Multi-channel ADC scanning in Embedded C - Mini Project: Build & Apply

Multi-channel ADC scanning

📖 Scenario: You are working with a microcontroller that reads analog signals from multiple sensors. These sensors are connected to different ADC channels. You want to write a program that reads all these channels one by one and stores their values.

🎯 Goal: Build a simple multi-channel ADC scanning program in C that reads values from 4 ADC channels and stores them in an array.

📋 What You'll Learn

Create an array to store ADC channel numbers

Create an array to store ADC readings

Write a loop to scan each ADC channel

Store the ADC reading for each channel

Print the ADC readings

💡 Why This Matters

🌍 Real World

Multi-channel ADC scanning is used in embedded systems to read sensors like temperature, light, or pressure from multiple inputs.

💼 Career

Embedded software engineers often write code to read and process sensor data from ADC channels for devices like IoT sensors, automotive systems, and industrial controllers.

1

Setup ADC channel array

Create an integer array called adc_channels with these exact values: 0, 1, 2, 3 representing the ADC channels to scan.

Embedded C

# Create an integer array called adc_channels with values 0, 1, 2, 3
// Your code here

Hint

Use curly braces to list the channel numbers inside the array.

2

Setup ADC readings array

Create an integer array called adc_readings with size 4 to store the readings from each ADC channel.

Embedded C

int adc_channels[4] = {0, 1, 2, 3};
// Create an integer array called adc_readings with size 4
// Your code here

Hint

Declare the array without initializing values.

3

Scan ADC channels and store readings

Write a for loop using int i from 0 to 3. Inside the loop, call the function read_adc(adc_channels[i]) and store the result in adc_readings[i]. Assume read_adc(int channel) is a function that returns an integer ADC value for the given channel.

Embedded C

int adc_channels[4] = {0, 1, 2, 3};
int adc_readings[4];
// Write a for loop to scan each ADC channel and store the reading
// Your code here

Hint

Use the loop variable i to index both arrays.

4

Print ADC readings

Write a for loop using int i from 0 to 3. Inside the loop, print the channel number and its ADC reading using printf in this exact format: Channel 0: 1234 where 0 is the channel and 1234 is the reading from adc_readings[i].

Embedded C

int adc_channels[4] = {0, 1, 2, 3};
int adc_readings[4];
for (int i = 0; i < 4; i++) {
    adc_readings[i] = read_adc(adc_channels[i]);
}
// Print each channel and its reading
// Your code here

Hint

Use printf("Channel %d: %d\n", adc_channels[i], adc_readings[i]); inside the loop.

Practice

1.

What is the main purpose of multi-channel ADC scanning in embedded C?

easy

A. To read multiple analog inputs one after another automatically

B. To write digital output signals to multiple pins

C. To increase the clock speed of the microcontroller

D. To store data in external memory

Solution

1. Step 1: Understand ADC scanning concept

   Multi-channel ADC scanning reads several analog inputs sequentially without manual switching.

2. Step 2: Identify the purpose

   This allows efficient sensor data collection from multiple sources automatically.

3. Final Answer:

   To read multiple analog inputs one after another automatically -> Option A

4. Quick Check:

   Multi-channel ADC scanning = reading multiple analog inputs [OK]

Hint: Remember ADC scanning reads many inputs in sequence [OK]

Common Mistakes:

• Confusing ADC scanning with digital output control
• Thinking it speeds up the microcontroller clock
• Assuming it stores data externally

2.

Which of the following code snippets correctly initializes a multi-channel ADC scan for 3 channels in embedded C?

void ADC_Init() {
    ADCON0 = 0x01; // Enable ADC
    ADCON1 = 0x00; // Configure ports as analog
    ADCON2 = 0xA9; // Right justify, 12 TAD, Fosc/8
    // Missing code here
}

easy

A. ADCHS = 0x00; // Select channel 0 only

B. ADCHS = 0x03; // Select channels 0 and 1 only

C. ADCHS = 0xFF; // Select all channels

D. ADCHS = 0x07; // Select channels 0 to 2 for scanning

Solution

1. Step 1: Understand ADCHS register role

   ADCHS selects which ADC channels to scan; bits 0-2 correspond to channels 0-2.

2. Step 2: Choose correct bitmask for 3 channels

   0x07 in binary is 00000111, enabling channels 0,1,2 for scanning.

