Start and stop conditions in Embedded C - Mini Project: Build & Apply

Start and Stop Conditions in I2C Communication

📖 Scenario: You are working with a microcontroller that communicates with a sensor using the I2C protocol. To start communication, you need to send a start condition, and to end communication, you need to send a stop condition.This project will guide you through writing code to generate these start and stop conditions using embedded C.

🎯 Goal: Write embedded C code to generate the I2C start and stop conditions by manipulating the SDA and SCL lines.

📋 What You'll Learn

Create variables representing SDA and SCL lines as integers.

Create a function to generate the start condition by setting SDA low while SCL is high.

Create a function to generate the stop condition by setting SDA high while SCL is high.

Print the states of SDA and SCL after start and stop conditions.

💡 Why This Matters

🌍 Real World

Microcontrollers use start and stop conditions to control communication on the I2C bus, which connects sensors and other devices.

💼 Career

Embedded systems engineers must understand how to generate and detect start and stop conditions to implement reliable I2C communication.

1

Setup SDA and SCL line variables

Create two integer variables called SDA and SCL and set both to 1 to represent the idle state of the I2C bus.

Embedded C

int SDA = 1;
int SCL = 1;
// Your code here

Hint

The I2C bus is idle when both SDA and SCL lines are high (1).

2

Create start condition function

Write a function called start_condition that sets SDA to 0 while SCL remains 1 to generate the I2C start condition.

Embedded C

int SDA = 1;
int SCL = 1;

void start_condition() {
    // Your code here
}

Hint

Start condition means SDA goes low while SCL stays high.

3

Create stop condition function

Write a function called stop_condition that sets SDA to 1 while SCL remains 1 to generate the I2C stop condition.

Embedded C

int SDA = 1;
int SCL = 1;

void start_condition() {
    SDA = 0;
    SCL = 1;
}

void stop_condition() {
    // Your code here
}

Hint

Stop condition means SDA goes high while SCL stays high.

4

Print SDA and SCL states after start and stop

Call start_condition() and then print the values of SDA and SCL. Then call stop_condition() and print the values of SDA and SCL again. Use printf with the format: "SDA: %d, SCL: %d\n".

Embedded C

int SDA = 1;
int SCL = 1;

void start_condition() {
    SDA = 0;
    SCL = 1;
}

void stop_condition() {
    SDA = 1;
    SCL = 1;
}

int main() {
    // Your code here
    return 0;
}

Hint

Call the functions and print the line states to see the start and stop conditions.

Practice

1.

What does a start condition indicate in embedded C communication protocols?

easy

A. It signals the beginning of data transmission.

B. It signals the end of data transmission.

C. It resets the device.

D. It pauses the communication temporarily.

Solution

1. Step 1: Understand start condition role

   The start condition marks when a device begins sending data on the communication line.

2. Step 2: Differentiate from stop condition

   Unlike the stop condition which ends communication, the start condition initiates it.

3. Final Answer:

   It signals the beginning of data transmission. -> Option A

4. Quick Check:

   Start condition = beginning signal [OK]

Hint: Start means begin communication, stop means end it [OK]

Common Mistakes:

• Confusing start with stop condition
• Thinking start resets the device
• Assuming start pauses communication

2.

Which of the following is the correct syntax to generate a stop condition in embedded C I2C communication?

void i2c_stop() {
    // Which line correctly generates stop condition?
    ?
}

easy

A. SDA = 1; SCL = 1;

B. SDA = 1; SCL = 0;

C. SDA = 0; SCL = 0;

D. SDA = 0; SCL = 1;

Solution

1. Step 1: Recall stop condition signal

   Stop condition occurs when SDA line goes from low to high while SCL is high.

2. Step 2: Match with code lines

   Setting SDA=1 and SCL=1 matches the stop condition signal.

3. Final Answer:

   SDA = 1; SCL = 1; -> Option A

4. Quick Check:

   Stop condition = SDA rising while SCL high [OK]

Hint: Stop condition: SDA rises while SCL is high [OK]

Common Mistakes:

• Setting SDA low instead of high for stop
• Setting SCL low during stop condition
• Confusing start and stop line states

3.

What will be the output of this code snippet simulating I2C start and stop conditions?

#include <stdio.h>
int start = 0, stop = 0;

void i2c_start() {
    start = 1;
    stop = 0;
}

void i2c_stop() {
    stop = 1;
    start = 0;
}

int main() {
    i2c_start();
    i2c_stop();
    if (start == 0 && stop == 1) {
        printf("Communication ended\n");
    } else {
        printf("Communication ongoing\n");
    }
    return 0;
}

medium

A. No output

B. Communication ongoing

C. Communication ended

D. Compilation error

Solution

1. Step 1: Trace function calls and variable changes

   Calling i2c_start() sets start=1, stop=0; then i2c_stop() sets stop=1, start=0.

2. Step 2: Evaluate if condition in main

   The condition start==0 and stop==1 is true, so it prints "Communication ended".

3. Final Answer:

   Communication ended -> Option C

4. Quick Check:

   start=0 & stop=1 means communication ended [OK]

Hint: Last called condition sets flags; check their final values [OK]

Common Mistakes:

• Assuming start remains 1 after stop call
• Confusing variable assignments order
• Expecting output before function calls

4.

Identify the error in this code snippet that tries to generate a start condition:

void i2c_start() {
    SDA = 1;
    SCL = 1;
    SDA = 0;
}

medium

A. SCL should be set to 0 after SDA goes low.

B. SCL should be low before SDA goes low to start communication.

C. SDA should go from low to high, not high to low.

D. SDA should go from high to low while SCL is high, so code is correct.

Solution

1. Step 1: Recall start condition definition

   Start condition requires SDA to go from high to low while SCL is high.

2. Step 2: Analyze code sequence

   The code first sets SDA=1, SCL=1 (establishing high-high state), then SDA=0 while SCL remains high, correctly generating the start condition.

3. Final Answer:

   SDA should go from high to low while SCL is high, so code is correct. -> Option D

4. Quick Check:

   Start condition = SDA high->low while SCL high [OK]

Hint: Start condition: SDA falls while SCL high [OK]

Common Mistakes:

• Confusing start (high->low) with stop (low->high)
• Thinking SCL must be low before SDA transition
• Ignoring that the code correctly times the transition

5.

You want to design a function that sends multiple data bytes over I2C with proper start and stop conditions. Which sequence correctly ensures communication integrity?

void send_data() {
    // Which sequence is correct?
    ?
}

hard

A. i2c_start(); send_byte(0x50); i2c_stop(); send_byte(0xA0);

B. i2c_start(); send_byte(0x50); send_byte(0xA0); i2c_stop();

C. send_byte(0x50); i2c_start(); send_byte(0xA0); i2c_stop();

D. i2c_stop(); send_byte(0x50); send_byte(0xA0); i2c_start();

Solution

1. Step 1: Understand I2C communication flow

   Communication must start with a start condition, then data bytes are sent, and finally a stop condition ends it.

2. Step 2: Evaluate each option sequence

   i2c_start(); send_byte(0x50); send_byte(0xA0); i2c_stop(); correctly starts communication, sends bytes, then stops. Others start or stop in wrong order.

3. Final Answer:

   i2c_start(); send_byte(0x50); send_byte(0xA0); i2c_stop(); -> Option B

4. Quick Check:

   Start -> data -> stop sequence is correct [OK]

Hint: Always start before sending data and stop after all bytes [OK]

Common Mistakes:

• Sending data before start condition
• Calling stop before all data sent
• Mixing order of start and stop calls