Single channel ADC reading in Embedded C - Time & Space Complexity

Time Complexity: Single channel ADC reading

O(n)

Understanding Time Complexity

We want to understand how the time to read from an ADC changes as we take more readings.

How does the number of readings affect the total time spent?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

int readADC(int channel) {
    // Start ADC conversion
    ADC_START_CONVERSION(channel);
    
    // Wait until conversion is done
    while(!ADC_CONVERSION_COMPLETE());
    
    // Return the ADC value
    return ADC_GET_VALUE();
}

int readMultiple(int channel, int times) {
    int sum = 0;
    for(int i = 0; i < times; i++) {
        sum += readADC(channel);
    }
    return sum / times;
}
    

This code reads an analog value from one ADC channel multiple times and averages the results.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: The for-loop in readMultiple calls readADC repeatedly.
• How many times: Exactly times times, which is the input size.

How Execution Grows With Input

Each additional reading adds one full ADC read cycle, so time grows directly with the number of readings.

Input Size (times)	Approx. Operations
10	10 ADC reads
100	100 ADC reads
1000	1000 ADC reads

Pattern observation: Doubling the number of readings doubles the total time.

Final Time Complexity

Time Complexity: O(n)

This means the total time grows linearly with the number of ADC readings requested.

Common Mistake

[X] Wrong: "Reading multiple times is just as fast as reading once because the ADC is fast."

[OK] Correct: Each ADC read takes time to start, convert, and finish, so multiple reads add up in time.

Interview Connect

Understanding how repeated hardware reads affect time helps you write efficient embedded code and explain your reasoning clearly.

Self-Check

"What if we read from multiple ADC channels instead of one? How would the time complexity change?"

Practice

1. What does a single channel ADC do in embedded C programming?

easy

A. It reads one analog input and converts it to a digital number.

B. It writes digital data to an analog output.

C. It reads multiple analog inputs simultaneously.

D. It converts digital signals to analog signals.

Solution

1. Step 1: Understand ADC function

   An ADC (Analog to Digital Converter) converts analog signals to digital numbers.

2. Step 2: Identify single channel meaning

   Single channel means it reads only one analog input at a time.

3. Final Answer:

   It reads one analog input and converts it to a digital number. -> Option A

4. Quick Check:

   Single channel ADC = One analog input read [OK]

Hint: Single channel ADC reads one input only [OK]

Common Mistakes:

• Confusing ADC with DAC
• Thinking ADC reads multiple inputs at once
• Mixing analog output with input reading

2. Which of the following is the correct way to start an ADC conversion in embedded C?

easy

A. ADC_Stop();

B. ADC_StartConversion();

C. ADC_Read();

D. ADC_Init();

Solution

1. Step 1: Identify function to start ADC

   Starting ADC conversion usually requires a function like ADC_StartConversion().

2. Step 2: Differentiate other functions

   ADC_Read() reads the value, ADC_Init() initializes hardware, ADC_Stop() stops ADC.

3. Final Answer:

   ADC_StartConversion(); -> Option B

4. Quick Check:

   Start ADC = ADC_StartConversion() [OK]

Hint: Start ADC with ADC_StartConversion() [OK]

Common Mistakes:

• Using ADC_Read() to start conversion
• Confusing initialization with start
• Calling ADC_Stop() instead of start

3. What will be the output of this code snippet if the ADC reads a value of 512?

int adcValue = ADC_Read();
printf("ADC Value: %d\n", adcValue);

medium

A. ADC Value: 0

B. ADC Value: 1023

C. ADC Value: 512

D. ADC Value: -512

Solution

1. Step 1: Understand ADC_Read() output

   ADC_Read() returns the current digital value from ADC, here given as 512.

2. Step 2: Print the value

   printf prints the integer value stored in adcValue, which is 512.

3. Final Answer:

   ADC Value: 512 -> Option C

4. Quick Check:

   ADC_Read() = 512 printed as is [OK]

Hint: ADC_Read() returns current ADC number [OK]

Common Mistakes:

• Assuming ADC_Read() returns max value always
• Confusing negative values in ADC reading
• Printing wrong variable or format

4. Identify the error in this ADC reading code:

ADC_Init();
int value = ADC_Read();
ADC_StartConversion();
printf("Value: %d", value);

medium

A. Variable 'value' should be float

B. ADC_Init() is missing

C. printf format specifier is wrong

D. ADC_StartConversion() should be called before ADC_Read()

Solution

1. Step 1: Check ADC function order

   ADC_StartConversion() must be called before ADC_Read() to start conversion.

2. Step 2: Analyze code sequence

   Here, ADC_Read() is called before starting conversion, so it reads old or invalid data.

3. Final Answer:

   ADC_StartConversion() should be called before ADC_Read() -> Option D

4. Quick Check:

   Start conversion before reading ADC [OK]

Hint: Start ADC conversion before reading value [OK]

Common Mistakes:

• Reading ADC before starting conversion
• Ignoring initialization step
• Wrong printf format for int

5. You want to read an analog sensor value using a single channel ADC and convert it to voltage (0-5V) with 10-bit resolution. Which code snippet correctly calculates the voltage?

int adcValue = ADC_Read();
float voltage = ?;

hard

A. voltage = (adcValue / 1023.0) * 5.0;

B. voltage = (adcValue * 5) / 1023;

C. voltage = adcValue * 5.0 / 1024.0;

D. voltage = (adcValue / 1024) * 5;

Solution

1. Step 1: Understand ADC resolution and voltage range

   10-bit ADC means values from 0 to 1023 represent 0 to 5 volts.

2. Step 2: Calculate voltage correctly

   Voltage = (adcValue / 1023.0) * 5.0; uses floating division and full scale 1023.

3. Final Answer:

   voltage = (adcValue / 1023.0) * 5.0; -> Option A

4. Quick Check:

   Voltage = ADC value / max * 5V [OK]

Hint: Divide by 1023.0 for 10-bit ADC voltage calc [OK]

Common Mistakes:

• Dividing by 1024 instead of 1023
• Using integer division instead of float
• Multiplying before dividing causing wrong result