Ake Hedman wrote:

> Jinx wrote:
>
>>> My fosc is 32 MHz.
>>> By using a 32 bit prescaler i get 1MHz
>>>  =20
>>
>>
>> I think you mean divide by 32. And your instruction clock rate is
>> only 8MHz, not 32MHz. A /32 pre-scaler reduces this to 250kHz
>> (=3D 4us per count)
>>
>> =20
>>
>>> I let the timer fire an interupt on overflow and reload it with=20
>>> 0xffff -
>>> 0x3E8
>>>  =20
>>
>>
>> So, your count rate is 250kHz and you're loading Timer0 with -1000d
>> =3D a rollover rate of 1000*4us =3D 4ms, or x4 the time you expected
>>
>> =20
>>
>>> But this give the wrong result. Instead if I change the prescaler to =
4
>>> the timing is OK
>>>  =20
>>
>>
>> I'm not sure if I've missed something in your description :
>>
>> A /4 pre-scaler puts the count rate at 2MHz =3D 0.5us/count. 1000
>> counts would =3D 0.5ms. It would appear to me that a /8 is needed
>> to produce a 1MHz count rate =3D 1ms from a 1000 count
>> =20
>>
> Hi Jinx,
>
> I test it the following way:
>
> I count 30000 timer overflows for 30 seconds. If I use a prescaler=20
> value of 4 i get the correct 30 seconds. If set to 8 I would have=20
> found it to be acceeptable and probably deduced the fosc/4 timer input=20
> clock (which I missed) .  But this is strange...
>
> Anyway thanks for your kind input Olin and Jinx.
>
> Regards
> /Ake
>

Problem solved! Prescaler set to eight is fine. I just messed up in=20
another place of the code. Thank you again for your input Olin & Jinx.

Regards
/Ake

--=20
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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
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Personal homepage: http://www.eurosource.se/akhe
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