Portenta H7: Interference of A3 pin on A5

Hardware Portenta Portenta H7
1

Hello, in a project, advancedADC were used, on pin A0 and A5, to read two sensors, and A5 pin to controll a sensor enable signal. Somehow the action on A3 pin seem to intefere the A5. Please see the following test code for details. Thanks in advance for your help.

#include <Arduino_AdvancedAnalog.h>

#define nBuffers 32 
#define nSamples 1000 
#define samplingFreq 100 

AdvancedADC adc1(A0);
AdvancedADC adc2(A5);

void setup() {
  Serial.begin(9600);
  while (!Serial);

  if (!adc1.begin(AN_RESOLUTION_16, samplingFreq*nSamples, nSamples, nBuffers)) { //this somehow overwrites D1 to HIGH and thus placed before pwm setup
    Serial.println("Failed to start adc1(A0)!");
    while (1);
  }
  if (!adc2.begin(AN_RESOLUTION_16, samplingFreq*nSamples, nSamples, nBuffers)) { //this somehow overwrites D1 to HIGH and thus placed before pwm setup
    Serial.println("Failed to start adc2(A5)!");
    while (1);
  }
  
  // digitalWrite(A3, HIGH);

}

void loop() {
  if (adc2.available()){
    SampleBuffer buf = adc2.read();
    
    for (int i=0; i < buf.size(); i++){
      Serial.println(buf.data()[i]);
    }
    buf.release();
  }
}

adc2 produces correct results in the code above. However, if line 21, "// digitalWrite(A3, HIGH);" is uncommented, A5 pin have 3.1 V read by a multimeter and not connected to anything else. And adc2's reading doesn't respond to signals applied to A5 pin anymore.

2

If pin does "not connected to anything else", it can has a perfectly random value.
It is absolutely normal if such free-flowing pin influenced by level of nearest pins.

3

Hi @b707, thanks for your feedback. It was a stable 3.1 V measured on A5 and stronger than normal cross-talk between pins. There appear to be some form of internal shared connection or configuration between A3 and A5. A second test case was conducted to help with the diagnosis. With the code as is, adc2 digitizes a signal in the 0-3 V range appided to A5 correctly, sinking or sourcing negligible amount of current, indicating A5 in a high impedance state. If line 21, "// digitalWrite(A3, HIGH)" is replaced by "pinMode(A3, OUTPUT)", the same signal applied to A5 will cause tens of mA of current flowing, indicating A5 has been changed to a low impedance state by the command pinMode(A3, OUTPUT).