From: Bo ([email protected])
Date: Mon Nov 20 2000 - 11:35:27 PST
I have an interesting problem that probably a lot of you have encountered
The bottom line to the problem is what to do when simulations show you much
worse results than measurements do. But lets start with facts...
PCI bus in question has 4 devices on it:
- Xilinx Virtexe FPGA
- Altera Apex 20K FPGA
- Intel (made originally by DEC) Ethernet controller
- Motorola PPMC750 card
The bus is running at 33 MHz and is 32 bits wide (Address/Data portion of it).
Xilinx FPGA is on board 1 while other three devices are on the board 2(Motorola
PPMC750 is connected to board 2 through connectors). All the devices can drive
The topology looks something like this:
X - Xilinx FPGA
C1 - Connector 1 that connects two boards (Mictor in this case)
C2 - Connector 2 connecting Motorola PMC750 card and Board 2
M - Motorola PPMC750
A - Altera FPGA
I - Intel
X->C1 = 3 inches
C1->C2 = 1 inch
C2->A = 3 inches
A->I = 2 inches
C2->M = 2.5 inches (where this distance represents distance between connector
and PCI bridge on the PPMC750 card).
Well from all this information most of people would think the same way I did:
"O this is easy, it is only 33 MHz (PCI is supposed to work at 66 MHz) and it
is only 32 bit wide". To make life even easier there were test cards done not
so long ago that had a similar setup (same devices but little different lengths
by a little I mean no more than 2 inches differences which may still me a lot
in some cases). And it worked. It all sounds like solved case. At least I
thought that way...
What happened next was that in early stages of design someone did simulations
that showed relatively nice waveforms. Few months later I was asked to redo
simulations since the topology changed from the last time simulations were
Someone also pointed out that one of the drivers didn't look good in the
simulations he performed earlier on another bus and that my bus should be
looked at more carefully.
I took the old simulations and modified them to represent current topology (the
simulations were done in Hspice). I noticed that previous simulations were
using an old U-model for transmission lines so I changed transmission lines to
use W-element. Also previous simulations modeled PPMC750 as a cap and resistor
in parallel so I replaced this model with an IBIS model of the PCI Bridge used
on this card. So I redid simulations and all of sudden I got severe undershoot
of 2V(this happens when both Xilinx and Altera drive the bus and highest
undershoot is seen on the PPMC750). My first guess was that I made mistake in
modeling the bus so I asked someone else to check the simulation deck. No
mistakes (the other person is VERY experienced in HSPICE simulations and tow of
us went through the deck line by line). Next step was to go to the lab and
look at the similar PCI bus. I went to the lab with another person and we
looked at the bus. What we saw was something that you can only see in
university undergraduate digital courses. Signal had the cleanest edges I have
ever seen. Nice perfect lines. There was though overshoot of around 400 mV and
undershoot of around 600 mV. At the time what I had was 600mV of the
undershoot in the lab and 2V in the simulations. Something didn't make sense.
I looked at my spice deck and started thinking what may be different. I looked
at devices to see if they may be using different I/O buffers than what I am
simulating. I found that Altera FPGA allows slew rate limited I/O buffers to
be put instead of normal ones. So I tried these. As I expected the fall time
and rise times increased and signal become much cleaner. The undershoot was
gone. So I went to look in the lab to see if this may be the case.
Unfortunately the fall/rise times in the simulations labs were not the same as
the ones in the lab. What I have seen in lab is 4 ns Fall time and 6 ns Rise
time. The simulations have 1.6 ns rise time and 1.3 ns. If I use slew rate
limited Fall time is 8 ns and rise time is 2 ns (all numbers are 0-100%).
This is were my problem lies: I can't guarantee that next set of devices will
have the same rise times. The device manufacturers do not want to guarantee
that their devices will work under conditions I see in the simulations. I am
not sure that device models are correct. Someone suggested putting a resistor
in series with the bus (on the board 1 in front of the connector 1). Is this
right thing to do? What is right thing to do? For a final note, to get a
approximately lab's rise/fall times from simulations I put a capacitive load of
180 pf to ground in front of a driver. This produced a clean edges and
comparable undershoot values (not exactly the same waveform as in the lab but
with similar quality). This leads me to think that the models or I are not
taking something in to account. But I can't imagine anything that would be 180
Well, I would like to thank anybody who read this email. I hope some of you
will be able help me in resolving my problem. Comments are greatly appreciated
(even the smallest ones).
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