Hi!! On 24/11/2011, at 13:48, Attila Kinali <att...@kinali.ch> wrote:
> On Thu, 24 Nov 2011 12:56:31 +0000 > Miguel Gonçalves <m...@miguelgoncalves.com> wrote: > >> Right! That is what I am doing. >> >> The problem is the drift between adjustments. Arduino's clock is slow 25 ms >> every minute and if temperature changes it will surely be different. >> >> The proposed algorithm wants to correct that. > > Oh.. sorry.. i misunderstood your question. > > Yes the algorithm would work, though i'd rather use a pll style sytem. > Ie, measure the differnce in time between your clock and your refrence. > Calculate from this a correction value for the clock frequency (should > be a simple multiplication by a constant factor). > Low pass filter that correction value. I will investigate PLL. I don't want something overly complicated because this is a time display. I want less that 10 ms error and this goal is much better than needed for a clock. > This should also do away with all those special cases in your algorithm, > as a PLL style algo is self stabilizing (unless you got the loop gain > too high*). You will get at most a constant difference between your clock > and the reference. This difference can be minimized by using a higher > gain in the feedback loop (at the cost of stability). This is exactly what I wanted. I really need to read about PLL. > Also as Javier said, 400ppm is a quite high offset for a crystal. > Or are you using a RC oscillator based RTC? (a lot of on chip > RTCs are build that way) I am using the oscillator that comes with the Arduino. > On the other hand, i once used a Atmel SAM7 RTC (or RTT in Atmel lingo) > that is based on a RC Oscillator with +/- 30% accuracy. Using a simple > calibration step (powering up the crystal oscillator every hour and > using the crystal to measure the RTT frequency, setting this as new > correction value, then going back to sleep for an hour) got me below > 100ppm over a day in my tests (office enviroment, temperature variation > probably below 2°C over a day, definitly less than 5°C, 30ppm crystal) I have a RTC that I am not using for this project that has 3.5ppm between -40C and 85C but I don't intend to use it because I want to keep this simple. Thanks a lot Atilla for your help. Cheers, Miguel _______________________________________________ time-nuts mailing list -- time-nuts@febo.com To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts and follow the instructions there.
