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 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 value=20
of 4 i get the correct 30 seconds. If set to 8 I would have found it to=20
be acceeptable and probably deduced the fosc/4 timer input clock (which=20
I missed) .  But this is strange...

Anyway thanks for your kind input Olin and Jinx.

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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