[time-nuts] Clock Correction Algorithm

Miguel Gonçalves mail at miguelgoncalves.com
Thu Nov 24 11:35:35 EST 2011


Hi Javier!

On 24/11/2011, at 15:15, Javier Herrero <jherrero at hvsistemas.es> wrote:

> El 24/11/2011 15:21, Miguel Gonçalves escribió:
>> 
>> 
>> I am using an Arduino Uno that presumably is running at 16 MHz.
>> 
>> I am using a 16 bit timer with a 256 pre-scaler:
>> 
>> 16000000 / 256 = 62500 Hz
>> 
>> 1 / 62500 = 16 us
>> 
>> 65536 - 62500 = 3036
>> 
>> I am setting the counter to 3036 and let it overflow after 65535.
>> 
>> If I check the error every second I see the clock getting behind correct time and after 60 seconds it's 25 ms apart from UTC.
>> 
>> Perhaps I should increase the initial value from 3036 to 3036+25E6/(60/16). I will make the second shorter this way.
>> 
> Hi!
> 
> Can you check the real frequency of the oscillator? If it is too far from 16MHz, there are two options: trim the capacitors that are in the crystal oscillator circuit (I'm not familiarized with the Arduino... but I suppose they will be there), or... software trim changing the counter reload value as you suggest :)

The software way is probably the best idea as it would account for small differences in different oscillators. I have 3 Arduino boards and will make 3 time displays. 

> Does the counter needs reloading every overflow or it is automatically reloaded? depending on how you are done this, it can also provide some delays that apparently slows the clock.

It's manually reloaded with the 3036 value every time the routine is called.

If I load it with a bigger value I will make a faster second. If with a small value the second will be longer. 

Thanks for your help!

Cheers,
Miguel

> 
> Regards,
> 
> Javier
> 
> 
> 
> 
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