Clock gating for power saving in Embedded C - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to enable clock gating for the peripheral.

Embedded C

CLK_CTRL_REG |= [1];  // Enable clock gating

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

ACLK_GATE_DISABLE

BCLK_GATE_ENABLE

CCLK_ALWAYS_ON

DCLK_RESET

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing the disable or reset option instead of enable.

Setting CLK_GATE_ENABLE enables clock gating, which saves power by stopping the clock to unused peripherals.

2fill in blank

medium

Complete the code to disable clock gating for the peripheral.

Embedded C

CLK_CTRL_REG &= ~[1];  // Disable clock gating

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

ACLK_GATE_ENABLE_BIT

BCLK_GATE_ENABLE

CCLK_GATE_ENABLE_MASK

DCLK_GATE_DISABLE

Attempts:

3 left

💡 Hint

Common Mistakes

Using the enable constant instead of the bit mask.

Using CLK_GATE_ENABLE_BIT with bitwise NOT disables the clock gating by clearing the bit.

3fill in blank

hard

Fix the error in the clock gating control code.

Embedded C

if ((CLK_CTRL_REG & [1]) == 0) {
    enable_clock_gating();
}

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

ACLK_GATE_DISABLE_BIT

BCLK_GATE_RESET_BIT

CCLK_GATE_STATUS_BIT

DCLK_GATE_ENABLE_BIT

Attempts:

3 left

💡 Hint

Common Mistakes

Checking the disable or reset bit instead of the enable bit.

The code checks if the clock gating enable bit is cleared (0) to enable clock gating. So it must check CLK_GATE_ENABLE_BIT.

4fill in blank

hard

Fill both blanks to create a clock gating control macro.

Embedded C

#define SET_CLOCK_GATING(reg, bit) \
    do { \
        (reg) [1]= (bit); \
    } while(0)

#define CLEAR_CLOCK_GATING(reg, bit) \
    do { \
        (reg) [2]= (bit); \
    } while(0)

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

A|

B&

C~

D^

Attempts:

3 left

💡 Hint

Common Mistakes

Using AND to set bits or OR to clear bits.

To set a bit, use bitwise OR |=. To clear a bit, use bitwise AND with the bitwise NOT &= ~. Here, CLEAR_CLOCK_GATING uses &= and expects the bit to be complemented before passing.

5fill in blank

hard

Fill all three blanks to implement a function that gates clock based on enable flag.

Embedded C

void clock_gate_control(volatile uint32_t *reg, uint32_t bit, bool enable) {
    if (enable) {
        *reg [1]= bit;  // Enable clock gating
    } else {
        *reg [2]= [3] bit;  // Disable clock gating
    }
}

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

A|=

B&=

C~

D^=

Attempts:

3 left

💡 Hint

Common Mistakes

Using XOR or other operators incorrectly for bit setting/clearing.

To enable clock gating, set the bit with OR assignment |=. To disable, clear the bit by AND assignment &= with the complement ~ of the bit.

Practice

1. What is the main purpose of clock gating in embedded systems?

easy

A. To enable all hardware modules simultaneously

B. To increase the clock speed of the processor

C. To reset the microcontroller

D. To save power by turning off clocks to unused modules

Solution

1. Step 1: Understand clock gating concept

   Clock gating disables the clock signal to parts of a chip that are not in use.

2. Step 2: Identify the main benefit

   Disabling clocks reduces power consumption and heat generation.

3. Final Answer:

   To save power by turning off clocks to unused modules -> Option D

4. Quick Check:

   Clock gating = power saving [OK]

Hint: Clock gating means stopping clocks to save power [OK]

Common Mistakes:

• Thinking clock gating speeds up the processor
• Confusing clock gating with resetting hardware
• Assuming clock gating enables all modules

2. Which of the following is the correct way to enable clock gating for a peripheral in embedded C?

easy

A. CLK_GATE_REG |= (1 << PERIPHERAL_BIT);

B. CLK_GATE_REG &= ~(1 << PERIPHERAL_BIT);

C. CLK_GATE_REG = 0;

D. CLK_GATE_REG ^= (1 << PERIPHERAL_BIT);

Solution

1. Step 1: Understand bit setting for enabling clock gating

   Setting a bit (using |=) enables clock gating for that peripheral.

