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Virtex4 DCM doesn't work unless freezing cold

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.

Reply to
msn444
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Hi Mike, It probably isn't NBTI.

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Google :- nbti group:comp.arch.fpga But, a bake at 150C for 48 hours would prove that it isn't!

You also might liek to read this thread from a few days back about reset. Google :- "Virtex 4 DCM problem" group:comp.arch.fpga

Good luck, Syms.

Reply to
Symon

Mike,

Have you double-checked the range settings of the DCM? That would be my first guess...

/Mikhail

Reply to
MM

Mike,

How is the "DCM Performance Mode" attribute set?

"MAX_SPEED" or "MAX_RANGE"?

"MAX_RANGE" should be used with an oscillator of 25 MHz. The process may have shifted to be slightly faster, and if the attribute was "MAX_SPEED" it could be the frequency input is too slow for that mode.

Austin

Reply to
austin

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table 45.

Also shows that 25 MHz is too low (per the specification). You may have just been lucky (process variations in last lot made it 'work').

The DCM delay line is temperature compensated, so it may be that when extremely cold, it is slowed down so much that it works with 25 MHz.

Aust> Mike,

Reply to
austin

Hi Austin,

That was exactly our problem, however for a slightly different reason than I originally indicated. We were actually using the CLKFX outputs, which don't have that limitation, but using the LOCKED signal to enable the rest of the system, and it wasn't asserting due to the input frequency being too low. The new batch is indeed from a newer lot of FPGAs and apparently they're much less tolerant than the previous batch.

Thanks for your help,

Mike.

Reply to
msn444

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