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.
My bad ! I messed up Bezier and Bresenham line algorithm, which I was
using for generating one average timed period from any other timed
period (like the PIC timer0 overflow period, for example).
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.
I am not familiar with Arduino. STM32, for sure, has interrupt priority.
But than it will not be 8$ project. ;-) The best results I achieve
using DMA. Timer updating the buffer variable(s) in background and its
not using interrupts for it. So, I could read the current value of
counter any time. At least I got good results for this approach when I
connect MCU clock to GPSDO and tried to measure the signal from external
OCXO. I got perfect 10 Mhz at that time.
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.
Hmm...
----- Original Message -----
From: "d0ct0r" <t...@patoka.org>
To: <time-nuts@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++;
}
}
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WBW,
V.P.
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