[sdiy] Why did this 100pF capacitor fix the SPI bus?

Spiros Makris spirosmakris92 at gmail.com
Tue Feb 16 16:36:15 CET 2021


Hello list,
I was recently helping a friend debug his prototype, that uses the DAC8564,
on the SPI bus of an atmega328p. I don't have the prototype here (and
haven't seen in in real life), but through trial and error we managed to
get it fixed.
The DAC is the only component on the SPI bus, and the exact same thing was
working on the breadboard, with the exact same code. No matter what he
tried and checked, the IC was just not responding to any commands at all. A
picture of the waveforms captured with a scope didn't show anything
suspicious either, like ringing, extreme overshoot etc. The length of each
signal was <2cm for all signals of the SPI port.
Eventually he discovered that when jumper wires were connected to the 6pin
SPI header, on the MOSI and GND lines, the damn thing magically worked. He
didn't have to connect them to anything on the other side, mind you, just
have them there.
I figured the only thing they could be changing electrically is acting as a
capacitor, and eyeballed the value as 100p, based on the capacitance
between two lines of the breadboard.
Well, what do you know, this actually worked. Now I am completely baffled
as to why. Any ideas?
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