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From: Ryan Gammon
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Subject: Re: Grounding advice
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Date: Mon, 07 Oct 2002 17:48:58 GMT
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mook johnson wrote:
> remove the voltmeter and add about 50pF where you had it clipped in. I'll
> bet you are getting crosstalk and the voltmeter is filtering it out.
Yeah, I agree. I wanted to make sure I had my grounds set up correctly
first before I started poking around with caps, though.
Right now, I have a short wire I pulled out of an old pc power supply
connecting the grounds of the regulators on both boards, and I've
disconnected all other grounds. I now get clock glitches all the time,
but I think that it's theoretically a step in the right direction.
> Perfboard is NOT the place for high speed ICs.
Are there any alternatives beyond pcb's?
I thought of this a bit when choosing the XC9536 -15, and decided that
I'd be ok. For other young aspiring hobbyists out there, the rise and
fall times of the xilinx cpld are, as far as I can figure out:
t_edge = t_slew_limiter + t_output_buffer_delay
= 5.0 ns + 4.5 ns
= 9.5 ns
For comparison purposes, my datasheet on the HC logic family says
t_low_high = 3.6 ns
t_high_low = 4.1 ns
I can't find the specs for the slew rate on the avr's outputs.
> At edge rates connecting wires look like inductors.
> Also be sure you bypasses the supply rails with
> some ceramic .1uF or .01uF caps.
I have that covered.
I think that getting this to work with just a voltmeter is going to be
impossible -- I'm going to have to invest in a cheap oscilloscope.
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