Setting (digitalWrite) pin y, unsets pin x

[FunDeckHermit]

• Arduino board: Custom board

• Arduino IDE version (found in Arduino -> About Arduino menu): PlatformIO

• List the steps to reproduce the problem below:
  Run below sketch, it is expected that pin5 is toggling every 2 seconds.
  It will however prematurely turn off setting another pin.

#include <Arduino.h>
#include <Adafruit_MCP23X08.h>

Adafruit_MCP23X08 mcp;
TwoWire my_wire(0);

void setup()
{
    Serial.begin(460800);

    my_wire.setPins(5,6);
    mcp.begin_I2C(0x27, &my_wire);
    mcp.pinMode(5, OUTPUT);
    mcp.pinMode(6, OUTPUT);
}

void loop()
{
    delay(2000);
    digitalWrite(1, HIGH);
    mcp.digitalWrite(6,LOW);
    
    delay(2000);
    digitalWrite(1, LOW);
    mcp.digitalWrite(6,HIGH);
    
    mcp.digitalWrite(5,HIGH); //This write also sets pin6 low ???
    delay(50);
    mcp.digitalWrite(5,LOW);
}

[xKaimac]

I'm having the same issue with a MKRWAN 1310.

With the following setup function I have no trouble:

#define MCP_I2C_ADDRESS 0x21
#define RESET_PIN = 4;
#define RELAY_ONE = 12;
#define RELAY_TWO = 13;
#define I2C_DIGITAL_PIN = 7;
#define INPUT_ONE = 0;

void setup() {
  Wire.begin();

  mcp.begin_I2C(MCP_I2C_ADDRESS);

  pinMode(RESET_PIN, OUTPUT);  
  digitalWrite(RESET_PIN, HIGH);

  mcp.pinMode(RELAY_TWO, OUTPUT);
  mcp.pinMode(RELAY_ONE, OUTPUT);

  pinMode(I2C_DIGITAL_PIN, INPUT);
  mcp.pinMode(INPUT_ONE, INPUT);
}

But if I set all of the pins before writing, ie:

  pinMode(RESET_PIN, OUTPUT);  

  mcp.pinMode(RELAY_TWO, OUTPUT);
  mcp.pinMode(RELAY_ONE, OUTPUT);

  pinMode(I2C_DIGITAL_PIN, INPUT);
  mcp.pinMode(INPUT_ONE, INPUT);
  
  digitalWrite(RESET_PIN, HIGH);

I have issues with the Arduino going into a boot loop (in my case it's the RESET_PIN being pulled low). I've reduced it down to only being the RELAY_ONE setting (12 in this library) that throws it all out of whack, the other ones are fine. This issue can be reproduced with any available digital pin for bridging the RESET_PIN in my case, not just digital pin 4.

I don't have a robust solution for you as I'm unsure what's causing this, but play around with the order in which you set pins and you might find one that gives you the desired result. Hopefully there's a patch or explanation for it eventually.