Arduino Nano 33 IOT A4 & A5 not returning expected values

I have a capacitive moisture sensor running on A5 and Photoresistor running on A4 but neither are acting as expected.

A5 just gives a constant reading of ~1023 and A4 seems to have a minimum value of ~600.

I have connected the same sensors to A0-A3 and gotten expected results, just A4 and A5 seem to be an issue, I have also tested on multiple Arduino Nano boards and got the same result.

My assumption is that is has something to do with the SDA SCI on those pins.

I don’t have easy ability to draw out the schematic so here is my best sketch.

Code snippet:

#define CMS3PIN A4
#define LRD3PIN A5

int m3;
int l3;

void setup() {
  Serial.begin(9600);
}

void loop() {
  delay(2000);
  m3 = analogRead(CMS3PIN);
  l3 = analogRead(LRD3PIN);
  Serial.print(" | Light: ");
  Serial.print(l3);
  Serial.print(" | Moisture: ");
  Serial.println(m3);
}

It looks like the A4 and A5 pins, which double as the I2C/Wire/TWI pins (just like on the UNO and Nano) are connected to three other chips on the board. Looks like an EEPROM chip, a wireless chip, and a crypto authentication chip. Because of that they both have 4.7k pull-up resistors which will outweigh some of your 10k pull-down resistor and give you different answers on your LDR.

Thanks John,

Any ideas on how to turn them off, I haven't had any success with pinMode or digitalWrite low

Here's the schematic: https://content.arduino.cc/assets/NANO33IoTV2.0_sch.pdf

Just unsolder the three chips and the two pull-up resistors connected to A4 and A5. That will allow you to use those two pins for analog input. You would definitely lose the radio capability.