Single channel ADC reading in Embedded C - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this ADC reading code?

Consider the following embedded C code snippet that reads from a single ADC channel and stores the result in a variable. What will be the value of adc_value after execution?

Embedded C

volatile uint16_t adc_value = 0;
void read_adc(void) {
    ADC->CHSELR = ADC_CHSELR_CHSEL0; // Select channel 0
    ADC->CR |= ADC_CR_ADSTART;       // Start conversion
    while (!(ADC->ISR & ADC_ISR_EOC)); // Wait for conversion complete
    adc_value = ADC->DR;             // Read data register
}

A adc_value is always zero because ADC is not started

B adc_value contains the 12-bit ADC conversion result from channel 0

C adc_value contains a random value due to missing conversion start

D adc_value contains the previous conversion result, not updated

💡 Hint

Check if the ADC conversion is started and if the code waits for completion before reading.

The code selects channel 0, starts the ADC conversion, waits until the conversion is complete, then reads the ADC data register. Thus, adc_value holds the latest 12-bit conversion result.

🧠 Conceptual

intermediate

Which statement best describes single channel ADC reading?

In embedded systems, what does single channel ADC reading mean?

A Reading digital inputs instead of analog

B Reading multiple ADC channels simultaneously

C Using ADC in differential mode with two channels combined

D Reading analog input from one specific ADC channel at a time

💡 Hint

Focus on the meaning of 'single channel' in ADC context.

Single channel ADC reading means the ADC reads the voltage from one selected input channel at a time, converting it to a digital value.

🔧 Debug

advanced

Why does this ADC reading code hang in the while loop?

Analyze the following code snippet. Why does the program get stuck in the while loop?

Embedded C

ADC->CHSELR = ADC_CHSELR_CHSEL1; // Select channel 1
ADC->CR |= ADC_CR_ADSTART;         // Start conversion
while (!(ADC->ISR & ADC_ISR_EOC)); // Wait for conversion complete
uint16_t val = ADC->DR;

A The ADC is not enabled before starting conversion, so EOC flag never sets

B The channel selection register is incorrect, causing conversion failure

C The data register is read before conversion starts

D The interrupt flag is cleared too early, causing infinite wait

💡 Hint

Check if ADC peripheral is enabled before starting conversion.

If the ADC peripheral is not enabled (e.g., ADC->CR |= ADC_CR_ADEN), the conversion never starts and the EOC flag never sets, causing the loop to hang.

📝 Syntax

advanced

Which option fixes the syntax error in this ADC reading code?

Identify the option that corrects the syntax error in the following code snippet:

uint16_t read_adc() {
    ADC->CHSELR = ADC_CHSELR_CHSEL2
    ADC->CR |= ADC_CR_ADSTART;
    while (!(ADC->ISR & ADC_ISR_EOC));
    return ADC->DR;
}

A Add a semicolon after the channel selection line

B Replace ADC_CHSELR_CHSEL2 with ADC_CHSELR_CHSEL3

C Change return type to void

D Remove the while loop

💡 Hint

Look for missing punctuation that causes syntax errors.

The line ADC->CHSELR = ADC_CHSELR_CHSEL2 is missing a semicolon at the end, causing a syntax error. Adding it fixes the code.

🚀 Application

expert

How many ADC readings are stored after this code runs?

Given this code that reads ADC channel 0 five times and stores results in an array, how many valid ADC readings does the array contain?

uint16_t adc_results[5];
for (int i = 0; i < 5; i++) {
    ADC->CHSELR = ADC_CHSELR_CHSEL0;
    ADC->CR |= ADC_CR_ADSTART;
    while (!(ADC->ISR & ADC_ISR_EOC));
    adc_results[i] = ADC->DR;
}

A Only 1 valid reading, repeated 5 times

B 0 readings because ADC is not enabled

C 5 valid ADC readings, one per loop iteration

D Undefined number due to missing delay between readings

💡 Hint

Consider the loop and waiting for conversion complete each time.

The loop runs 5 times, each time starting a conversion, waiting for it to finish, and storing the result. So the array contains 5 valid readings.