Low-power design patterns in Embedded C - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to enter low-power sleep mode.

Embedded C

void enter_sleep_mode() {
    __[1]();
}

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

AWFI

BNOP

CSVC

DWFE

Attempts:

3 left

💡 Hint

Common Mistakes

Using NOP does not put the CPU to sleep.

Using SVC triggers a supervisor call, not sleep.

WFE waits for an event, but WFI is more common for sleep.

The WFI instruction puts the processor into sleep mode until an interrupt occurs, saving power.

2fill in blank

medium

Complete the code to disable the peripheral clock to save power.

Embedded C

void disable_peripheral_clock() {
    RCC->APB1ENR &= ~[1];
}

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

ARCC_APB1ENR_TIM2EN

BRCC_APB1ENR_I2C1EN

CRCC_APB1ENR_USART1EN

DRCC_APB1ENR_SPI2EN

Attempts:

3 left

💡 Hint

Common Mistakes

Disabling the wrong peripheral clock.

Using the wrong register bit mask.

Disabling the I2C1 peripheral clock saves power when the I2C1 peripheral is not used.

3fill in blank

hard

Fix the error in the code to correctly configure the microcontroller to enter STOP mode.

Embedded C

void enter_stop_mode() {
    PWR->CR |= PWR_CR_LPDS;
    PWR->CR &= ~[1];
    SCB->SCR |= SCB_SCR_SLEEPDEEP_Msk;
    __WFI();
}

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

APWR_CR_ULP

BPWR_CR_CWUF

CPWR_CR_LPSDSR

DPWR_CR_PDDS

Attempts:

3 left

💡 Hint

Common Mistakes

Clearing the wrong bit causes the MCU to enter STANDBY mode.

Not setting SLEEPDEEP bit prevents deep sleep.

Clearing PWR_CR_PDDS bit configures the device to enter STOP mode instead of STANDBY mode.

4fill in blank

hard

Complete the code to create a dictionary comprehension that maps sensor names to their power states only if the sensor is active.

Embedded C

sensor_power = {name: state for name, state in sensors.items() if state [1] 1}

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

B==

C!=

D>

Attempts:

3 left

💡 Hint

Common Mistakes

Adding an operator after state in the value expression.

Using != or > instead of == in the condition.

The comprehension includes sensors where state == 1, meaning active sensors. No operator is needed after state in the value expression.

5fill in blank

hard

Fill all three blanks to create a dictionary comprehension that maps device IDs (uppercase) to their power levels only if the level is above zero.

Embedded C

power_levels = [1]: [2] for id, level in devices.items() if level [3] 0}

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

Aid.upper()

Blevel

C>

Did.lower()

Attempts:

3 left

💡 Hint

Common Mistakes

Using id.lower() instead of uppercase.

Using < or == instead of >.

Swapping keys and values in the comprehension.

The comprehension maps uppercase device IDs to their power levels only if the level is greater than zero.

Practice

1. Which of the following best describes the purpose of low-power design patterns in embedded C programming?

easy

A. To reduce battery consumption by putting the CPU and peripherals into sleep modes

B. To increase the clock speed of the CPU for faster processing

C. To add more features to the embedded device

D. To improve the graphical user interface responsiveness

Solution

1. Step 1: Understand low-power design patterns

   These patterns focus on saving battery by reducing power use in the CPU and peripherals.

2. Step 2: Compare options to the purpose

   Only To reduce battery consumption by putting the CPU and peripherals into sleep modes talks about reducing battery consumption by sleep modes, which matches the purpose.

3. Final Answer:

   To reduce battery consumption by putting the CPU and peripherals into sleep modes -> Option A

4. Quick Check:

   Low-power design = save battery by sleep modes [OK]

Hint: Low-power means saving battery by sleeping parts [OK]

Common Mistakes:

• Confusing low-power with performance improvement
• Thinking it adds features instead of saving power
• Assuming it improves UI responsiveness

2. Which of the following is the correct way to put the CPU into sleep mode using a standard instruction in embedded C?

easy

A. enter_sleep_mode();

B. sleep();

C. power_down();

D. __WFI();

Solution

1. Step 1: Identify the standard CPU sleep instruction

   The __WFI() instruction is a common CPU instruction to wait for interrupt and enter sleep mode safely.

