Olin Lathrop wrote:

>Ake Hedman wrote:
> =20
>
>>My fosc is 32 MHz.
>>By using a 32 bit prescaler i get 1MHz
>>   =20
>>
I meant that the prescaler was set to 32 of course....

>You don't have anywhere near 32 bits of prescaler.
>
> =20
>
>>I let the timer fire an interupt on overflow and reload it with
>>0xffff - 0x3E8
>>
>>But this give the wrong result. Instead if I change the prescaler to 4
>>the timing is OK. This indicates that I have a 4MHz clock in to the
>>timer!? If it was 8MHz  I could guess whats up but 4 MHz...?  Im lost?
>>
>>Anyone see *the fault* in my thinking.
>>   =20
>>
>
>The descriptions of your prescaler settings are muddled, which probably
>indicates the thinking behind them is too.  One thing to keep in mind is
>that timer 0 increments on the instruction clock, not the oscillator.  T=
he
>instruction clock is Fosc/4, which is 8MHz in your case.
>
>However, you will have some error creep into your timing due to interrup=
t
>latency, a few instructions needed after the interrupt to reload timer 0=
,
>that timer 0 stops incrementing for a few cycles after it's modified, an=
d
>that the prescaler gets cleared on a write to timer 0.  All in all this =
is
>not a good way to get a 1mS clock.
>
>An easier and more reliable way is to use one of the timers with built i=
n
>period registers.  That's what they are for.  You set it up once then yo=
u
>automatically get periodic interrupts.
> =20
>
I can live with the error introduced by this  method. That is not *the=20
problem*.  What I did not know was that the timer is updated with the=20
instruction clock. Thats good information .

So I have 8 MHz into the timer. Set the prescaler to 4 (where I get the=20
correct result) which give me 2 MHz.  1000 countes   I would expect half=20
the time of which I measure.....

Regards
/Ake

--=20
 ---
Ake Hedman (YAP - Yet Another Programmer)
eurosource, Brattbergav=E4gen 17, 820 50 LOS, Sweden
Phone: (46) 657 413430 Cellular: (46) 73 84 84 102
Company home: http://www.eurosource.se     =20
Kryddor/Te/Kaffe: http://www.brattberg.com
Personal homepage: http://www.eurosource.se/akhe
Automated home: http://www.vscp.org


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