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Default DCO frequency on MSP430F2002

Carlos Gonzalez Avelino
Prodigy 190 points

Hello,

Reading through the datasheet of the family 2xxx to make a application I encountered two frequencies that are supposed to be by default on the DCO, I'm trying to make an application with a counter as a clock having seconds, minutes, hours, days and months so I'm using various counters as variables and the timer A as the core of it all triggering an interruption every second to update the counters. So far I haven't been able to make it work because sometimes the timer goes faster and sometimes it goes slower, supposedly the default frequency is either 12KHz or ~1MHz, have tried with both and none work properly and when it does i try to make the counters more and more realistic it just doesn't work, so far my months are of about 15 minutes but it's not working with longer periods. 

I'm using ACLK in up mode btw.

Can anyone help me with this?

Cheers!

  • 0
    Guru 58965 points

    The data-sheet tells you that the frequency of the on-chip DCO and VLO vary from chip to chip and also depend on operating voltage and temperature. Even if you use the "calibrated" settings of the DCO, you still get 2% to 3% error.

    Thus you cannot depend on DCO or VLO to do accurate timing. A 2% to 3% error means about half an hour of error per day.

    You need to use an external "watch" crystal with an accuracy of a few parts per million.

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