[time-nuts] First success with very simple, very low cost GPSDO, under $8

Tom Van Baak tvb at LeapSecond.com
Thu Apr 10 19:39:17 UTC 2014

Hi Vlad,

Correct, your code example is the traditional way to keep track of elapsed time without long-term rounding error, although it's usually not attributed to Bézier.

Not sure if you've been following the whole thread, though -- the problem with timer interrupt code is that it can, and will on rare occasion, conflict with 1PPS rising edge pin interrupts. On a microcontroller, for best precision, the solution is to get rid of the timer interrupts, or get rid of the 1PPS interrupts, or both. Secondly, you cannot share multi-byte variables between interrupt level and main level without synchronization or arbitration.

Your code snippet is a good example of what is subtle and dangerous, and dare I say, wrong. You are updating long t_besier in an interrupt handler. Any main level code using t_besier faces byte carry corruption in the multi-byte value of t_besier. True, this works fine on a 32- or 64-bit cpu, but not on an 8- or 16-bit cpu.


----- Original Message ----- 
From: "d0ct0r" <time at patoka.org>
To: <time-nuts at febo.com>
Sent: Thursday, April 10, 2014 10:12 AM
Subject: Re: [time-nuts] First success with very simple, very low cost GPSDO, under $8

> I am not sure about Arduino, but probably it too has so-called "timer 
> overflow interrupt". If so, then its possible to use that other 
> interrupt and some "long" (lets say 32-bit) RAM variable to accumulate 
> real value of counter.
> In one of my project I was using timer overflow and Besier method to 
> make good 1 minutes intervals.
> volatile signed long t_besier = 12000000L;
> interrupt void TPM2OV_ISR(void)
> {
>         TPM2SC = TPM2SC;
>         if (TPM2SC_TOF)
>                 TPM2SC_TOF=0;
>         t_besier -= 65536L;
>         if(t_besier < 0) {
>                         t_besier += 12000000L;
>                         t_time++;
>         }
> }

More information about the Time-nuts_lists.febo.com mailing list