Bob, I have versions of the picPET that capture an ADC reading along with a timestamp. I used this with one of Richard's simpler interpolator designs. There's some info at http://leapsecond.com/pic/picpet2.htm and the projects aren't final yet but you can contact me or Richard directly.
/tvb ----- Original Message ----- From: "Bob Stewart" <b...@evoria.net> To: "Discussion of precise time and frequency measurement" <time-nuts@febo.com> Sent: Saturday, February 15, 2014 5:19 PM Subject: Re: [time-nuts] TIC model > Hi Lars, > > I'm lucky in that I'm starting with a PIC running internally at 40MHz. So I > think timing is not going to be a real problem. But after getting a response > from Richard, I'm concerned about flaws in the spice chip models. I'm also a > bit concerned about breadboarding, but come to think of it, in that link you > gave me the guy used dead-bug assembly. So maybe that's not a big concern. > > I suppose the next thing I need to do is experiment with using the ADC to > grab the voltage from the incoming 1PPS pulse. Mine stays high for 50ms, so > it should be an easy place to start. In fact, there are lot of things I can > do with the 1PPS and an RC to get some experience working with the ADC. > > Bob > > > > > >>________________________________ >> From: Lars Walenius <lars.walen...@hotmail.com> >>To: "time-nuts@febo.com" <time-nuts@febo.com> >>Sent: Saturday, February 15, 2014 5:13 PM >>Subject: Re: [time-nuts] TIC model >> >>What Bruce says is really important. >> >> >>For the ATmega328 the datasheet says 14pF sampling capacitance and nothing >>about temperature coefficient. >> >>It also specifies a series resistance 1..100k. So not very precise. If it is >>100k the time constant is 1400ns! >> >>I have tested several boards and they seem to behave similar with my 1nF NPO >>capacitor. With a 47pF I guess it is more uncertain. >> >>I also recommend you to test your model in the real world. I have used two >>good OCXOs and/or rubidiums with a small offset. Say 1E-9 offset that gives >>1nS per sec. One of the channels have had a divider for example HC390s or the >>excellent PICDIVs from Tom Van Baak to output 1PPS. >> >>I have also applied a heat gun near the circuit to test that the circuit >>doesn´t drift with temperature. A reasonable goal is to have less than 1LSB >>drift with a couple of degrees change. This test I have done with the same >>source for both 10MHz and 1PPS and a high reading from the ADC. >> >>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. >> >> >>Lars >> _______________________________________________ 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.
