Generating precise delays with timers in Embedded C - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to start the timer.

Embedded C

TIM2->CR1 |= [1];

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

ATIM_CR1_DIR

BTIM_CR1_CEN

CTIM_CR1_UDIS

DTIM_CR1_OPM

Attempts:

3 left

💡 Hint

Common Mistakes

Using TIM_CR1_UDIS disables the timer update event.

Using TIM_CR1_DIR changes the counting direction.

Using TIM_CR1_OPM sets one pulse mode instead of starting timer.

Setting TIM_CR1_CEN bit starts the timer counter.

2fill in blank

medium

Complete the code to set the prescaler value for the timer.

Embedded C

TIM3->PSC = [1] - 1;

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

Aprescaler_value

Bcounter_value

Cdelay_time

Dtimer_frequency

Attempts:

3 left

💡 Hint

Common Mistakes

Using delay_time instead of prescaler_value.

Setting counter_value in PSC register.

Confusing timer_frequency with prescaler_value.

The prescaler divides the timer clock frequency by (PSC+1).

3fill in blank

hard

Fix the error in the code to wait until the timer update event occurs.

Embedded C

while (!(TIM4->SR & [1])) {}

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

ATIM_SR_CC1IF

BTIM_SR_TIF

CTIM_SR_CC2IF

DTIM_SR_UIF

Attempts:

3 left

💡 Hint

Common Mistakes

Using capture/compare flags instead of update flag.

Using undefined or incorrect flag bits.

TIM_SR_UIF is the update interrupt flag indicating timer overflow.

4fill in blank

hard

Fill both blanks to configure the timer for a 1s delay assuming 16MHz clock.

Embedded C

TIM5->PSC = [1] - 1;
TIM5->ARR = [2] - 1;

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

A16000

B1000

C1600

D100

Attempts:

3 left

💡 Hint

Common Mistakes

Swapping prescaler and ARR values.

Using wrong values that don't match clock frequency.

Prescaler divides 16MHz to 1kHz (16000), ARR counts 1000 ticks for 1s delay.

5fill in blank

hard

Fill all three blanks to create a delay function using TIM6 with prescaler and auto-reload values.

Embedded C

void delay_ms(uint16_t ms) {
    TIM6->PSC = [1] - 1;
    TIM6->ARR = [2] * ms - 1;
    TIM6->EGR = [3];
    TIM6->SR = 0;
    TIM6->CR1 |= TIM_CR1_CEN;
    while (!(TIM6->SR & TIM_SR_UIF)) {}
}

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

A16000

B1

CTIM_EGR_UG

D1000

Attempts:

3 left

💡 Hint

Common Mistakes

Not generating update event after changing PSC and ARR.

Using wrong multiplier for ARR.

Forgetting to clear status register before starting timer.

Prescaler divides 16MHz to 1kHz (16000), ARR counts ms ticks, EGR_UG generates update event.

Practice

1. What is the main purpose of using timers in embedded C programming for delays?

easy

A. To create precise and accurate wait times without blocking the CPU

B. To increase the CPU clock speed automatically

C. To store large amounts of data temporarily

D. To handle user input events directly

Solution

1. Step 1: Understand timer functionality

   Timers count clock pulses to measure time intervals precisely without busy-waiting.

2. Step 2: Identify timer use for delays

   Using timers allows the program to wait exact times while freeing CPU for other tasks.

3. Final Answer:

   To create precise and accurate wait times without blocking the CPU -> Option A

4. Quick Check:

   Timers = precise delays [OK]

Hint: Timers count clock pulses for exact delays [OK]

Common Mistakes:

• Thinking timers speed up the CPU
• Confusing timers with memory storage
• Assuming timers handle user input

2. Which of the following is the correct way to start a hardware timer in embedded C?

easy

A. beginTimer();

B. start_timer();

C. timer_start();

D. initTimer();

Solution

1. Step 1: Recognize common timer function names

   Standard embedded C libraries often use timer_start() to begin timers.

2. Step 2: Check syntax correctness

   timer_start(); is a valid function call matching typical timer APIs.

3. Final Answer:

   timer_start(); -> Option C

4. Quick Check:

   Correct timer start syntax = timer_start() [OK]

Hint: Look for standard function names like timer_start() [OK]

Common Mistakes:

• Using incorrect function names like start_timer()
• Confusing initialization with starting
• Missing parentheses in function calls

3. Given the code snippet:

timer_set(1000); // Set timer for 1000 ms
timer_start();
while(!timer_expired()) {
  // wait
}
LED_toggle();

What happens after this code runs?

medium

A. LED toggles immediately without delay

B. LED toggles after approximately 1 second delay

C. LED never toggles because timer_expired() is always false

D. LED toggles after 1000 CPU cycles, not milliseconds

Solution

1. Step 1: Understand timer_set and timer_start

   The timer is set for 1000 milliseconds and then started to count down.

2. Step 2: Analyze the while loop waiting for timer expiration

   The loop waits until timer_expired() returns true, which happens after ~1 second.

3. Final Answer:

   LED toggles after approximately 1 second delay -> Option B

4. Quick Check:

   timer_set(1000) = 1 second delay [OK]

Hint: timer_set(ms) delays for ms milliseconds [OK]

Common Mistakes:

• Assuming timer_expired() never becomes true
• Confusing CPU cycles with milliseconds
• Thinking LED toggles immediately

4. Identify the error in this delay code:

timer_set(500);
while(timer_expired()) {
  // wait
}
LED_on();

medium

A. timer_expired() function does not exist

B. timer_set() should be called after the while loop

C. LED_on() should be called before timer_set()

D. The while condition should be !timer_expired() to wait correctly

Solution

1. Step 1: Analyze the while loop condition

   The loop waits while timer_expired() is true, but timer just set, so expired is false initially.

2. Step 2: Correct the condition to wait until timer expires

   It should wait while timer is NOT expired, so condition must be negated: !timer_expired().

3. Final Answer:

   The while condition should be !timer_expired() to wait correctly -> Option D

4. Quick Check:

   Wait while not expired = !timer_expired() [OK]

Hint: Wait while timer not expired: use !timer_expired() [OK]

Common Mistakes:

• Using timer_expired() without negation in wait loop
• Calling timer_set() after waiting
• Assuming LED_on() order affects delay

5. You want to toggle an LED every 250 ms precisely using a timer interrupt. Which approach is best?

hard

A. Use a timer interrupt set for 250 ms to toggle LED inside ISR

B. Use a busy-wait loop with delay function for 250 ms repeatedly

C. Toggle LED once and never use timer

D. Use a timer interrupt but toggle LED outside ISR only

Solution

1. Step 1: Understand precise timing with interrupts

   Using timer interrupts allows exact timing without blocking CPU.

2. Step 2: Identify best place to toggle LED

   Toggling LED inside the timer ISR ensures it happens exactly every 250 ms.

3. Final Answer:

   Use a timer interrupt set for 250 ms to toggle LED inside ISR -> Option A

4. Quick Check:

   Timer ISR toggles LED precisely every 250 ms [OK]

Hint: Toggle LED inside timer ISR for precise periodic delay [OK]

Common Mistakes:

• Using busy-wait loops causing CPU blocking
• Toggling LED outside ISR losing timing precision
• Not using timer interrupts for periodic tasks