analogWrite on R4 boards generates spikes

[kevinjwalters]

An arduino program trying to use analogWrite() to accurately generate a waveform suffers from a lot of brief spikes. I noticed these when trying to make a low frequency triangle wave. A recent Playduino YouTube video (by @olivereybel) shows these although it's possible they aren't shown in full by the oscilloscope. Even setting the midpoint value repeatedly can do this.

A more advanced oscilloscope shows this for a sine wave.

Perhaps the issue discussed in the "Hairy output on R4 DAC" forum topic was never fixed? @TriodeGirl provided some code to deal with this in https://github.com/TriodeGirl/Arduino-UNO-R4-code-DAC-ADC-Ints-Fast_Pins/

Arduino Forum discussion: Spikes on UNO R4 DAC output using analogWrite()

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Other analogWrite issues:

• AnalogWrite creates a PWM when value is 255 #388