Re: gEDA-user: series terminators

[ldoolitt@xxxxxxxxxxxxxxx]

DJ -

On Thu, Sep 21, 2006 at 11:47:38AM -0400, DJ Delorie wrote:
> I'm thinking I should design in some series resistors on the address
> and data lines for my RAM expansion board (30MHz, about 6" of 8 mil
> trace on DS FR4, 5v).

5 Volts?  How quaint.

You didn't give the thickness of the FR4 between the trace and the
ground plane.

Series resistors for a bidirectional trace go near the middle of
their length.  For unidirectional traces, place the resistor anywhere
from the middle to the source end.

> How does one go about calculating an ideal
> resistance?  The mcu spec doesn't have a min rise time spec, the ram
> chips are 20ns (50MHz) and imply they want a 3ns rise time.

Think of it this way: you want to charge a parasitic capacitance
(guess 10 pF) with a rise time of 3 ns.  That means you need a
series resistance less than 3.0 ns / 10 pF = 300 Ohms.

> I figure I can put in zero-ohm resistors for now, and experiment with
> one of the lines, but I'd rather just come close enough on the first
> try ;-)

Any resistor in the range of 25 to 150 Ohms will damp the ringing.
It takes a good FET probe and a fast 'scope to characterize the
system well enough to optimize on the bench.  Whenever I have this
concern, I just put in 100 Ohms, observe that it works, and go on
to the next chore.

    - Larry


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