[SI-LIST] Re: SDRAM Blowup!
- From: "Doug White" <dowhite@xxxxxxxxx>
- To: <andrew.seddon@xxxxxxxxxxxx>, <si-list@xxxxxxxxxxxxx>
- Date: Thu, 2 Dec 2004 11:13:29 -0500
I would expect, if both data lines are held at the same value, that if =
you
were to measure the impedance between them on the board you would see
roughly twice the on-resistance of the I/Os. If the drivers are very, =
very
strong (on the order of 5 or 10 ohms on-resistance), then you could get
close to 10 or 20 ohms. Then if you hold them at different values, you
should see a reactive impedance; the capacitance between your power and
ground rails charging up.
I guess the question might be whether the impedance between the two =
lines
changes drastically during your test time. I assume from your posting
you've checked that and found a significant change. How much does it
change?
Doug
-----Original Message-----
From: si-list-bounce@xxxxxxxxxxxxx [mailto:si-list-bounce@xxxxxxxxxxxxx] =
On
Behalf Of Andrew Seddon
Sent: Tuesday, November 30, 2004 1:09 PM
To: si-list@xxxxxxxxxxxxx
Subject: [SI-LIST] SDRAM Blowup!
Ok, slight exageration, but an interesting problem non the less!
I have a board with a uP connected to a 133Mhz SDRAM and a connector. =
The
connector brings out the address/data bus which is then passed on to =
another
board. Total trace lengths across both boards for the address/data bus
(including connector) are about 4 inches. The connector is a simple =
1.27mm
spacing type. All specific SDRAM signals stay on the first board(CLK,
CAS,RAS etc). Both boards are 50ohm controlled impedance but no traces =
are
terminated and the connectors could have any impedance??
Now we've had a few back that have been working nicely with no SI =
problems
what so ever. However the latest lot are showing a really weird problem.
After running fine reading from SDRAM for a couple of hours data lines
become shorted together (about 7 ohms, eg D7+D9 etc) removing the SDRAM
cures the short. However there is no short on the bare removed SDRAM.
Secondly forceably driving the data lines to different values takes the
short resistance upto about 200ohms, and the SDRAM then works fine for
another couple of hours.
This problem occurs even at low 50mhz bus freq's.
So my question is, has anybody ever seen anything like this before/have =
any
suggestions? I'm trying to get hold of a high speed scope at the minute =
to
see what's going on but any tips would be really usefull. My current =
theory
is some sort of ringing effect that is overdriving the pins and causing =
an
internal brake-down in the SDRAM's output drivers.
The confusing thing is another batch of supposidly identical boards runs
fine for day's on end.
Cheers guys, any help much appreciated!
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