Date: Sat, 11 Jul 1998 10:52:38 +1000 (EST) From: Peter Jeremy <peter.jeremy@alcatel.com.au> To: hackers@FreeBSD.ORG Subject: PPS device Message-ID: <199807110052.KAA07523@gsms01.alcatel.com.au>
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Is anyone using a PPS device with FreeBSD? I notice that -current includes support for a PPS source attached via a parallel port. For some time, I have been using a PPS source via DCD on a serial port (if anyone wants the patches to sio.c, let me know). One problem with this approach is jitter - firstly, there's the general interrupt latency (I'm not sure how to measure this, but I'd assume it's in the 10's of usec on a 486) and secondly interrupt masking via spl(). One approach I have been considering is to have a hardware counter counting the number of (eg) usec from the 1pps transition time and read it as an 8-bit number via the parallel port. This would reduce the unfiltered jitter to <1usec. Based on a quick read of draft-mogul-pps-api-02.txt, this either hasn't been considered, or has been discarded. Does anyone have any comments on this idea. BTW: Anyone looking for a good stand-alone reference might like to look at the master reference oscillators in digital cellular base stations - the GSM standard (which also covers DCS-1800 and DCS-1900) requires a 0.05ppm or better reference in the base station. I suspect DAMPS and CMDA also have similarly tight tolerances. Whilst these references are expensive new, if you know someone in the industry, you might be able to acquire one from a junked board. Peter -- Peter Jeremy (VK2PJ) peter.jeremy@alcatel.com.au Alcatel Australia Limited 41 Mandible St Phone: +61 2 9690 5019 ALEXANDRIA NSW 2015 Fax: +61 2 9690 5247 To Unsubscribe: send mail to majordomo@FreeBSD.org with "unsubscribe freebsd-hackers" in the body of the message
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