Graham, Thank you for your input. I have reached out to inquire about the TICC kit and am awaiting a response. I don't mind spending some money if I need to in order to accomplish this task. If I were to utilize the PRU's, how would you envision this working. (I have zero experience here).
Once we detect the leading edge of the contact closure switch, how would we get a time stamp? I assume there is a fixed 200,000,000 cycles per second on the PRU. Is this constant or have drift? I am wondering about detecting the leading edge, then counting the number of cycles until a 1 PPS signal is received. Or is this way over complicated? Looking forward to hearing your thoughts... On Wed, Jan 30, 2019 at 3:32 PM Graham <[email protected]> wrote: > > Thomas: > > You are at the limit of a combination of the OS and Python response time. > > I think you will also find that the absolute accuracy of the internal time > base in the BBB, set by timesyncd (or optionally ntpd) will meander +/- ten > milliseconds across several hours. > (Put the 1PPS output from a GPS into the port and plot what happens to the > time stamps over a 24 hour period.) > > Depending on your requirements, I suggest you consider something like a > "TICC" for the edge timing capture, it will provide a serial stream with the > time stamps, easily read by the BBB via the USB port, that has relative > accuracy into the 100 ps range, and absolute time a function of the 10 MHz > reference signal you give it. > > https://tapr.org/kits_ticc.html > > Your other alternative is to write some code for the PRU's that can do edge > capture in real-time, then hand off the results to the BBB main cpu. > > So, depends on your need to trade off money versus your time. > > --- Graham > > == > > > > > > On Wednesday, January 30, 2019 at 3:05:23 PM UTC-6, Thomas Remmert wrote: >> >> Hi Robert, >> Thanks for the quick reply. I am already using a custom overlay and >> using the PPS/GPS tools... >> >> trying PPS source "/dev/pps0" >> found PPS source "/dev/pps0" >> ok, found 1 source(s), now start fetching data... >> source 0 - assert 1548882288.001021883, sequence: 3727 - clear >> 0.000000000, sequence: 0 >> source 0 - assert 1548882289.001020522, sequence: 3728 - clear >> 0.000000000, sequence: 0 >> ^C >> >> On Wed, Jan 30, 2019 at 3:01 PM Robert Nelson <[email protected]> wrote: >> > >> > On Wed, Jan 30, 2019 at 2:54 PM Thomas Remmert >> > <[email protected]> wrote: >> > > >> > > Hello, >> > > I have a contact closure switch connected to P8_14. When the switch is >> > > activated, P8_14 is brought to ground. I need to apply a time stamp >> > > when this occurs with millisecond accuracy. >> > > >> > > 1) I have an adafruit GPS with 1 pps output. I see the NMEA strings and >> > > have confirmed the 1 PPS is working. >> > > >> > > 2) I have NTP running with my local GPS and 1PPS source configured. >> > > (Maybe chrony is a better choice?) >> > > >> > > 3) Just for testing, I am using the following python code: >> > > >> > > import Adafruit_BBIO.GPIO as GPIO >> > > import time >> > > import datetime >> > > >> > > GPIO.setup("P8_14", GPIO.IN, pull_up_down=GPIO.PUD_UP) >> > > >> > > #GPIO.add_event_detect("P8_14", GPIO.RISING, callback=my_callback, >> > > bouncetime=1000) >> > > >> > > while True: >> > > GPIO.wait_for_edge("P8_14", GPIO.RISING) >> > > ttime = datetime.datetime.now() >> > > print(ttime) >> > > >> > > >> > > When I compare my time with a very expensive DAQ, my time stamp is >> > > always off by 2-4ms and it is not a consistent offset. Does anyone have >> > > any other thoughts at to how I might approach this? I am needing to get >> > > the timestamp accurate to 1ms. >> > >> > Use the pps driver, setup an overlay like shown: >> > >> > https://github.com/beagleboard/bb.org-overlays/blob/master/src/arm/PB-UART4-TESEO-LIV3F.dts#L63-L71 >> > >> > Then you can use the pps/gps tools... >> > >> > Regards, >> > >> > -- >> > Robert Nelson >> > https://rcn-ee.com/ >> >> >> >> -- >> Thomas Remmert >> Cell: 936-647-6030 >> Office: 281-203-0855 > > -- > For more options, visit http://beagleboard.org/discuss > --- > You received this message because you are subscribed to the Google Groups > "BeagleBoard" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to [email protected]. > To view this discussion on the web visit > https://groups.google.com/d/msgid/beagleboard/0d397706-1b6d-4665-9928-4e69a73dc7c3%40googlegroups.com. > For more options, visit https://groups.google.com/d/optout. -- Thomas Remmert Cell: 936-647-6030 Office: 281-203-0855 -- For more options, visit http://beagleboard.org/discuss --- You received this message because you are subscribed to the Google Groups "BeagleBoard" group. To unsubscribe from this group and stop receiving emails from it, send an email to [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/beagleboard/CAGiPVCuN3%3DbXoEeNnnnx-CSwbhkVOXTW8S%2BNy%2Bb%3Dc7aB54ia2A%40mail.gmail.com. For more options, visit https://groups.google.com/d/optout.
