[time-nuts] Frequency subtraction with D-flip flops

Javier Serrano javier.serrano.pareja at gmail.com
Tue Jun 25 03:15:46 EDT 2013


On Tue, Jun 25, 2013 at 4:36 AM, ed breya <eb at telight.com> wrote:

>
> 4. It seems to me that whenever fd is much higher than fc (fd>>fc), that
> fd could be used instead to trigger the second DFF, which would reduce the
> metastability of the first DFF somewhat, and also synchronize the output
> signal closer to the edges of fd - but with some metastability from that
> too.
>
>
Clocking the two FFs of a synchronizer with different clock signals will
not work against metastability. When the edges of fd and fc are very close
in time, there is a slight chance that the output of the first FF will be
in a metastable state for some time. By clocking the second FF with fc you
allow for a full (guaranteed) period of fc for that output to stabilize to
a solid '0' or '1'. If you clock the two FFs with two different clock
signals you don't have that guarantee. There should be a big gain to be had
somewhere else to do something like that, but I can't see it. I must say
all my experience is with fc very close to fd in frequency, so maybe I am
missing something about the fd>>fc case.

Cheers,

Javier


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