2. Step 2: Analyze each option

   CLK_GATE_REG |= (1 << PERIPHERAL_BIT); sets the bit; CLK_GATE_REG &= ~(1 << PERIPHERAL_BIT); clears it; CLK_GATE_REG = 0; clears all bits; CLK_GATE_REG ^= (1 << PERIPHERAL_BIT); toggles bits.

3. Final Answer:

   CLK_GATE_REG |= (1 << PERIPHERAL_BIT); -> Option A

4. Quick Check:

   Set bit to enable clock gating [OK]

Hint: Use |= to set bits for enabling clock gating [OK]

Common Mistakes:

• Using &=~ which clears bits instead of setting
• Assigning 0 clears all bits
• Using ^= toggles bits unpredictably

3. Given the code snippet:

uint32_t CLK_GATE_REG = 0x0;
#define PERIPH1_BIT 2
CLK_GATE_REG |= (1 << PERIPH1_BIT);
CLK_GATE_REG &= ~(1 << PERIPH1_BIT);

What is the final value of CLK_GATE_REG?

medium

A. 0x4

B. 0x0

C. 0x2

D. 0x6

Solution

1. Step 1: Set bit 2 in CLK_GATE_REG

   CLK_GATE_REG |= (1 << 2) sets bit 2, so value becomes 0x4.

2. Step 2: Clear bit 2 in CLK_GATE_REG

   CLK_GATE_REG &= ~(1 << 2) clears bit 2, so value returns to 0x0.

3. Final Answer:

   0x0 -> Option B

4. Quick Check:

   Set then clear bit 2 = 0x0 [OK]

Hint: Set then clear same bit results in zero [OK]

Common Mistakes:

• Forgetting that clearing bit resets value
• Confusing bit positions
• Assuming bits accumulate without clearing

4. Identify the error in this clock gating code snippet:

uint32_t CLK_GATE_REG = 0xFF;
#define PERIPH2_BIT 3
CLK_GATE_REG = (1 << PERIPH2_BIT);

medium

A. It toggles bit 3 instead of setting it

B. It correctly enables clock gating for peripheral 2

C. It clears all bits except bit 3, disabling other clocks

D. It causes a syntax error due to missing semicolon

Solution

1. Step 1: Analyze the assignment operation

   Assigning CLK_GATE_REG = (1 << 3) overwrites previous bits, clearing all except bit 3.

2. Step 2: Understand impact on clock gating

   This disables clock gating for other peripherals unintentionally.

3. Final Answer:

   It clears all bits except bit 3, disabling other clocks -> Option C

4. Quick Check:

   Direct assignment clears other bits [OK]

Hint: Use |= to set bits, not = to avoid clearing others [OK]

Common Mistakes:

• Using = instead of |= to set bits
• Ignoring impact on other bits
• Thinking this toggles bits

5. You want to save power by gating clocks to peripherals only when they are idle. Which approach correctly implements this in embedded C?

void update_clock_gating(bool peripheral_active) {
    if (peripheral_active) {
        CLK_GATE_REG &= ~(1 << PERIPH_BIT); // Disable gating
    } else {
        CLK_GATE_REG |= (1 << PERIPH_BIT);  // Enable gating
    }
}

hard

A. This code correctly enables gating when peripheral is idle

B. This code enables gating when peripheral is active, which wastes power

C. This code disables gating in both cases, no power saving

D. This code causes syntax errors due to missing variable declarations

Solution

1. Step 1: Understand gating logic in code

   When peripheral_active is true, gating is disabled (clock runs). When false, gating is enabled (clock off).

2. Step 2: Confirm power saving behavior

   Enabling gating only when peripheral is idle saves power correctly.

3. Final Answer:

   This code correctly enables gating when peripheral is idle -> Option A

4. Quick Check:

   Gating enabled when idle = power saving [OK]

Hint: Enable gating only when peripheral is idle [OK]

Common Mistakes:

• Reversing active and idle logic
• Forgetting to use |= and &=~ correctly
• Assuming code has syntax errors without checking