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Timer ticks occurring much more faster

Started by ratemonotonic July 9, 2008
Hi All ,

I am implementing RTC code on an AVR Atmega2560  I have a 32Khz clock
crystal to TSOSC1 which is used by timer2 in async mode. I have
configured the clock divide to 8 so the clock to the timer is 32768/8
= 4096. It is a 8bit so a interrupt should be generated every 16hz
(4096/256).

So on 16 ticks make one second. If I add up second to make 5 minutes
and light an LED on this event , it is happening within 2minutes. Why
is this. I have checked the crystal is oscillation at 32Khz.

Has any on encountered this?


BR
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ratemonotonic escribi�:
> Hi All , > > I am implementing RTC code on an AVR Atmega2560 I have a 32Khz clock > crystal to TSOSC1 which is used by timer2 in async mode. I have > configured the clock divide to 8 so the clock to the timer is 32768/8 > = 4096. It is a 8bit so a interrupt should be generated every 16hz > (4096/256). > > So on 16 ticks make one second. If I add up second to make 5 minutes > and light an LED on this event , it is happening within 2minutes. Why > is this. I have checked the crystal is oscillation at 32Khz.
�Exactly 2 minutes, or perhaps 1 minute and 40 seconds?
On Jul 10, 1:43 pm, "Ignacio G.T." <igtorque.rem...@evomer.yahoo.es>
wrote:
> ratemonotonic escribi=F3: > > > Hi All , > > > I am implementing RTC code on an AVR Atmega2560 I have a 32Khz clock > > crystal to TSOSC1 which is used by timer2 in async mode. I have > > configured the clock divide to 8 so the clock to the timer is 32768/8 > > =3D 4096. It is a 8bit so a interrupt should be generated every 16hz > > (4096/256). > > > So on 16 ticks make one second. If I add up second to make 5 minutes > > and light an LED on this event , it is happening within 2minutes. Why > > is this. I have checked the crystal is oscillation at 32Khz. > > =BFExactly 2 minutes, or perhaps 1 minute and 40 seconds?
Investigating further I have varified the it is seeing a clock of exactly 200Khz. Just as an experiment I freeze sprayed the chip and it stopped seeing the 200khz clock. It looks like it is some kind of switching noise it was picking up.