3. Final Answer:

   ADCHS = 0x07; // Select channels 0 to 2 for scanning -> Option D

4. Quick Check:

   0x07 = channels 0,1,2 selected [OK]

Hint: Use bitmask 0x07 to select first 3 ADC channels [OK]

Common Mistakes:

• Selecting only one channel instead of multiple
• Using 0xFF which may select invalid channels
• Confusing ADCHS with ADCON registers

3.

Consider the following embedded C code snippet for multi-channel ADC scanning:

int adc_values[3];
for(int ch = 0; ch < 3; ch++) {
    ADCON0 = (ch << 2) | 0x01; // Select channel and turn on ADC
    __delay_ms(2); // Acquisition time
    ADCON0 |= 0x02; // Start conversion
    while(ADCON0 & 0x02); // Wait for conversion
    adc_values[ch] = (ADRESH << 8) | ADRESL;
}

What will be the content of adc_values after this loop?

medium

A. Array with zeros because ADC is not started

B. Array with ADC readings from channels 0, 1, and 2

C. Array with same value repeated 3 times

D. Compilation error due to wrong bit operations

Solution

1. Step 1: Analyze loop operation

   The loop runs for ch = 0 to 2, selecting each ADC channel and starting conversion.

2. Step 2: Understand ADC reading storage

   After conversion, the 10-bit ADC result is stored in adc_values[ch] for each channel.

3. Final Answer:

   Array with ADC readings from channels 0, 1, and 2 -> Option B

4. Quick Check:

   Loop reads channels 0-2 storing results in array [OK]

Hint: Loop reads and stores ADC values channel-wise [OK]

Common Mistakes:

• Assuming ADC conversion never starts
• Thinking all values are identical
• Misreading bitwise operations as errors

4.

Identify the error in this multi-channel ADC scanning code snippet:

for(int i = 0; i < 3; i++) {
    ADCON0 = (i << 2); // Select channel
    ADCON0 |= 0x01; // Turn on ADC
    ADCON0 |= 0x02; // Start conversion
    while(ADCON0 & 0x02);
    adc_results[i] = (ADRESH << 8) | ADRESL;
}

medium

A. The ADC result is stored incorrectly

B. The conversion start bit is never set

C. ADCON0 is overwritten before turning on ADC, losing previous bits

D. The loop runs one time too many

Solution

1. Step 1: Check ADCON0 assignment

   ADCON0 is assigned with (i << 2), which clears other bits including ADC enable.

2. Step 2: Understand bitwise OR usage

   Turning on ADC with |= 0x01 works, but previous bits are lost due to overwrite.

3. Final Answer:

   ADCON0 is overwritten before turning on ADC, losing previous bits -> Option C

4. Quick Check:

   Overwriting ADCON0 clears important bits [OK]

Hint: Use |= to set bits, not = to overwrite [OK]

Common Mistakes:

• Ignoring that ADCON0 bits get reset
• Thinking start bit is not set
• Miscounting loop iterations
• Misunderstanding ADC result storage

5.

You want to scan 4 analog sensors using a microcontroller ADC that supports hardware scanning. Which approach best improves efficiency?

// Option 1: Manually switch channels in software loop
// Option 2: Enable hardware scan mode and read results from buffer
// Option 3: Use single channel repeatedly
// Option 4: Use digital inputs instead

hard

A. Use hardware scan mode and read results from buffer

B. Manually switch channels in software loop

C. Use single channel repeatedly with delays

D. Use digital inputs instead of ADC

Solution

1. Step 1: Compare manual vs hardware scanning

   Manual switching requires CPU cycles and delays; hardware scan automates channel switching.

2. Step 2: Identify efficiency gain

   Hardware scan mode reads multiple channels automatically, storing results in buffer for quick access.

3. Step 3: Evaluate other options

   Using single channel repeatedly or digital inputs does not meet multi-channel analog reading needs efficiently.

4. Final Answer:

   Use hardware scan mode and read results from buffer -> Option A

5. Quick Check:

   Hardware scan mode = efficient multi-channel ADC reading [OK]

Hint: Hardware scan automates multi-channel ADC reading [OK]

Common Mistakes:

• Choosing manual switching which wastes CPU time
• Using single channel repeatedly causing delays
• Confusing digital inputs with analog ADC inputs