[ntp:questions] ESR looking for good GPS clocks
bruce.lilly at gmail.com
Thu Mar 8 22:26:27 UTC 2012
On Mon, 05 Mar 2012 20:18:31 +0000, unruh wrote:
> The problem is that parallel ports are far rarer than serial ports these
> days, and even rarer than usb ports. And you would then have to get the
> output of that counter into the computer. A bit more than $100 for the
> whole thing I suspect.
Here's an outline of how this can be easily done for far less than $100
of hardware: any parallel output for which a suitable driver exists can
be used. Most general-purpose computers have more than one of these (e.g.
LED drivers, IR drivers, as well as serial and/or parallel ports). At
some time, the computer takes a timestamp, sets an output port line to a
specific state and takes another timestamp. The two timestamps bracket
the setting of the output (the output state can be reset at some later
non-critical time). A simple microcontroller such as a Microchip dsPIC
series device does the rest, viz. measure the time interval between the
aforementioned computer output pin and the GPS PPS timing reference edge,
read and echo NMEA messages from the GPS serial data, and convert the
measured time interval into an NMEA sentence which is inserted into the
serial output along with echoed GPS NMEA sentences. The computer reads
the NMEA data (USB is fine as timing of the serial data is not critical);
the measured time interval is encoded in the NMEA data and the computer
has before and after timestamps bracketing the output port state
transition, from which the local clock timing offset to GPS PPS timing
reference is easily computed. Echoed GPS NMEA sentences provide the date
and time. Microcontroller and support components, I/O signal
conditioning, serial line transceivers, etc. total cost is likely to be
well under $100 even in small quantities. Microcontroller requirements
are modest; a reasonable timer, serial I/O, and enough memory for a
fairly simple program.
Of course, there are many boards suitable for use as a router (e.g.
Soekris products) that not only have parallel I/O but also can use the on-
board CPU for timing, eliminating the need for a microcontroller. And
interfacing GPS timing receivers to a Soekris board has already been done
(Google can point you to some examples).
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