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 <robert...@gmail.com > <javascript:>> wrote: > > > > On Wed, Jan 30, 2019 at 2:54 PM Thomas Remmert > > <trem...@snaggleboards.com <javascript:>> 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 beagleboard+unsubscr...@googlegroups.com. 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.
