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Observer daveb1
Observer
5,795 Views
Registered: ‎04-08-2016

Do multiple DCM clock outputs need to be synchronised ?

Hi, My Spartan-6 based design uses a 140MHz system clock generated by a DCM. A separate 70MHz clock is generated from the same DCM to clock fairly complex logic that's not easy to pipeline. My question is: To sample signals generated by the 70MHz clock using the 140MHz clock, do I need to treat the two clocks as though they are 2 asynchronous clock domains i.e. use FF synchroniser circuits or can I rely on the phase relationship maintained by the DCM ? Thanks Dave
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4 Replies
Scholar austin
Scholar
5,784 Views
Registered: ‎02-27-2008

Re: Do multiple DCM clock outputs need to be synchronised ?

d,

 

If you use the CLK0 output, and the CLKDV ouput with divide=2, the pjase relationship ia guaranteed to +/- 100 ps.  The rising edge of the 70 MHz, will happen at every other 140 MHz clock rising edge.

 

So, yes, use of the DCM's multiple outputs is a good thing to insure the timing is known, and does not vary.

 

A caution though:  which rising edge of the 140 MHz designates the 70 MHz rising edge is not selectable:  after a reset of two like configured DCM's their 70 MHz may be in phase, or exactly not in phase, as the divide by two may get initialized differently between the two DCM's.  That is why all such outputs need to be from one DCM.

 

 

Austin Lesea
Principal Engineer
Xilinx San Jose
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Observer daveb1
Observer
5,773 Views
Registered: ‎04-08-2016

Re: Do multiple DCM clock outputs need to be synchronised ?

Austin, Thanks for the info. Just to clarify your word of caution. When you say 2 DCMs, do you mean 2 separate clock generating blocks, one generating 70MHz & the other generating 140MHz, that conceptually reside in a single, multiple output, block or two DCM blocks each with their own 70MHz / 140MHz outputs ? Dave
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Scholar austin
Scholar
5,771 Views
Registered: ‎02-27-2008

Re: Do multiple DCM clock outputs need to be synchronised ?

Correct,

 

Two separate DCM, one with 140, the other with 70 MHz are not expected to start up the same every time.  If driven by the same CLKIN, outputs from both will be matched to each other, but the divisor may be 0 or 1 initially, so the phase of the 70 MHz may change from reset to reset with respect to the 140:  instead keep all such clocks devired from the same DCM.

 

 

Austin Lesea
Principal Engineer
Xilinx San Jose
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Observer daveb1
Observer
5,764 Views
Registered: ‎04-08-2016

Re: Do multiple DCM clock outputs need to be synchronised ?

Ah I see what you mean, don't use one DCM to generate one clock and another DCM to generate another clock (event though they have the same input clock); use a single DCM with multiple outputs. Thanks Dave
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