[time-nuts] Phase, One edge or two? (was Digital mixing with a D Flip Flop)

pablo alvarez pabloalvarezsanchez at gmail.com
Wed Nov 26 08:32:15 EST 2014

>  ...The problem is that this decoded data clock is locked to the incoming
> data by means of a PFD in the Spartan6/Virtex6 GTP. The PFD normaly only
> looks at rising edges, so any change in the clock duty cycle will translate
> in a phase change in the falling edge and not in the rising edge. I am not
> sure this is really the case, but we certainly had this discussion at the
> time, but I don't remember if there was any real measurement made.

Well, I don't like correcting myself but this is not right. The PFD is only
used in the TX path. In the RX path the clock is decoded using CDR using a
VCO which operates at the 1.25Gb/s and then is divided down to 125, so the
duty cycle problem is not really a big issue here. In any case it is the
typical bug one tries to avoid when doing precise timing. Notice that in
this case it would not have been a good idea to sample with the system
clock falling edge to increase the performance.



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