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From: Ryan Gammon
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Subject: Re: Grounding advice
References: <3D9DFA4D.firstname.lastname@example.org> <3D9E1B5A.209DA53D@xympatico.ca>
Date: Sun, 06 Oct 2002 18:06:20 GMT
NNTP-Posting-Date: Sun, 06 Oct 2002 15:06:20 ADT
> This sounds like a timing problem - the capacitance of the voltmeter is
> slowing the output transition just enough for the data to be clocked
> into wherever it is going. Where is it going anyway?
On the rising edge of a load signal (sent from microcontroller to
cpld-acting-as-a-shift-register on the perfboard), the cpld changes
three of its outputs, all going to a single sram address lines with no
other fanout. The sram is on the perfboard. Putting a voltmeter between
one of these cpld outputs (aka address line inputs) and ground seems to
suppress the extra clock pulses on the cpld input
I can see timing problems corrupting sram data, but not causing extra
clock pulses on the cpld.
I have three general things I tend to expect (anyone have any other
1. low slew rate on the input clock signal going to the cpld
3. ground bounce
1 seems unlikely -- the signal is driven from a cmos output on the avr
chip directly over a 3 inch wirewrap-gauge wire to the cpld's plcc
socket, with no fanout.
2 seems unlikely -- the board is wired in a spagehtti-ish,
point-to-point way, and not many of the wires are running in parallel.
3 seems possible, but I would expect putting a capacitance across the
output to make things worse. (My reasoning: a capacitance give the
higher freqencies of the switching signal a low-impedance path to
ground, which puts more current on the ground line, which causes
> Of course, you may also have a bad ground, but if the boards are close
> together and the wires are short (say 3 inches long) just connect the
> grounds of the boards together near the voltage regulators and pretend
> it is one big board.
... and leave the assorted ground pin headers unconnected. This makes
the most sense to me too.
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