Communication Protocols

Microcontrollers need to talk to sensors, displays, and other microcontrollers. Standard communication protocols make this possible.

[Diagram: Master-Slave connection topologies for SPI and I2C]

UART (Universal Asynchronous Receiver-Transmitter)

Simple, point-to-point communication. Uses two wires: TX (Transmit) and RX (Receive). No clock line is used (asynchronous), so both sides must agree on a baud rate.

SPI (Serial Peripheral Interface)

Synchronous, full-duplex communication. Uses 4 wires: SCK (Clock), MOSI (Master Out Slave In), MISO (Master In Slave Out), and SS (Slave Select). Very fast.

I2C (Inter-Integrated Circuit)

Synchronous, half-duplex, multi-master bus. Uses 2 wires: SDA (Data) and SCL (Clock). Devices have unique addresses. Slower than SPI but requires fewer pins.

Comparison

Protocol	Wires	Speed	Type
UART	2 (TX, RX)	Slow	Asynchronous
SPI	4 (SCK, MOSI, MISO, SS)	Fast	Synchronous
I2C	2 (SDA, SCL)	Medium	Synchronous

Example: UART Transmit

void uart_init(unsigned int baud) {
    unsigned int ubrr = F_CPU/16/baud - 1;
    UBRR0H = (unsigned char)(ubrr>>8);
    UBRR0L = (unsigned char)ubrr;
    UCSR0B = (1<// Enable RX and TX
}

void uart_send(unsigned char data) {
    while (!(UCSR0A & (1<// Wait for buffer to be empty
    UDR0 = data;
}
            