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++;
}
}
On 2014-04-09 23:10, Chris Albertson wrote:
Bob,
Yes, that is kind of how it works. The timer is only read once per
second. After reading it we subtract whatever was the count in the
previous sample to get the number of cycles in this last second.
There is no accurate way to reset the timer at the start of the
second. So we let it run "forever". The 16-bit timer actually over
flows many times per second. The maximum value it ever gets to is
about 65,536. (actually 2^16 - 1) The counter will never reach
10,000,000. (but actually we count after a divide by 2 so 5,000,000
is the target number)
As it turns out even in a perfect world the counter value every second
is some random value between zero and 65535. Because when the
overflows happen depend in the exact microsecond I applied power to
the system., that is my arbitrary "zero" and I count up from there
overflowing back to zero about 76.6 times per second Every second we
capture whatever the timer value is and compute "delts cycles"
You are right in the I don't even need data cycles. All I want is the
error which is 5,000,000 minus the count. this is hopefully zero.
This would be easier if we have a 32 bit counter that could be reset
to zero each second. In the past I think people have built counters
like this but now I can buy a $3.80 Arduino that does the counting,
ADC and DAC and computer USB interface. So I put up with a harder to
use 16-bit nonresetable counter
On Wed, Apr 9, 2014 at 6:33 PM, Bob Stewart <[email protected]> wrote:
Have you considered reading the timer only at PPS? You don't need to
keep track of the actual count. You just need to keep track of the
difference between counts at each PPS. Resolution isn't a problem
since the difference in the lower 16 bits is a fixed number for your
purpose. IOW, 10,000,000 is 0x989680. You're only interested in the
0x9680. If there's a difference in two successive timer counts of
0x9680, then you know you counted 10,000,000, unless your oscillator
is capable of running at one of the other values that gives 0x9680 as
the lower two bytes.
Bob
________________________________
From: Chris Albertson <[email protected]>
To: Discussion of precise time and frequency measurement
<[email protected]>
Sent: Wednesday, April 9, 2014 6:35 PM
Subject: Re: [time-nuts] First success with very simple, very low
cost GPSDO, under $8
On Wed, Apr 9, 2014 at 1:04 PM, Tom Harris <[email protected]>
wrote:
Another point with the software is that your handler for the PPS
just reads
the counter. This gives an offset between the PPS edge and the value
read,
as your software takes time to respond to the interrupt and read the
counter. In your code, it doesn't matter as you only have one
interrupt.
Actually there are two interrupts. One is for PPS and the other is
for overflow of the 16-bit counter. This over flow happens about 76
times per seconds.
However, if you have another interrupt enabled, this could run after
the
PPS pulse but before the handler runs, giving you a very rare
jitter.
A better way would be to use the input capture feature to read the
timer
into a capture register. Then the interrupt handler has until the
next edge
to read the capture register.
Do you know how to do this. I don't see any way to capture the timer
value other then reading it with software. The timer capture
register
is 16 bits and is set atomically after each timer increment but I
don't see a way to make an external interrupt pin capture a timer.
The two interrupts do bump into each other about roughly every 100
seconds but I can detect that. I think I'll just ignore that second.
--
Chris Albertson
Redondo Beach, California
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--
WBW,
V.P.
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