[SI-LIST] Ground bounce issue in FPGA?
- From: Chris Bobek <cbobek@xxxxxxxxxx>
- To: si-list@xxxxxxxxxxxxx
- Date: Wed, 16 Jan 2002 16:28:56 -0800
Hi,
I have an Altera APEX 20K160E in a 484pin BGA package on a board. I
have 4 POS-PHY busses (two in, two out) and a 100bit ZBT SRAM bus on the
FPGA. When I run random, full-rate traffic through the board with a
fan, I can run error free for several days. If I take away the fan, the
case temperature ends up at 60degrees C (well within 85degree spec), yet
I receive several errors per second. I tried recompiling the code to
try to get the Fmax up, which it did, however I got errors with and
without the fan. After recompiling several other times, I get different
error rates with and without the fan. Finally, I compilied an image
without SRAM (leaving 4 PL3 busses) and ran overnight without errors and
without a fan.
Here are some facts:
1) The internal resources are only 30% utilized.
2) All external timing on the FPGA, SRAM, PL3 chips, etc. have high
measured margins, with and without fan.
3) I've tried images with/without the internal PLL enabled. Similar
results.
4) If I apply traffic with mostly all 1's (except for CRC), I run error
free without fan.
5) I have 1 .1uF cap per pin, directly under each power pin (I used
via-in-pad to fit them underneath). Plus, several bulk caps surrounding
the BGA.
6) I did not add any programmable grounds (wish I did).
7) I looked at a static, active-high output on the FPGA and noticed
about a 1V negative spike (dips to ~2.3V) coincident with the clock
period. Similar is true for active-low output (rises to ~1V). These
spikes never showed up on other signals that I was doing timing on.
8) I've tried different output drivers (LVTTL, LVCMOS). LVCMOS had
lower errors, but higher ground bounce! Why would that be?
9) I've tried slow slew rate and I get the same mixed results...
Of course, I suspect ground bounce is an issue. Are there any other
measurements I can make? Are there any possible solutions or ideas
short of choosing a larger part and making every other pin a ground?
Sorry for the long message, this has been one hairy problem!
Thanks,
Chris
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