begin()

Serial communication on pins TX/RX uses TTL logic levels (RP2040 uses 3.3V TTL Logic Levels).

Serial is used for communication between the Arduino board and a computer or other devices. All Arduino boards have at least one serial port (also known as a UART or USART): Serial. It communicates on digital pins 0 (RX) and 1 (TX) as well as with the computer via USB. You can use the Arduino environment's built-in serial monitor to communicate with an Arduino board. Click the serial monitor button in the toolbar and select the same baud rate used in the call to begin().

The Arduino RP2040 board uses Serial1 to communicate via TTL (3.3V) serial on pins 0 (RX) and 1 (TX). Serial is reserved for USB CDC communication.