[time-nuts] DMTD to MMTD

Bruce Griffiths bruce.griffiths at xtra.co.nz
Fri Feb 19 02:54:52 UTC 2010

Magnus Danielson wrote:
> Bruce Griffiths wrote:
>> Magnus Danielson wrote:
>>> Bruce Griffiths wrote:
>>>> 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.
>>> It doesn't help at all for this application. The 8 channels is more 
>>> likely to be spread out and as Jim pointed out, the next bin is 
>>> sufficient for needing a unique time-stamp. The flag-solution is 
>>> less efficient (8 bits rather than 3) and the FIFO need is always 
>>> there, but may be implemented in various ways. For the flag system 
>>> to be efficient, a high probability for the same time-bins to be 
>>> used needs to exist, and a high resolution system can expect to 
>>> actually see noise spread out the channels over the time-bins.
>> Depending on the system constraints it may be the difference between 
>> being able to do implement it or not.
> With your clarifying comment yes... because it is until now that you 
> have clarified the merit of the flag system, lowered implementation 
> complexity vs. lowered signalling capacity.
>>> A DMTD systems have a low rate of events per channel, but the 
>>> nominal distance of events for each channel is fairly long, 
>>> worst-case burst is when all channels time-stamps. For a 100 Hz 
>>> beating and 100 MHz clock, the nominal rate of rise/fall events is 
>>> 200 Hz or 5 ms. Letting the locked value stay put for at least 4 ms. 
>>> If all channels could be emptied within these 4 ms (just another way 
>>> of saying that it has enough transport capacity) then a fairly 
>>> simple schedule system can loop through the channels to find a new 
>>> sample to transmit.
>>> I think the re-occurring flag system should be put to rest, it 
>>> doesn't contribute and is a red herring, at least for this application.
>> Nonsense, it requires simpler logic and for a device with limited 
>> internal connection/routing capability and a large number of channels 
>> the data path interconnections may be simpler and easier to route. It 
>> may also run with a higher clock frequency.
>> It should even be possible to impement in a relatively small CPLD 
>> albeit with an external FIFO or equivalent (eg a PPI port on a 
>> Blackfin DSP).
>> Each additional channels requires one input pin, one output pin, a 2 
>> bit synchroniser and a 1 bit wider data path and little else.
> But it produces more data, which was what I was commenting on. Indeed 
> it is very simple to implement, but it's a complexity which is still 
> on the low-end.
> So, finally you made the point of the merit of the approach in such a 
> way that it became clear to me why you have maintained that standpoint.
> Cheers,
> Magnus
One can always reduce the data sent to the PC by removing redundant 
information, or increase it by Ascii coding of channels and slope 
In general in the absence of significant constraints either method will 
work, they just have different tradeoffs.

Its tempting to see if one can just synchronously clock the synchronised 
ZCD output signals directly into a processor and derive the timestamps 
entirely in software.
The only question being what is the maximum clock frequency at which 
this can be done without missing any ZCD transitions.
A Blackfin DSP for example should be able to do this with a maximum ZCD 
sampling clock frequency of at least 50MHz.


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