#include <SPI.h>
#include <Wire.h>

#define MCP23017 0x27

bool stateISR = true;
unsigned long period = 1000;
unsigned long last_time = 0;



void setup() {
  // put your setup code here, to run once:
  Wire.begin();

  Wire.beginTransmission(MCP23017);
  Wire.write(0x00);
  Wire.write(0x00);
  Wire.endTransmission();
  delay(500);

}

void loop() {
  // put your main code here, to run repeatedly:
  if(millis()-last_time > period){
    last_time = millis();
    if(stateISR == true){
      Wire.beginTransmission(MCP23017);
      Wire.write(0x12);
      Wire.write(0xAA);
      Wire.endTransmission();
    }
    else{
      Wire.beginTransmission(MCP23017);
      Wire.write(0x12);
      Wire.write(0x55);
      Wire.endTransmission();
      
    }
    stateISR = !stateISR;
  }
}