Multi-channel ADC scanning in Embedded C - Step-by-Step Execution

Concept Flow - Multi-channel ADC scanning

Start ADC Scan

↓

Select Channel 0

↓

Start Conversion

↓

Wait for Conversion Complete

↓

Read ADC Value

↓

Store Value

↓

More Channels?

Yes→Select Next Channel

No +---> Back to Start Conversion↓

End Scan

↩No +---> Back to Start Conversion

The ADC scan starts by selecting each channel one by one, converting, reading the value, storing it, and moving to the next channel until all are done.

Execution Sample

Embedded C

for (int ch = 0; ch < 3; ch++) {
    ADC_SelectChannel(ch);
    ADC_StartConversion();
    while (!ADC_ConversionComplete());
    values[ch] = ADC_ReadValue();
}

This code scans 3 ADC channels, reads their values one by one, and stores them in an array.

Execution Table

Iteration	Channel Selected	Conversion Started	Conversion Complete?	Value Read	Stored Value
1	0	Yes	Yes	512	values[0] = 512
2	1	Yes	Yes	678	values[1] = 678
3	2	Yes	Yes	345	values[2] = 345
4	-	-	-	-	Loop ends, all channels scanned

💡 Loop ends after channel 2, condition ch < 3 becomes false

Variable Tracker

Variable	Start	After 1	After 2	After 3	Final
ch	0	1	2	3	3
values[0]	undefined	512	512	512	512
values[1]	undefined	undefined	678	678	678
values[2]	undefined	undefined	undefined	345	345

Key Moments - 3 Insights

Why do we wait for conversion to complete before reading the value?

Because ADC conversion takes time, reading before completion gives wrong data. See execution_table rows where 'Conversion Complete?' is 'Yes' before reading.

Why does the loop stop after channel 2 and not continue?

The loop condition is ch < 3, so when ch becomes 3, it stops. See variable_tracker where ch final is 3 and exit_note in execution_table.

Why do we store the value in an array indexed by channel?

To keep each channel's reading separate and accessible later. See execution_table 'Stored Value' column showing values stored per channel.

Visual Quiz - 3 Questions

Look at the execution_table, what is the stored value for channel 1 at iteration 2?

A 512

B 345

C 678

D undefined

💡 Hint

Check the 'Stored Value' column at iteration 2 in execution_table

At which iteration does the variable 'ch' become 3?

A After iteration 3

B After iteration 2

C After iteration 1

D Never

💡 Hint

Look at variable_tracker row for 'ch' and see when it reaches 3

If we change the loop to ch < 4, how many iterations will the execution_table show?

A 3

B 4

C 5

D 2

💡 Hint

The loop runs while ch is less than the limit, so increasing limit to 4 adds one more iteration

Concept Snapshot

Multi-channel ADC scanning:
- Loop through each ADC channel
- Select channel, start conversion
- Wait until conversion done
- Read and store value
- Repeat for all channels
- Stops when all channels scanned

Full Transcript

This example shows how to scan multiple ADC channels one by one. The program loops over channels 0 to 2. For each channel, it selects the channel, starts the ADC conversion, waits until the conversion is complete, reads the ADC value, and stores it in an array. The loop stops after scanning all channels. Variables like 'ch' track the current channel, and 'values' array stores the readings. Waiting for conversion completion is important to get correct data. The execution table shows each step clearly, and the variable tracker shows how variables change after each iteration.

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