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AT91SAM7A3 real-time timer

Hi,

I see in the datasheet of the AT91SAM7A3 that the real-time timer (RTT) is fed by the Slow Clock, which is an internal RC oscillator (f=22 kHz to 42 kHz). The RTT has a 16-bit prescaler and a 32-bit counter.

How are we supposed to use such an inaccurate clock as the source for a real-time timer? The only way I can think of is to add a crystal (which will not always be running (otherwise, there would be no reason to use the RC oscillator)) and use it to "calibrate" the RC oscillator every once in a while. The 16-bit prescaler of the RTT allows to do this. This way, the 32-bit counter could count more or less exact seconds. The datasheet (preliminary), however, does not say anything about the drift of such an RC oscillator. How often would you say it would be needed to perform such a calibration? Has anyone done this?

Is there a simpler method to use the RTT as what it is supposed to be?

Thanks,

Reply to
Mochuelo
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A test was done against a GPS receiver (which is pretty exact) over ~ 5 days. Difference was 36 ppms...

5 days is 432 000 seconds and 36 ppm is about 15 seconds or 3 seconds per day. 
--
Best Regards,
Ulf Samuelsson
ulf@a-t-m-e-l.com
This message is intended to be my own personal view and it
may or may not be shared by my employer Atmel Nordic AB
Reply to
Ulf Samuelsson

Thank you for the information.

That was under constant temperature, right? Do you know approximately what's the temperature coefficient of those RC oscillators?

There's something I don't fully understand, though. Even in the worst case (22 kHz), a difference of one unit in the prescaler load value implies 1/22000=45.5 ppm, which would mean that there is an optimal choice for the prescaler value that would give you a max error of

+-22.7 ppm. My question is "were you able to find any other prescaler load value that would give you an error better than 36 ppm, or that was the best one you found?" Did you try N-2, N-1, N+1, N+2?

Best,

Reply to
Mochuelo

In the office, so not wildy varying. It may be good to have an external temp sensor and recalibrate...

No clue.

No, (I didnt do the test myself)

--
Best Regards,
Ulf Samuelsson
ulf@a-t-m-e-l.com
This message is intended to be my own personal view and it
may or may not be shared by my employer Atmel Nordic AB
Reply to
Ulf Samuelsson

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