Serin2 and Timer1

[retepsnikrep]

Thanks got the timer working now but have one more question.

T1CON.0 = 0           'Stops Timer
    
    TMR1RunOn.Highbyte = TMR1H         'Load the Run-On (Over-Run) value (if any)
    TMR1RunOn.Lowbyte = TMR1L        'Load the Run-On (Over-Run) value (if any)
    TMR1RunOn = PreLoad + TMR1RunOn        'Calculate new Preload setting to include any timer overrun    
    TMR1H = TMR1RunOn.HighByte        'Load the timer with new value
    TMR1L = TMR1RunOn.LowByte           'Load the timer with new value

    PIR1.0 = 0        'Clear TMR1 int flag 
    T1CON.0 = 1               'Starts Timer

In the above code i stop the timer then work out any TMR1 overun and add it to my timer reload value.
But how do i compensate for the instructions above which clearly take some time to execute?
I must need to add a few more timer ticks to my reload value, but how many? My clock is running at 8mhz.

[HenrikOlsson]

You could just do it by trial and error, let it run for a while and see if and how much it drifts. Otherwise you need to dig into the generated .lst file and count the actual instructions. I'm guessing somewhere around 16 cycles.

Or, why not use a second timer (TMR0 etc) to actually time it. Stop TMR1, start TMR0, do the reload, stop TMR0, start TMR1. TMR0 will now give you your answer.

If you tick the timer with the main oscillator it matter if you run at 8kHz or 8MHz, a cycle is a cycle.

/Henrik.

[retepsnikrep]

I'm confused now by the relationship between the crystal/int osc at 8mhz and the various clocks :?

So at 8mhz with Fosc/4 & 1/8 prescaler timer1 is ticking at 250,000hz? OK I get that.

But the sytem instruction clock is operating at? 8mhz instruction cycles a second or Fosc/4 = 2mhz? or something else?

[retepsnikrep]

The asm listing shows these instructions.

    MOVE?CT    000h, _T1CON??0
    MOVE?BB    TMR1H, _TMR1RunOn??HIGHBYTE
    MOVE?BB    TMR1L, _TMR1RunOn??LOWBYTE
    ADD?WWW    _PreLoad, _TMR1RunOn, _TMR1RunOn
    MOVE?BB    _TMR1RunOn??HIGHBYTE, TMR1H
    MOVE?BB    _TMR1RunOn??LOWBYTE, TMR1L
    MOVE?CT    000h, _PIR1??0
    MOVE?CT    001h, _T1CON??0

So how much to add to timer1 reload to compensate I wonder?
Looking at the data sheet these instructions all seem to need 1 cycle each? So I make that 8 cycles how much is that in TMR1 ticks?

[HenrikOlsson]

Hi,
You are correct, when using any other prescaler than 1:1 one instructioncycle is not equal to one timer tick.

If you're using 1:8 prescaler then the timer is incremented every 8th instructioncycle but it still doesn't matter if the Fosc is 8kHz or 8Mhz, the relationship between instructioncylce and timertick/count is still the same and that relationship is the prescaler ratio.

The problem with the ASM-listing that is generated is that it's not "true" ASM, those commands you have in your listing are macros which you then need to "expand" into the "real" ASM instructions. (Did you really find those instruction in the datasheet?) But if it does work out to 8 instructions and your prescaler is 1:8 then it equals exactly 1 timer tick.

/Henrik.