3 cycle delay, but it's still two words. You might as well do this: BRA $+2 NOP Note that a $+2 in the 18F family is analogous to a $+1 in the 16X family. In the first example, the goto will branch to the second half of the goto instruction which is executed as a NOP. You can do a single word 4-cycle delay: CALL some_spare_return There's bound to be a RETURN somewhere in your program that can be labeled. Scott -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

On 30 Mar 2003 at 13:55, Scott Dattalo wrote: > > You can do a single word 4-cycle delay: > > CALL some_spare_return > > There's bound to be a RETURN somewhere in your program that can be > labeled. > This can be a 3-cycle delay when following some movlw instruction, which can be changed to a retlw. Mark -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

On Sun, 30 Mar 2003, Marcelo Puhl wrote: > On 30 Mar 2003 at 13:55, Scott Dattalo wrote: > > > > > You can do a single word 4-cycle delay: > > > > CALL some_spare_return > > > > There's bound to be a RETURN somewhere in your program that can be > > labeled. > > > > This can be a 3-cycle delay when following some movlw instruction, which > can be changed to a retlw. Are you saying that instead of: MOVLW K GOTO $+1 NOP That you write: CALL retlw_K ... RETLW K ? -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.

On 30 Mar 2003 at 16:27, Scott Dattalo wrote: > Are you saying that instead of: > > MOVLW K > GOTO $+1 > NOP > > That you write: > > CALL retlw_K > > ... > > RETLW K > Yes, and you even gain one instruction. Mark -- http://www.piclist.com hint: The PICList is archived three different ways. See http://www.piclist.com/#archives for details.