Serial output with UART

The UART is the simplest serial port and the first peripheral to set up on any board. It gives you printf: your program prints text, and a terminal on your computer displays it. From now on, every project you build should print what it is doing.

A UART link uses 2 wires plus ground: TX sends data and RX receives it. Connect your TX to the other side's RX, and their TX to your RX. Connecting TX to TX is a common beginner mistake. Because each direction has its own wire, both sides can send at the same time. This is called full duplex.

There is no clock wire. Both sides agree on a speed in advance, the baud rate, such as 115200 bits per second. Each byte is sent in a frame. The line rests high. A start bit pulls it low for one bit time, so the receiver knows a byte is coming. Then come 8 data bits and a stop bit that returns the line high. With no parity bit, this format is written 8N1. At 9600 baud each of the 10 bits takes about 104 microseconds, so a byte takes about 1 ms.

The signal on the MCU pins is at logic level, 3.3 V or 5 V. This is not RS-232. A true RS-232 port, the older 9-pin PC connector, swings between plus and minus 5 to 15 V, often plus and minus 12 V, with inverted signalling. Wiring it directly to an MCU pin can destroy the pin. Use a USB to serial adapter, which works at logic level, or the USB port already on your board.