[time-nuts] No State Of The Art Counter

Magnus Danielson magnus at rubidium.dyndns.org
Thu Jan 6 23:24:31 UTC 2011

On 01/06/2011 11:54 PM, Gerhard Hoffmann wrote:
> Am 06.01.2011 22:45, schrieb Gerhard Hoffmann:
>> I was thinking about abusing the built-in SERDES-Units in the Spartan6
>> as a prescaler. It would
>> be necessary to sort out the transitions after the 1:16 gearbox, but
>> given the amount of logic in a Spartan6
>> that should be possible. While I have not seriously tried that, I have
>> spotted no show stopper
>> in the data sheet.
>> The SERDES units are used for PCI-express, USB3 or fiber optic links
>> otherwise.
>> It should be possible to reach 3 or 4 GHz.
> To clarify this:
> That would not be a prescaler but a full range extension.
> Timestamping would be possible at 3 or 4 GHz.
> And I meant SATA-3, not USB3.

For a SERDES to be useable as a high frequency input, the Rx SERDES 
needs to be able to use the reference clock as clock source rather than 
doing clock recovery. While the SERDESes I have seen is able to do that, 
one always want to check that detail before going further into the 
design. For reference you typically use a SAW-oscillator which in this 
case probably should have a VC input so you can lock it up. Should not 
be too hard to find a suitable one.

I've considered this approach myself, but just not got around to cook 
something up.

It's a bit of a design-challenge to do a parallel trigger detection and 
event counter, but it is doable.


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