2. Step 2: Check other options

   Other options like sleep(), power_down(), enter_sleep_mode() are not standard embedded C instructions.

3. Final Answer:

   __WFI(); -> Option D

4. Quick Check:

   Use __WFI() to sleep CPU [OK]

Hint: Use __WFI() to enter CPU sleep mode [OK]

Common Mistakes:

• Using non-standard function names
• Forgetting the double underscore in __WFI()
• Confusing sleep() with standard CPU instructions

3. Consider the following code snippet in embedded C:

void enter_low_power() {
    disable_peripherals();
    __WFI();
    enable_peripherals();
}

What is the expected behavior when this function runs?

medium

A. The CPU sleeps until an interrupt occurs, peripherals are disabled before sleep and enabled after wake-up

B. The CPU runs continuously without sleeping, peripherals remain disabled

C. The CPU sleeps but peripherals stay enabled, causing high power use

D. The function causes a syntax error and does not compile

Solution

1. Step 1: Analyze the function steps

   First, peripherals are disabled to save power, then __WFI() puts CPU to sleep waiting for interrupt, finally peripherals are re-enabled after waking.

2. Step 2: Understand expected behavior

   The CPU sleeps until an interrupt wakes it, peripherals are off during sleep to save power, then turned back on.

3. Final Answer:

   The CPU sleeps until an interrupt occurs, peripherals are disabled before sleep and enabled after wake-up -> Option A

4. Quick Check:

   Disable peripherals + __WFI() = sleep with low power [OK]

Hint: Disable peripherals before __WFI() to save power during sleep [OK]

Common Mistakes:

• Assuming peripherals stay enabled during sleep
• Thinking CPU does not sleep after __WFI()
• Confusing function behavior with syntax errors

4. The following code is intended to put the CPU into low-power mode, but it does not work as expected:

void sleep_mode() {
    __WFI;
}

What is the error and how to fix it?

medium

A. The function should disable peripherals before __WFI; add disable_peripherals();

B. __WFI is not a valid instruction; replace with sleep();

C. __WFI is missing parentheses; fix by writing __WFI();

D. The function needs a return statement; add return 0;

Solution

1. Step 1: Identify syntax error in __WFI usage

   The code uses __WFI without parentheses, which is incorrect syntax for a function-like instruction.

2. Step 2: Correct the syntax

   Add parentheses to call __WFI as a function: __WFI(); fixes the error.

3. Final Answer:

   __WFI is missing parentheses; fix by writing __WFI(); -> Option C

4. Quick Check:

   __WFI() needs parentheses to call [OK]

Hint: Always use parentheses with __WFI() instruction [OK]

Common Mistakes:

• Omitting parentheses on __WFI
• Replacing __WFI with non-standard functions
• Ignoring peripheral disable step (not syntax error here)

5. You want to design a low-power routine that puts the CPU to sleep only if no peripherals are active. Given the functions are_peripherals_active() returning 1 if active, 0 if not, and __WFI() to sleep, which code snippet correctly implements this logic?

hard

A. if (are_peripherals_active()) { __WFI(); }

B. if (!are_peripherals_active()) { __WFI(); }

C. while (are_peripherals_active()) { __WFI(); }

D. do { __WFI(); } while (are_peripherals_active());

Solution

1. Step 1: Understand the condition to sleep

   The CPU should sleep only if no peripherals are active, so condition is peripherals inactive (0).

2. Step 2: Match condition with code

   Using if (!are_peripherals_active()) means if peripherals are not active, then call __WFI() to sleep.

3. Final Answer:

   if (!are_peripherals_active()) { __WFI(); } -> Option B

4. Quick Check:

   Sleep only if peripherals inactive = if (!active) [OK]

Hint: Sleep only when peripherals inactive: use if (!are_peripherals_active()) [OK]

Common Mistakes:

• Sleeping when peripherals are active
• Using loops causing repeated sleep calls
• Confusing condition negation