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-   -   DCM Question (http://www.fpgacentral.com/group/showthread.php?t=58352)

motty 04-17-2006 01:27 AM

DCM Question
I have an embedded project (MicroBlaze) that is currently using
external oscillator to drive a DCM. The clkfx output of that DCM is
used to drive all of the logic in the system. The parts that our
company makes (Transceivers) have reference clock outputs that we want
to use to clock some logic in the system. We have two "flavors" of
parts that out current hardware needs to support. The problem is that
one part outputs a 38.4 MHz clock, and the other outputs a 26 MHz
clock. The 38.4 MHz clock can be used at 1x speed. We "talk" to out
parts via a 3 wire serial protocol that is rated at 19.2 MHz top speed.
This works nicely with the 38.4 clock since the flops driving the
external protocol output 19.2 MHz, or slower.

The part that ouputs the 26MHz clock has the same 3 wire serial
interface (the protocol is different) but it is rated at 26 MHz top
speed. So the logic that drives the interface needs to be 52MHz to
reach this output speed.

So basically what we need to do is pick either a 1x version of the
input clock (38.4MHz) or a 2x version (to bump the 26MHz to 52MHz). I
implemented a mux connected to the OPB. A software write will select
which output of the DCM to use.

My concern is with the input frequency parameter of the DCM. Does this
parameter make or break the core? The input frequency to the DCM will
either be 38.4MHz or 26MHz. But I want to use the same DCM for both
freqs. Is this OK? I can easily try it, but wanted to ask here to see
what folks had to say.

I can also use two different DCM's and switch between the two if need


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