# Serial (UART) for Debugging and Data — Arduino & Embedded C

Source: https://www.geekswithgeeks.com/en/arduino-embedded-c/uart-serial

> Serial.begin, print, read.

## Asynchronous serial

A **UART** sends bytes over two wires, TX and RX, with no shared clock, so both sides must agree on the **baud rate** (for example 9600 or 115200) and frame format (usually 8N1). On the Uno R3 the hardware UART is on pins 0 (RX) and 1 (TX) and is also connected to the USB-to-serial chip, which is why you should avoid using those pins for other things while using USB. `Serial.print()` is the main debugging tool in Arduino work; `Serial.available()` and `Serial.read()` receive data. Connect TX to the other device's RX, share a ground, and match voltage levels (5 V versus 3.3 V).

## Talking to PCs, sensors and displays

UART, I2C and SPI are the three serial buses you will meet in almost every project.

![Three ideas: Serial/UART, I2C, SPI and libraries.](assets/figures/arduino-embedded-c/section-4-map.svg) — Figure 4.1 — UART, I2C and SPI links from one microcontroller.

## A tiny command interface

Type a line such as "led on" in the serial monitor (newline line ending).

```cpp
String line;

void setup() {
  Serial.begin(115200);
  pinMode(LED_BUILTIN, OUTPUT);
  Serial.println(F("Ready. Commands: led on | led off"));
}

void loop() {
  while (Serial.available() > 0) {
    char c = Serial.read();
    if (c == '\n') {
      line.trim();
      if (line == "led on")       digitalWrite(LED_BUILTIN, HIGH);
      else if (line == "led off") digitalWrite(LED_BUILTIN, LOW);
      else Serial.println(F("Unknown command"));
      line = "";
    } else if (line.length() < 32) {
      line += c;                       // cap length to protect tiny RAM
    }
  }
}
```

## Garbage in the monitor?

Random characters usually mean a baud-rate mismatch between Serial.begin() and the serial monitor setting.

**Quiz:** What must both ends of a UART link agree on?

- [x] The baud rate and frame format
- [ ] The I2C address
- [ ] A shared clock line
- [ ] The SPI mode

*Answer:* The baud rate and frame format. UART is asynchronous: timing comes from the agreed baud rate.