What is UART in microcontroller?

UART = Universal Asynchronous Receiver/Transmitter. It’s the hardware block inside a microcontroller that turns bytes in memory into a serial bitstream on a pin (TX) and turns a bitstream on another pin (RX) back into bytes—without a shared clock line.

What it does (in plain terms)

• Sends: shifts out bits LSB-first with a start bit (low), data bits (usually 8), optional parity, then stop bit(s) (high).

• Receives: watches RX for a falling edge (start), samples the line (often 16× oversampling), reconstructs the byte, checks parity/stop, raises flags/interrupts.

Idle=1 ──┐  Start  Data bits (LSB→MSB)   Parity   Stop
         └─0─────b0─b1─b2─b3─b4─b5─b6─b7─(P)──────1──

Typical pins & wiring

• TX (transmit), RX (receive), GND (ground). Cross them: TX ↔ RX and share ground.

• Optional RTS/CTS for hardware flow control.

• Some MCUs support single-wire half-duplex (TX/RX on one pin).

Electrical levels (important!)

• TTL/CMOS UART: 0–3.3 V or 0–5 V logic. For short PCB/short cable only.

• RS-232: ±3…±12 V and inverted logic → needs a level shifter (e.g., MAX232).

• RS-485/422: differential, long cables, multi-drop → needs an RS-485/422 transceiver.

Key configuration terms

• Baud rate: bits/s (9 600, 115 200, etc.). Both sides must match closely.

• Frame: data bits (7/8/9), parity (None/Even/Odd), stop bits (1/2).

• Flow control: none, RTS/CTS (hardware), or XON/XOFF (software).

• Errors you may see: framing (bad stop bit), parity, overrun (RX FIFO full).

Common uses

• Debug logs/printf, bootloaders, firmware updates.

• Modules: GPS/GNSS, Bluetooth (HC-05/06), Wi-Fi/Cell modems, motor drives, sensors.

• Inter-MCU communication when speed/length is moderate.

UART vs. USART (and vs. SPI/I²C)

• UART: asynchronous (no clock pin).

• USART: can do UART and synchronous serial (with a clock pin).

• SPI/I²C: synchronous buses with shared clock (higher speed, multi-device support).

Minimal code snippets

Arduino (ATmega328P, 115200-8N1)

void setup() {
  Serial.begin(115200);         // 8 data, No parity, 1 stop by default
  Serial.println("Hello UART");
}
void loop() {
  if (Serial.available()) {
    int c = Serial.read();
    Serial.write(c);            // echo
  }
}

STM32 HAL (e.g., USART2 on 115200-8N1)

UART_HandleTypeDef huart2;

void MX_USART2_UART_Init(void) {
  huart2.Instance          = USART2;
  huart2.Init.BaudRate     = 115200;
  huart2.Init.WordLength   = UART_WORDLENGTH_8B;
  huart2.Init.StopBits     = UART_STOPBITS_1;
  huart2.Init.Parity       = UART_PARITY_NONE;
  huart2.Init.Mode         = UART_MODE_TX_RX;
  huart2.Init.HwFlowCtl    = UART_HWCONTROL_NONE;
  HAL_UART_Init(&huart2);
}

void app(void){
  const char *msg = "Hello\r\n";
  HAL_UART_Transmit(&huart2,(uint8_t*)msg,strlen(msg),100);
}

Practical tips

• Match baud/parity/stop on both ends; start with 115200-8N1 if unsure.

• Share ground; cross TX/RX; avoid long flying leads at TTL levels.

• For long/noisy runs use RS-485 with twisted pair and termination.

• Use interrupts or DMA and a ring buffer for reliable reception at higher baud.

• If text looks garbled, check baud mismatch, inverted logic, or wrong voltage level.