Re: [oc] interfacing 16Mhz and I Mhz clock registers

["Jim Dempsey" <tapedisk@ameritech.net>]

----- Original Message -----
From: "John Sheahan" <jrsheahan@optushome.com.au>
To: <cores@opencores.org>
Sent: Thursday, June 19, 2003 5:45 PM
Subject: Re: [oc] interfacing 16Mhz and I Mhz clock registers


> On Fri, 2003-06-20 at 02:34, Umair Farooq Siddiqi wrote:
> > I can adjust skew between two clocks, at present there is no skew
between them, 1 Mhz Clock can be considered as 16Mhz clock divided by 16.
> >
> > Thanks
>
> (its early and my coffee is not working yet. please think thru these
> points)
>
> Sounds like the clocks are synchronous then - which simplifies things a
> lot. Otherwise memories and junk may be required.
>
> If the 16M and the 1M are exactly in phase, there is no issue - you can
> connect them directly q -> D

Not so quickly. If the leading edge of the 1M and the leading edge of one of
16 of the 16M's then your statement is true for only that one out of the 16
clock ticks. Note, depending on how things are wired up you may be
restricted to using that one (in phase) tick for intercommunication. Also,
nothing has been said about the dwell time that the clock pulse is held high
(and/or low). You may need to handshake on the slower clock when moving data
between domains.

> (I assume the min clock ->Q delay plus any interconnect delay is more
>   than the hold time.
>   If this is wrong, shift registers dont work either)
>
> If the 1M clock is slightly advanced on the 16M, even more margin.
> With reasonable clock control, this is probably where you want to be.
>
> If the 1M clock is delayed WRT the 16M - timing can get nasty.
> And this is the common case when  1M is just divided from 16M.
> Then a delay in all 16 datapaths longet than the clock skew is probably
> the required solution, which is implemntation specific and slightly
> backend nasty.
>
> all of these delays I hope are a tiny fraction of a 16mhz clock period.
> john
>
>
>
> >
> >
> > John Sheahan <jrsheahan@optushome.com.au> wrote:
> > On Thu, 2003-06-19 at 19:42, Umair Farooq Siddiqi wrote:
> > >
> > > Hi
> > >
> > > I am interfacing two 16-bit wide registers, one running at 16 Mhz to
another register running at 1 Mhz. On 16 Mhz side though the clock is at 16
Mhz but data only changes after 16 clocks, hence I am thinking of directly
connecting both clock frequency registers. Would you kindly suggest me
whether my approach is correct or not? Is there any special care required to
meet setup and hold timings ?
> > >
> > whats the skew between the 16MHz nd the 1MHz clocks?
> >
> >
> >
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