UART protocol fundamentals in Embedded C

Introduction

UART helps two devices talk to each other by sending data one bit at a time. It is simple and common for communication.

Connecting a microcontroller to a GPS module to get location data.

Sending sensor readings from a device to a computer for logging.

Communicating between two microcontrollers in a robot.

Debugging code by sending messages from a microcontroller to a PC.

Controlling a device like a motor driver from a microcontroller.

Syntax

Embedded C

void UART_Init(unsigned int baud_rate);
void UART_SendChar(char data);
char UART_ReceiveChar(void);

UART_Init sets up the communication speed (baud rate).

UART_SendChar sends one character at a time.

Examples

Start UART communication at 9600 bits per second.

Embedded C

UART_Init(9600);

Sends the letter 'A' over UART.

Embedded C

UART_SendChar('A');

Waits and reads one character received via UART.

Embedded C

char received = UART_ReceiveChar();

Sample Program

This program sets up UART at 9600 baud, sends 'H' and 'i', then receives a character and prints it.

Embedded C

#include <stdio.h>
#include <stdlib.h>

// Simulated UART functions for demonstration
void UART_Init(unsigned int baud_rate) {
    printf("UART initialized at %u baud\n", baud_rate);
}

void UART_SendChar(char data) {
    printf("Sent: %c\n", data);
}

char UART_ReceiveChar(void) {
    // Simulate receiving character 'X'
    return 'X';
}

int main() {
    UART_Init(9600);
    UART_SendChar('H');
    UART_SendChar('i');
    char received = UART_ReceiveChar();
    printf("Received: %c\n", received);
    return 0;
}

Important Notes

UART sends data one bit at a time, so speed and timing must match on both devices.

Common baud rates are 9600, 19200, 38400, 115200.

UART uses start and stop bits to mark the beginning and end of each byte.

Summary

UART is a simple way for devices to send data one bit at a time.

Set baud rate to match both devices for correct communication.

Use UART functions to send and receive characters easily.

Practice

1. What does UART stand for in embedded systems communication?

easy

A. Universal Analog Radio Transmitter

B. Universal Automatic Register Transfer

C. Unified Asynchronous Remote Terminal

D. Universal Asynchronous Receiver/Transmitter

Solution

1. Step 1: Understand UART acronym

   UART is a communication protocol used in embedded systems for serial communication.

2. Step 2: Match full form

   UART stands for Universal Asynchronous Receiver/Transmitter, describing its function.

3. Final Answer:

   Universal Asynchronous Receiver/Transmitter -> Option D

4. Quick Check:

   UART = Universal Asynchronous Receiver/Transmitter [OK]

Hint: Remember UART is about asynchronous serial communication [OK]

Common Mistakes:

• Confusing UART with synchronous protocols
• Mixing up the acronym words
• Thinking UART is analog communication

2. Which of the following is the correct way to initialize UART in embedded C?

easy

A. uart_init(9600);

B. UART_Init(baudrate=9600);

C. uartStart(9600);

D. init_uart(9600);

Solution

1. Step 1: Identify common UART init function

   In embedded C, uart_init with baud rate parameter is a common initialization call.

2. Step 2: Check syntax correctness

   uart_init(9600); uses correct function name and parameter style without extra keywords.

3. Final Answer:

   uart_init(9600); -> Option A

4. Quick Check:

   Correct UART init syntax = uart_init(9600); [OK]

Hint: Look for simple function call with baud rate only [OK]

Common Mistakes:

• Using incorrect function names
• Adding parameter names inside call
• Using camelCase instead of snake_case

3. What will be the output of this UART send code snippet?

uart_init(9600);
uart_send_char('A');
uart_send_char('B');

medium

A. BA

B. A B

C. AB

D. Error: uart_send_char undefined

Solution

1. Step 1: Initialize UART at 9600 baud

   The uart_init(9600) sets baud rate for communication.

2. Step 2: Send characters in order

   uart_send_char('A') sends 'A', then uart_send_char('B') sends 'B' sequentially.

3. Final Answer:

   AB -> Option C

4. Quick Check:

   Characters sent in order = AB [OK]

Hint: Characters send in call order, no spaces added [OK]

Common Mistakes:

• Assuming characters reverse order
• Expecting spaces between characters
• Thinking uart_send_char is undefined

4. Identify the error in this UART initialization code:

void setup() {
  uart_init(115200);
  uart_init(9600);
}

medium

A. uart_init requires no parameters

B. Multiple uart_init calls cause baud rate conflict

C. Function name should be UART_Init

D. No error, code is correct

Solution

1. Step 1: Review uart_init calls

   Two uart_init calls with different baud rates cause conflict in configuration.

2. Step 2: Understand UART config rules

   UART baud rate must be set once before communication; multiple calls overwrite settings.

3. Final Answer:

   Multiple uart_init calls cause baud rate conflict -> Option B

4. Quick Check:

   Multiple init calls cause conflict = A [OK]

Hint: Only one uart_init call needed to set baud rate [OK]

Common Mistakes:

• Thinking multiple init calls are allowed
• Assuming uart_init has no parameters
• Ignoring case sensitivity in function names

5. You want to send the string "HELLO" over UART. Which code snippet correctly sends each character one by one?

hard

A. for(int i=0; i<5; i++) uart_send_char("HELLO"[i]);

B. for(int i=0; i<5; i++) uart_send_char('H' + i);

C. for(int i=0; i<5; i++) uart_send_char('HELLO'[i]);

D. for(int i=0; i<5; i++) uart_send_char("HELLO" + i);

Solution

1. Step 1: Understand string indexing in C

   In C, single quotes ' ' denote characters, double quotes \" \" denote strings (char arrays).

2. Step 2: Check each option for correct character access

   for(int i=0; i<5; i++) uart_send_char(\"HELLO\"[i]); correctly indexes the string literal to access each character 'H','E','L','L','O'.

3. Final Answer:

   for(int i=0; i<5; i++) uart_send_char("HELLO"[i]); -> Option A

4. Quick Check:

   Correct string indexing uses double quotes \"HELLO\"[i] [OK]

Hint: Use double quotes for string literal indexing in C [OK]

Common Mistakes:

• Using single quotes for multi-character string literals
• Adding integers to characters ('H' + i sends HIJKL)
• Using pointer arithmetic without dereferencing