In message <[EMAIL PROTECTED]>, John Pettitt writes: >I read the work phk did (http://phk.freebsd.dk/soekris/pps/) and came up >with the following product idea. > >The spec: > >PCI 3.3v board with: > 10Mhz OCXO (provision for external clock source?) > Uart (serial is an endangered on many PC's) > Free running counter driven from the OCXO and readable by PC > inputs to latch the counter (how many?) with the latched result also >readable (for PPS)
hehe, I did more or less exactly this five years ago. I used a "Hot-1" card with a Xilinx 6216 from Vcc.com (now defunct). They had an onboard PLL chip (can't remember type) so I banged the input clock frequency up to 100 Mhz and ran a 26 bit counter (that was as fast as it would go) and 10 latches. You should probably aim at something similar to your clock because 100nsec resolution is pretty lumpy for PLL work. Five of the latches had programmable dividers in front of them, so that I could pass in frequencies, divide them down to 1Hz and timestamp them at that rate. One of the problems I ran into was of course noise. I found that optocouplers generally have acceptable jitter for low quality signals (like the mains frequency etc). Another thing you have to be aware of is something called 'meta-stability' when you latch a signal in one synchronous clock domain on a signal which is not in that domain. The usual trick is to synchronize the external signal to the clockdomain through a sequence (2-3) of latches. (the delay is constant and known, so it can be handled in software). Consider adding a D/A converter for tweaking OCXO. Using the two-tier trick which SRS uses in the PRS10 may be a good and cheap way. >So does the basic idea make sense? Worked fine for me :-) -- Poul-Henning Kamp | UNIX since Zilog Zeus 3.20 [EMAIL PROTECTED] | TCP/IP since RFC 956 FreeBSD committer | BSD since 4.3-tahoe Never attribute to malice what can adequately be explained by incompetence. _______________________________________________ time-nuts mailing list time-nuts@febo.com https://www.febo.com/cgi-bin/mailman/listinfo/time-nuts
