[time-nuts] TIC model
Tom Van Baak
tvb at LeapSecond.com
Sat Feb 15 20:10:16 EST 2014
> What Bruce says about interrupts is also worth to check in real life as “jitter” due to unexpected interrupts or different timing may give problem. In the Arduino GPSDO the timer1 overflow interrupt may delay the 1PPS interrupt about 3us and delay the ADC conversion 3us. This is not so critical as it sounds as the ADC input is not changing at this time. For me this jitter gives more problem with the timer1 Reading. This jitter is not so easy to to test as it in the Arduino GPSDO program only happens every 1024secs and if you are (un)lucky it may not be seen at all depending on startpoint of timer1 relative to the 1PPS.
For those working on microcontroller-based interpolators, you might want to contact Richard McCorkle; I know he has developed a number of other versions since he released the PICTIC II to time-nuts some years ago. One of the reasons he and I and others enjoy working with PIC's is that they have extremely short interrupt latency and no interrupt jitter. Now, I would not recommend anyone switching from Arduino to a PIC, but a working PIC solution may provide an example of what can be done, if nothing else.
For Arduino and other less fortunate uC you can always use external chips to obtain optimal and jitter-free charge/discharge timing. I'm not that familiar with Atmel chips; could capture/compare be used instead of interrupts somehow?
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