MCP23017 to read hall sensors

[alto777]

@arduinobotting

  if (!mcp.begin_I2C()) {
 // if (!mcp.begin_SPI(CS_PIN)) {

Wait. Are you trying to use I2C?

Then you have to hook SCL and SDA to specific pins on the Mega.

What pins to use for I2C bus with AVR Mega2560

Documentation seems to say that I2C bus uses pins 20 and 21. In Mega. However in the schemati I2C bus seems to be on pins 43 and 44. What are the correct pins to use.

a7