RE: [SI-LIST] : PCI topology

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From: JNH (John@quantatw.com)
Date: Tue Jan 30 2001 - 22:40:07 PST


Patel,
I remember that PCI spec mention that if you violate the PCI specs --
topology, stub length, or others, then you have to do simulation to check if
the SI and timing can meet spec.

Therefore, in your simulation, you have proved the non-daisy chain topology
is workable. Then you may use the one. Make sure you are using right models
for simulation.

rgds,

John Lin
Senior SI Engineer, Server Team, ARD4
Quanta Computer Inc.,Taiwan, R.O.C.
Email: John@quantatw.com
Tel: 886+3+3979000 ext. 5183

-----Original Message-----
From: Bhavesh Patel [mailto:bpatel@cyras.com]
Sent: Wednesday, January 31, 2001 10:03 AM
To: 'si-list@silab.eng.sun.com'
Subject: [SI-LIST] : PCI topology

Hi! I had a question regarding PCI bus topology that I am trying to
simulate. I have a PCI controller and three PCI compliant interface devices.
The topology is
 <<...OLE_Obj...>>

P---> PCI controller
C--> connector
P1, P2, P3 are network processor with PCI interfaces. This bus is running at
33Mhz

P can drive/talk to any device on the bus and P1, P2, P3 can only talk to P
but not to each other, but when P1 is driving P2, P3 also will be listening.
Now, when I simulate with these trace lengths using idela 60ohm transmission
lines the signal especially when either P1 or P2 is driving is non-monotonic
at each of the receivers and takes a long time for the signal to settle
down.
Hence, I said with the given trace lengths this topology does not work. It
works if I reduce the trace lengths which will affect other things on the
board.
So, I used a topology with dual PCI driver concept i.e one PCI connected to
two chips P1 and P2 and the other connected to P3. The topology looks like

 <<...OLE_Obj...>>
P2 is connected before the connector. This topology works with the above
trace lengths.
The other topology is P connected to P3
 <<...OLE_Obj...>>
The PCI controller does support dual concept.

Now, my question:

1) If in simulations the signals look monotonic, settle delay is good should
I still worry about thing relevant to PCI because I know PCI is strict on
the stub lengths and the total trace lenght etc?
2) Also, I forgot to mention that I am using 10ohm resistor in series at
each device and this does help me in achieving the extra trace length. Am I
violating anything ?
3)What else should I look for?

Thanks and sorry for such a long email

Bhavesh

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