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

Ullrich Peter Peter.Ullrich at kapsch.net
Tue Feb 16 16:47:00 CET 2021


Data is sampled on the Din pin at the negative edge. So maybe you have your SPI code for the positive edge then then capacitor helps to store your data for some microseconds so that it also works with the negative edge…


http://rc100-usb.at.tt/   (Roland sampler tool)
http://synpro.heimat.eu/datadial.htm (Synth addon)

Von: Synth-diy <synth-diy-bounces at synth-diy.org> Im Auftrag von Spiros Makris
Gesendet: Dienstag, 16. Februar 2021 16:36
An: synth-diy mailing list <synth-diy at synth-diy.org>
Betreff: *EXT* [sdiy] Why did this 100pF capacitor fix the SPI bus?

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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