I think that both of your concerns stem from the same root cause - your driver
output impedance is too low for the impedance of your net. I am assuming that
you drive 1 or 2 closely spaced DRAMs. My guess is that you chose an AC type
part for it's speed and your net impedance is 60+ ohms. AC parts can have an
output impedance in the neighborhood of 20 ohms. Pick an part with a built in
series damping resistor or add one yourself to your net.
One way to check out this kind of problem is to place a probe at the far-end of
the net like you have done, and simultaneously place a probe on the driver
output pin. This will give you the transmission line view of what is happening
on the net. You can see how the driver drives the net and you can see how the
net drives the load.
CEC Analysis and I/O Design
Phone: 507.253.0762 Fax: 507.253.4966
owner-si-list@silab.Eng.Sun.COM on 08/27/98 12:02:16 AM
Please respond to owner-si-list@silab.Eng.Sun.COM
Subject: [SI-LIST] : overshoot
Ok, I'm a rookie, so go easy on me. I am having trouble with a signal
on a board we just built. The signal is a CE for a memory device.
During certain conditions, there is a short duration pulse on this line.
Two things are happening that causes me concern.
1) When it pulses high, it tops out at about 3.6 - 3.8V (operating
voltage of board is 3.3V)
2) When the signals is once again driven low, there is ringing.
My assumptions is that reflections are causing the ringing, however I'm
have no clue about the additional 300 mV on the high part of pulse.
Any ideas on either of the above?
Thanks in advance for any help.
Motorola LinkWorks Division
(800)-759-8352 (#2454833) (pager)
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