glen,
another possibility is that the FPGA doesn't meet
timing because there's clock jitter.
if you compile for 250 MHz (4 nS clock), and
there's 1 nS P-P of clock jitter, then the FPGA sometimes
sees a 3.5 nS clock, and sometimes sees a 4.5 nS clock,
and the shorter 3.5 nS clock violates timing.
does is work reliably at low clock frequencies?
if so, it's not a clock jitter problem.
dan
Vinayak Nagpal wrote:
If I understand correctly Glenn is not sampling data using the ADC and
using a data source internal to the FPGA. Any transmission line effects
external to the FPGA may cause clock glitches etc but those should not
affect the results because the clock to all blocks should glitch together.
Such a problem should arise only when the clock distribution network
inside the FPGA is doing something funny. That this should happen and
synth/par tools not report it - is indeed a long shot.
On May 15, 2008, at 9:47 AM, David MacMahon wrote:
On May 15, 2008, at 9:22 , Dan Werthimer wrote:
this problem of non-repeat of power spectra
might be a problem with how often you put in a sync pulse.
it might also be a problem that your test pattern is
not a multiple of pfb taps * fft length.
Hi, Dan, while these ideas can explain odd behaviors, I think they
would affect the design regardless of clock frequency (so long as it's
stays in reasonable range).
One thing to double check, Glenn, is that the design is really getting
built for the clock frequency you think it is. When clocking in
through the ADC blocks, the clock frequency needs to be specified in
the parameters of the ADC block(s). In this case, the clock frequency
in the XSG block is ignored. Given that your design does run at 1024
MHz ADC clock (but not 1023 MHz ADC clock), however, I would guess
that you are already doing this correctly.
If you have two ADC cards, you could try swapping them. Maybe
something has gone bad with the current ADC0's clock signal path
integrity? I know it's a long shot, but this problem sounds rather
odd. As far as I know, an design built for one frequency should work
fine at a slower (but not *too* slow) frequency unless there are some
sort of transmission line effects (external to the FGPA) that make it
work better/worse at different frequencies.
Dave
