SHIFTOUT Clock rate

[Charles Linquis]

An assembly-language subroutine would certainly allow for a faster SHIFTOUT.

[Brock]

It would be no small task inserting an assembly routine... I would have to insert
many routines. I also don't know assembly very well. Speaking of assembly, is
it possible to modify the PBPPIC18.LIB file (shiftout routine)?

If I could double the speed of the shiftout clock, that would do it.

[SteveB]

Speaking of assembly, is it possible to modify the PBPPIC18.LIB file (shiftout routine)?

I'm sure it is possible. If you do a Find... on the term "ifdef SHIFTTOGGLE_USED", you should be able to find the right section.

    ifdef SHIFTTOGGLE_USED
  LIST
shifttoggle
        movf    RM2, W          ; Get clock bit mask
        xorwf   INDF1, F        ; Toggle clock
     ifdef SHIFT_PAUSEUS
	movf	R0, W		; Save shift data
	movwf	R1
	movf	R0 + 1, W
	movwf	R1 + 1
        movlw   (SHIFT_PAUSEUS) >> 8    ; Slow down transfer
        movwf   R0 + 1
        movlw   low (SHIFT_PAUSEUS)
        call    PAUSEUSL
	movf	R1, W		; Restore shift data
	movwf	R0
	movf	R1 + 1, W
	movwf	R0 + 1
        movf    RM2, W          ; Get clock bit mask
PAUSEUS_USED = 1
     else
        nop                     ; Slow down transfer
      if (OSC > 4)
        bra     $ + 2
      endif
      if (OSC > 8)
        bra     $ + 2
      endif
      if (OSC > 12)
        bra     $ + 2
      endif
      if (OSC > 16)
        bra     $ + 2
      endif
      if (OSC > 20)
        bra     $ + 2
      endif
      if (OSC > 25)
        bra     $ + 2
        bra     $ + 2
      endif
      if (OSC > 33)
        bra     $ + 2
        bra     $ + 2
      endif
      if (OSC > 40)
        bra     $ + 2
      endif
     endif
        xorwf   INDF1, F
        return
  NOLIST
    endif

The bold text is the stuff you need to tweek. If I were doing it, I would start with the "if (OSC > XX)...endif" statement which would be just below the OSC value your using (if (OSC > 16)), comment it out (add a semi-colon), and see how it runs. Then do the same for the next "if (OSC > XX)...endif", and so on. You coud even remove the "nop ; Slow down transfer".

Don't see why it wouldn't work. But the suggestion comes with all the standard disclaimers.
Save a copy of the original file if you plan on doing this.
Just had to say it.

Let us know how it goes!
Steve

[Brock]

Success!!!

First, I tried deleting the whole "else" section with the delays. My circuit worked
OK, but the active pulse width was less than 400ns. Too short for me to feel
comfortable about reliability. So then I tried changing all the "2"s to "1"s in the
"else" section. I got errors when I compiled. So, I deleted the "if" statements
for the lower speed oscillators and it looks pretty good now. I still wish it was
faster, but speeding it up seems to shrink the active pulse width too much.

For reference:

No delays (removing all the "else" statements): 3.6us clock (278kHz)
No delays up to 16MHz: 4us clock (250kHz)

Here is what I have:

ifdef SHIFTTOGGLE_USED
LIST
shifttoggle
movf RM2, W ; Get clock bit mask
xorwf INDF1, F ; Toggle clock
ifdef SHIFT_PAUSEUS
movf R0, W ; Save shift data
movwf R1
movf R0 + 1, W
movwf R1 + 1
movlw (SHIFT_PAUSEUS) >> 8 ; Slow down transfer
movwf R0 + 1
movlw low (SHIFT_PAUSEUS)
call PAUSEUSL
movf R1, W ; Restore shift data
movwf R0
movf R1 + 1, W
movwf R0 + 1
movf RM2, W ; Get clock bit mask
PAUSEUS_USED = 1
else
nop ; Slow down transfer
if (OSC > 16)
bra $ + 2
endif
if (OSC > 20)
bra $ + 2
endif
if (OSC > 25)
bra $ + 2
bra $ + 2
endif
if (OSC > 33)
bra $ + 2
bra $ + 2
endif
if (OSC > 40)
bra $ + 2
endif
endif
xorwf INDF1, F
return
NOLIST
endif

[SteveB]

Success!!!

First, I tried deleting the whole "else" section with the delays

This eliminates the xorwf INDF1, F statement, so not a great idea.

So then I tried changing all the "2"s to "1"s in the "else" section. I got errors when I compiled.

Yea, the BRA $ + 2 are relative branches and it won't work properly on a PIC18F with $ + 1. The reason for this is the PC18F instructions are two bytes long, so a $+1 will branch to the middle of an instruction. $+2 branches to the next instruction. It has the simple effect of causeing a 2 instruction cycle delay with one command (in lieu of using 2 nop commands).

So, I deleted the "if" statementsfor the lower speed oscillators and it looks pretty good now. I still wish it was faster, but speeding it up seems to shrink the active pulse width too much.

This is what I was getting at. I think with a little tinkering, the active pulse could be lengthened, while keeping the inactive state short, but I can't go there now. Maybe if I get to it in the future I'll follow up in this thread (who knows when that may be!). Besides, I'd want to take a good look at the timing specs before fiddling with this too much. But it is good to know what you have confirmed already.

Steve

[SteveB]

After re-reading the code, my previous statement was incorrect:

This eliminates the xorwf INDF1, F statement, so not a great idea.

the xorwf falls just after the if..else..endif statement, otherwise when you deleted the whole else portion it would not have worked.

[Brock]

After re-reading the code, my previous statement was incorrect:

the xorwf falls just after the if..else..endif statement, otherwise when you deleted the whole else portion it would not have worked.

Yeah, I didn't delete that part.

Thanks for the help!