Start and stop conditions in Embedded C - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to generate a start condition on the I2C bus.

Embedded C

I2C->CR1 |= [1];  // Generate start condition

Drag options to blanks, or click blank then click option'

AI2C_CR1_START

BI2C_CR1_ACK

CI2C_CR1_STOP

DI2C_CR1_PE

Attempts:

3 left

💡 Hint

Common Mistakes

Using the stop condition bit instead of the start condition bit.

Confusing the peripheral enable bit with the start condition bit.

The I2C_CR1_START bit is set to generate a start condition on the I2C bus.

2fill in blank

medium

Complete the code to generate a stop condition on the I2C bus.

Embedded C

I2C->CR1 |= [1];  // Generate stop condition

Drag options to blanks, or click blank then click option'

AI2C_CR1_STOP

BI2C_CR1_ACK

CI2C_CR1_PE

DI2C_CR1_START

Attempts:

3 left

💡 Hint

Common Mistakes

Using the start condition bit instead of the stop condition bit.

Confusing the acknowledge bit with the stop condition bit.

The I2C_CR1_STOP bit is set to generate a stop condition on the I2C bus.

3fill in blank

hard

Fix the error in the code to correctly generate a start condition and wait for it to be set.

Embedded C

I2C->CR1 |= [1];
while(!(I2C->SR1 & I2C_SR1_SB));  // Wait for start bit set

Drag options to blanks, or click blank then click option'

AI2C_CR1_PE

BI2C_CR1_STOP

CI2C_CR1_ACK

DI2C_CR1_START

Attempts:

3 left

💡 Hint

Common Mistakes

Setting the stop bit instead of the start bit.

Not waiting for the start bit flag in the status register.

Setting I2C_CR1_START generates the start condition. The code waits until the start bit (SB) is set in the status register.

4fill in blank

hard

Fill both blanks to generate a stop condition and wait until the bus is free.

Embedded C

I2C->CR1 |= [1];
while(I2C->SR2 & [2]);  // Wait until bus busy flag clears

Drag options to blanks, or click blank then click option'

AI2C_CR1_STOP

BI2C_SR2_BUSY

CI2C_CR1_START

DI2C_SR1_SB

Attempts:

3 left

💡 Hint

Common Mistakes

Waiting on the wrong status register flag.

Setting the start bit instead of the stop bit.

Set I2C_CR1_STOP to generate a stop condition. Then wait until the busy flag I2C_SR2_BUSY clears in the status register 2.

5fill in blank

hard

Fill all three blanks to generate a start condition, wait for it, and then generate a stop condition.

Embedded C

I2C->CR1 |= [1];  // Start condition
while(!(I2C->SR1 & [2]));  // Wait for start bit
I2C->CR1 |= [3];  // Stop condition

Drag options to blanks, or click blank then click option'

AI2C_CR1_START

BI2C_SR1_SB

CI2C_CR1_STOP

DI2C_CR1_ACK

Attempts:

3 left

💡 Hint

Common Mistakes

Not waiting for the start bit flag before generating the stop condition.

Using the acknowledge bit instead of start or stop bits.

First, set I2C_CR1_START to generate the start condition. Then wait for the start bit flag I2C_SR1_SB to be set. Finally, set I2C_CR1_STOP to generate the stop condition.

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