Low-power design patterns in Embedded C - Mini Project: Build & Apply

Low-power design patterns

📖 Scenario: You are working on a small embedded device that needs to save battery power. To do this, you want to use low-power design patterns like putting the device to sleep and waking it up only when needed.

🎯 Goal: Build a simple embedded C program that sets up a low-power mode, configures a wake-up event, and then enters sleep mode. When the device wakes up, it should print a message.

📋 What You'll Learn

Create a variable to represent device state

Define a wake-up threshold variable

Use a function to check if the device should wake up

Print a message when the device wakes up

💡 Why This Matters

🌍 Real World

Embedded devices like sensors and wearables need to save battery by sleeping and waking only when necessary.

💼 Career

Understanding low-power design patterns is important for embedded systems engineers working on battery-powered devices.

1

DATA SETUP: Create device state variable

Create an integer variable called device_state and set it to 0 to represent the device being awake.

Embedded C

# Create an integer variable called device_state and set it to 0
# Your code here

Hint

Use int device_state = 0; to create the variable.

2

CONFIGURATION: Define wake-up threshold

Create an integer variable called wake_up_threshold and set it to 5 to represent the condition to wake the device.

Embedded C

int device_state = 0;
// Create an integer variable called wake_up_threshold and set it to 5
# Your code here

Hint

Use int wake_up_threshold = 5; to create the threshold variable.

3

CORE LOGIC: Check wake-up condition

Write a function called check_wake_up that takes an integer sensor_value and returns 1 if sensor_value is greater than or equal to wake_up_threshold, otherwise returns 0. Then, call this function with 7 and assign the result to device_state.

Embedded C

int device_state = 0;
int wake_up_threshold = 5;

// Write check_wake_up function and call it with 7, assign result to device_state
# Your code here

Hint

Define the function with int check_wake_up(int sensor_value) and use an if-else to return 1 or 0.

4

OUTPUT: Print wake-up message

Write a printf statement that prints "Device awake!" if device_state is 1, otherwise print "Device sleeping.".

Embedded C

int device_state = 0;
int wake_up_threshold = 5;

int check_wake_up(int sensor_value) {
    if (sensor_value >= wake_up_threshold) {
        return 1;
    } else {
        return 0;
    }
}

device_state = check_wake_up(7);

// Print "Device awake!" if device_state is 1, else print "Device sleeping."
# Your code here

Hint

Use if (device_state == 1) and printf("Device awake!\n").

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