Hi everyone,
We've been shipping a Virtex4 FX20 based product for a few months now with relatively few problems. However, we're seeing a 60+% failure rate in our latest batch of boards, characterized by the V4 DCMs not locking or providing any output at all unless the chip is freezing cold. Neither the design, board house or CM shop has changed at all from our previous working batch, which generally runs pretty hot (~60C) with no known problems.
The details I have found are: when powering up from room temperature, other logic implemented in the chip seems to work, but the DCMs have no output and are not locked. If I give the FPGA a good shot of cold spray and power up the device, it becomes fully functional, until the temperature rises somewhat and the DCM ceases completely. Among the samples I've tried, the temperature at which it stops working ranges from really cold to slightly below room temperature.
The clock source is a 25MHz crystal oscillator -- I tried a function generator and nothing changes. I tried twiddling VCCAux as I've seen suggested and no difference there, either. Has anyone seen such a dramatic DCM failure before, or have any ideas what might be causing this?
Thanks, Mike.