waitcnt?

Leon

Why is it necessary to add 9 to Time when using waitcnt for a delay:
··

       mov       Time, cnt
       add       Time, #9  
:loop  waitcnt   Time, Delay             

I can't find an explanation in the Manual.

Leon

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Amateur radio callsign: G1HSM
Suzuki SV1000S motorcycle

Paul Baker

Because the addition of delay to time by waitcnt doesn't occur until after the wait occurs, so you need to compute the first delay before entering the waitcnt (unless you want to wait nearly a minute).

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Paul Baker
Propeller Applications Engineer

Parallax, Inc.

deSilva

see also deSilva's posting near the end http://forums.parallax.com/showthread.php?p=699845

deSilva

Thanks to Google I could also find an instructive example I posted some months ago (to a certain "Aempirei"), showing the working of the "instruction pipe"

deSilva said...
This is something Mike addressed:

        mov Time, CNT           'Calculate delay time
        add Time, #9            'Set minimum delay here
        waitcnt Time, Delay     'Wait
        xor   OUTA, Pin

Does #9 suffice?

Yes, in fact this is the case whithout any "wait" at all (5 ticks of WAITCNT only)!

Tick   GetInstr  empty  GetDest  GetSrc  Operate   StoreBack
-3     MOV
-2                ---
-1                      Time
0                                CNT
+1     ADD                                ---
+2                ---                              Time
+3                      Time
+4                               #9
+5     WAITCNT                            add
+6                ---                              Time
+7                      Time
+8                               Delay
+9                                        Wait     Time
+10    XOR
+11               ---
+12                     OUTA
+13                             Pin
+14                                       xor
+15                                               OUTA   -> Strope detectable

I am sure aempirei ("nomen est omen") checked this out