Generating precise delays with timers in Embedded C - Mini Project: Build & Apply

Generating precise delays with timers

📖 Scenario: You are programming a microcontroller to control an LED. To make the LED blink at a precise interval, you need to create a delay using a hardware timer.

🎯 Goal: Build a program that sets up a timer to generate a precise delay of 1 second, then toggles an LED.

📋 What You'll Learn

Create a variable to hold the timer count value

Define a constant for the timer reload value for 1 second delay

Write a function to initialize the timer with the reload value

Write a loop that waits for the timer flag and toggles the LED

Print or output the LED state change (simulated)

💡 Why This Matters

🌍 Real World

Precise timing is essential in embedded systems to control hardware like LEDs, motors, or sensors accurately.

💼 Career

Embedded software engineers often write timer-based delays to manage hardware timing and synchronization.

1

Set up the timer count variable

Create an unsigned int variable called timer_count and set it to 0.

Embedded C

# Your code here

Hint

Use unsigned int timer_count = 0; to create the variable.

2

Define the timer reload value

Define a constant #define TIMER_RELOAD 1000 to represent the timer reload value for a 1 second delay.

Embedded C

unsigned int timer_count = 0;
# Your code here

Hint

Use #define TIMER_RELOAD 1000 to set the reload value.

3

Write the timer initialization function

Write a function called init_timer() that sets timer_count to TIMER_RELOAD.

Embedded C

unsigned int timer_count = 0;
#define TIMER_RELOAD 1000

// Your code here

Hint

Define void init_timer() and inside set timer_count = TIMER_RELOAD;.

4

Create the main loop to wait and toggle LED

Write a main() function that calls init_timer(), then uses a while loop to decrement timer_count until it reaches 0, then prints "LED toggled" and resets timer_count to TIMER_RELOAD. Repeat this 3 times.

Embedded C

unsigned int timer_count = 0;
#define TIMER_RELOAD 1000

void init_timer() {
    timer_count = TIMER_RELOAD;
}

// Your code here

Hint

Use nested loops: outer loop runs 3 times, inner loop decrements timer_count to 0, then print and reset.

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