On Wed, 6 Aug 2003, Samo Benedicic wrote:

> I think ten or so days ago someone posted code
> fragment that deals with this 1 cycle jitter issue.

I must've missed it among the flurry of rediculously off topic posts we've
had lately. (What's the deal with asking questions about Excel on a
microcontroller mailing list? My question is rhetorical *please* don't
waste time answering it!)

But back on topic....

Was it something like this:

   ; interrupt code .....

   ; If TMRO is not the value we expect it to be then (presumably)
   ; we were delayed one extra cycle when the interrupt occurred

       MOVLW   TMR0_KNOWN_VALUE_RIGHT_NOW
       SUBWF   TMR0,W
       SKPZ                           ;If TMR0 is not what we expect
        GOTO   $+1                    ;then delay one cycle.

Changing the subject slightly...

I very often have to deal with timing; e.g. converting instruction cycles
into milliseconds or whatever. It's always easy to make a mistake doing
these tedious conversions. So as general advice, I suggest carrying units
along with the numbers during your conversions. It's a trick you're taught
(or should've been taught) in Freshman Physics.

So for example, let's suppose you want to know how many instructions
execute in 1/60'th of a second on a midrange PIC clocked at 10MHz.

fosc = 10Mhz = 10million cycles/second
Tosc = 1/fosc = 100 nSecond/ cycle
Tinst = 4 cycles/instruction *Tosc
      = 4 cycles/instruction * 100nSeconds/cycle
      = 400 nSeconds/Instruction
Tinterval = 1/60 seconds/Interval

Now, to find the number of instructions per the 1/60'th of a second
interval we need an answer that has the units of Instruction/Interval:

                   1    seconds      400  nSeconds
Tinterval/Tinst = --- * --------  / --------------
                   60   Interval      Instruction

                   1    seconds       Instruction
                = --- * --------  *  -------------
                   60   Interval      400 nSeconds

                        1        Instruction
                = ------------ * -----------
                   60 * 400e-9    Interval

                = 41666.666...  Instructions/Interval

In summary, identify the units of all your variables and carry them along
in your analysis. This approach works well with timer prescales and
rollovers too.

Scott

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