Start and stop conditions in Embedded C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a start condition in I2C communication?

A start condition signals the beginning of a data transfer on the I2C bus. It is created when the data line (SDA) goes from high to low while the clock line (SCL) is high.

beginner

What does a stop condition indicate in I2C communication?

A stop condition signals the end of a data transfer on the I2C bus. It happens when the data line (SDA) goes from low to high while the clock line (SCL) is high.

intermediate

How do you generate a start condition in embedded C for I2C?

To generate a start condition, first ensure SDA and SCL are high. Then pull SDA low while SCL stays high. This tells all devices on the bus that communication is starting.

intermediate

How do you generate a stop condition in embedded C for I2C?

To generate a stop condition, keep SCL high and change SDA from low to high. This signals the end of communication and releases the bus for other devices.

beginner

Why are start and stop conditions important in I2C communication?

They mark the boundaries of data transfer. Start condition tells devices to listen, and stop condition tells them communication is done. Without them, devices can't sync properly.

What happens to the SDA line during a start condition in I2C?

A It goes from high to low while SCL is high

B It goes from low to high while SCL is low

C It stays high while SCL goes low

D It stays low while SCL goes high

The start condition is defined by SDA changing from high to low while SCL remains high.

During a stop condition, what change occurs on the SDA line?

A SDA stays high while SCL goes low

B SDA goes from low to high while SCL is high

C SDA stays low while SCL goes low

D SDA goes from high to low while SCL is low

The stop condition happens when SDA changes from low to high while SCL is high.

Why must SCL be high during start and stop conditions?

A Because SCL must be low to start communication

B Because SCL controls the data speed

C Because SCL is ignored during start and stop

D Because data changes on SDA are only valid when SCL is high

In I2C, data line changes are recognized only when the clock line (SCL) is high.

What does a start condition tell devices on the I2C bus?

A Data is invalid

B Communication is ending

C Communication is beginning and they should listen

D Bus is free

Start condition signals devices to prepare for data transfer.

What is the main purpose of the stop condition?

A To signal the end of communication and release the bus

B To start data transfer

C To reset the device

D To pause communication

Stop condition marks the end of communication and frees the bus for others.

Explain how start and stop conditions are generated on the I2C bus and why they are important.

Think about how the data and clock lines behave during these conditions.

Describe the role of start and stop conditions in coordinating communication between devices on an I2C bus.

Consider how devices know when to send or receive data.

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