[time-nuts] DMTD to MMTD

Bruce Griffiths bruce.griffiths at xtra.co.nz
Wed Feb 17 23:46:00 UTC 2010

Lux, Jim (337C) wrote:
>> -----Original Message-----
>> From: time-nuts-bounces at febo.com [mailto:time-nuts-bounces at febo.com] On Behalf Of Bruce Griffiths
>> Sent: Wednesday, February 17, 2010 2:10 PM
>> To: Discussion of precise time and frequency measurement
>> Subject: Re: [time-nuts] DMTD to MMTD
>> The latest version actually records time stamps from a continuously
>> running counter clocked at some at a constant frequency (100Mhz??)for
>> all channels simultaneously.
>> They may use a flag bit for each for each channel to indicate to which
>> channel or channels the zero crossing time stamp belongs.
> Simpler than that.. it grabs 20 bit numbers and shoves them out in ASCII over a com port with an indication of which channel it was for.
> The FPGA has a 20 bit free running counter at 100 MHz. When an input changes state, it latches the counter, and shoves it out along with the channel number.  They use an offset frequency>100 Hz so that you don't have to disambiguate the counter rollovers. (20 bits rolls over every 10+milliseconds counting at 100 MHz)
> I don't know if there's a FIFO in front of the UART (e.g. what if you get simultaneous zero crossings).. but I would expect there is.
> The "hard work" is in the zero crossing detector ahead of the FPGA. (and perhaps in the latching of the ZCD inputs into the FPGA).
> Given how long ago it was made, that FPGA isn't a huge one.
Using 8 flag bits (one per channel) together with the associated time 
stamp is a little more efficient and very easy to do and it doesn't 
require a FIFO to ensure that simultaneous zero crossings aren't missed.


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