Hello,
Although i have gone through the Discussion on this forum regarding the DLL usage, i would like to clarify a point.
I.e, I am having a single clock. I don't have phase shifting, clock division, clock multiplication requirements. And ofcoures i don't bother about the clock jitter.
Now do i require DCM? If so why ?
Thanks in advance
Regards, Muthu
That depends of the clock frequency and timing requirement to meet. I'd say no, else, you'd know you want it ;)
The DLL is normally used to reduce the clock-out delay. Is that interesting to you?
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For data into the FPGA:
The FPGA IOB's input setup/hold time requirement is defined with respect to the clock at the IOB registers.
If your input clock (to the FPGA) is connected to an IBUFG (a clock-capable IOB), then to a DLL or DCM, then to a BUFG (with the output of the BUFG connected to the DLL/DCM's feedback port), then the edges of your input clock will be coincident with the clock edges on the FPGA's internal clock net (i.e., the output of the BUFG). This is known as "de-skewing" an input clock. This clock net can be used to clock the IOB registers -- and when you do this, the timing relationship, at the IOB's register input, is known with respect to your external clock. This is critical when your data rate is fairly high.
You'll have to do a timing analysis to see if de-skewing your input clock is required. If you don't de-skew, then you'll have a more unpredictable relationship between your input clock and the internal clock net (that is clocking your input data).
For data out of the FPGA:
You can route the BUFG-to-DLL/DCM feedback path outside of the FPGA (using an external trace). This will have the effect of making your output data edges coincident with those of your input clock.
Hope this helps.
Bob
Good point. Thanks.
Note that any delay in the clock distribution turns into hold time requirements at the input to the IOB. Hold times are evil. They hurt even if the clock is not running fast.
Older Xilinx chips had a delay in the IOB that was long enough to cover that hold time, and an option to bypass it in order to reduce the setup time.
Thanks all for you Inputs.
Regards, Muthu S
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