Re: Virtex-4FX DCM autoshutdown failure, any suggestions

Antti wrote:
Hi

if anyone has seen the same or has any ideas how to avoid the issue I
am faced please help - I am trying to get it solved myself (my deadline
is today 21:00 german time) but kinda stuck currently.

problem:

Virtex-4FX12
2 DCMs in series
DCM1 uses only FX output for PPC system clock (to get the clock into
DLL input range)
DCM2 generates 3X clock proc clock for PPC

it all works for 360 milliseconds after configuration. then the first
DCM will remove lock, output stop, everythings stops. the delay 360
milliseconds is not dependand on the first DCM clock ratio settings. if
PPC is held in reset then the DCMs still shut down after the same 360
milliseconds.

any ideas? what to check? I have Lecroy 2GS/s DSO on some signals and
power supply lines but not seeing anything at the time where the DCM
shut off.

thanks in advance for any suggestions,

Antti


Antti,

I don't think this is your problem since you said it is the first DCM losing lock, however check to make sure the jitter out of the first DCM is within the max jitter input specs for the second DCM. In most cases, you violate the max jitter spec when trying to drive a second DCM with the clkfx output of the first. Perhaps you can change it around so that the first is doing clkx2 and the second is doing the clkfx? Otherwise, at least check the jitter for your M and D values to satisfy yourself that the jitter is not too much for the second DCM

Then again, I'm sure you already know all this ;-)

Second question: are you using the NBTI macro? I don't know what changes Xilinx has made to it in the past 16 months. The original version for the devices that don't have the DCM changes had race conditions that made it unreliable for higher clock input frequencies, at least on paper. We had no end of problems getting it to react properly when starting a clock up after the FPGA had been programmed. There were several revisions to the circuit since then, but we have not looked at them here to see if the race problem was addressed or not. We are leaving the NBTI circuit out for designs that have a crystal on the board now.
.

Relevant Pages

  • Re: Virtex-4FX DCM autoshutdown failure, any suggestions
    ... DCM1 uses only FX output for PPC system clock (to get the clock into ... DCM will remove lock, output stop, everythings stops. ... I don't think this is your problem since you said it is the first DCM ... however check to make sure the jitter out of the first DCM ...
    (comp.arch.fpga)
  • Re: Virtex-4 VLX25 DCM problem
    ... I used a DCM in my ... multiple clock domains, the logic seems to be working so far. ... The PCI spec is not compatible with the DCMs. ... The PCI spec allows the PCI clock frequency to be changed. ...
    (comp.arch.fpga)
  • Re: DCM Jitter
    ... I've read that the DCM DLL function is actually very clean as far as phase ... A DCM does not reduce jitter under any circumstance ... As a delay line, the DCM does not have any oscillators. ... if you stop the input clock, you will need to re-lock the DCM ...
    (comp.arch.fpga)
  • How to phase align a 10MHz clock using V4LX60 DCM
    ... I am now working with new board based on ARM processor which is ... possible i thought to advance the clock by 4ns using the DCM on the V4 ... 100Mhz are phase aligned also the input to second DCM and the output ...
    (comp.arch.fpga)
  • Re: Virtex4 CLKX2 DCM Jitter
    ... Try to distinguish whether the DCM input clock is ... affected when the I/O switches; ... without DCM) and see if the jitter changes ...
    (comp.arch.fpga)