Broken ADC readings on esp32c3 with read_raw() and read() #468
Description
Test setup
- esp32c3, connected to wifi (random dev board from aliexpress)
- Using adc1 and gpio0
- applying external voltage (~0.9V) to pin using lab PSU, checking voltage with good quality DMM. also have a small (couple nF) capacitor directly on the pin
- issue reproduced on two out of two tested devices, one fresh from the packaging, so unlikely hardware issue
- setup code:
let adc = AdcDriver::new(peripherals.adc1).unwrap();
let adc_pin = peripherals.pins.gpio0;
let adc_cfg = AdcChannelConfig::new();
let mut adc_driver = AdcChannelDriver::new(&adc, adc_pin, &adc_cfg).unwrap();Problem
When I use read_raw() in a loop to collect 16 samples and then averaging over those
loop {
let mut samples = [0; 16];
for sample in &mut samples {
*sample = adc.read(&mut adc_driver).unwrap();
}
// uncomment to fix:
//info!("samples: {samples:?}");
let sample = samples.into_iter().sum::<u16>() / samples.len() as u16;
info!("sample: {sample}");
thread::sleep(Duration::from_secs(1));
}I get unreliable results. The applied voltage of 0.9V seems to max out the ADC since I see no fluctuation in the values anymore. However, the value I get is not 4095, but instead 3839, which converts to binary 0b111011111111. If I slightly change the code, and, for example log the individual sub-samples in the for-loop, or log the samples array, or add a small 10ms sleep to the for loop, then I suddenly see the correct reading of 4095.
The read() function, however, seems to behave correctly, returning reliable and accurate voltage readings. A user in IRC suggested, this might be some timing issue, that is magically right in read() all the time, but in my read_raw() setup sometimes is wrong.
If you need any more information, please ask.