Timer prescaler and clock division in Embedded C - Mini Project: Build & Apply

Timer Prescaler and Clock Division

📖 Scenario: You are working with a microcontroller timer that counts pulses from a clock source. To control the timer speed, you use a prescaler that divides the clock frequency.Understanding how to set the prescaler and calculate the timer tick frequency is important for timing tasks like blinking an LED or measuring intervals.

🎯 Goal: Build a simple program that sets a timer prescaler and calculates the timer tick frequency based on the system clock and prescaler value.

📋 What You'll Learn

Create a variable for the system clock frequency in Hz

Create a variable for the timer prescaler value

Calculate the timer tick frequency by dividing the system clock by the prescaler

Print the timer tick frequency

💡 Why This Matters

🌍 Real World

Microcontrollers use timers with prescalers to create precise delays and measure time intervals in embedded systems like blinking LEDs, motor control, or communication protocols.

💼 Career

Understanding timer prescalers and clock division is essential for embedded software engineers working on hardware control, real-time systems, and device drivers.

1

Set the system clock frequency

Create an unsigned int variable called system_clock_hz and set it to 16000000 (16 MHz).

Embedded C

# Create an unsigned int variable called system_clock_hz and set it to 16000000
// Your code here

Hint

The system clock frequency is usually given in Hertz (Hz). 16 MHz means 16 million cycles per second.

2

Set the timer prescaler value

Create an unsigned int variable called prescaler and set it to 64.

Embedded C

unsigned int system_clock_hz = 16000000;
// Create an unsigned int variable called prescaler and set it to 64
// Your code here

Hint

The prescaler divides the system clock frequency to slow down the timer.

3

Calculate the timer tick frequency

Create an unsigned int variable called timer_tick_hz and set it to the result of dividing system_clock_hz by prescaler.

Embedded C

unsigned int system_clock_hz = 16000000;
unsigned int prescaler = 64;
// Calculate timer_tick_hz by dividing system_clock_hz by prescaler
// Your code here

Hint

Use the division operator / to calculate the timer tick frequency.

4

Print the timer tick frequency

Use printf to print the text "Timer tick frequency: " followed by the value of timer_tick_hz and then a newline.

Embedded C

unsigned int system_clock_hz = 16000000;
unsigned int prescaler = 64;
unsigned int timer_tick_hz = system_clock_hz / prescaler;
// Print the timer tick frequency
// Your code here

Hint

Use printf("Timer tick frequency: %u\n", timer_tick_hz); inside main().

Practice

1. What is the main purpose of a timer prescaler in embedded C programming?

easy

A. To generate an interrupt on every clock cycle

B. To increase the CPU clock speed

C. To divide the main clock frequency and slow down the timer counting

D. To reset the timer counter automatically

Solution

1. Step 1: Understand the role of a prescaler

   A prescaler divides the main clock frequency to slow down the timer's counting speed.

2. Step 2: Compare options with prescaler function

   Only To divide the main clock frequency and slow down the timer counting correctly describes this function; others describe unrelated timer features.

3. Final Answer:

   To divide the main clock frequency and slow down the timer counting -> Option C

4. Quick Check:

   Prescaler divides clock = To slow timer [OK]

Hint: Prescaler always divides clock to slow timer [OK]

Common Mistakes:

• Confusing prescaler with timer reset
• Thinking prescaler speeds up the clock
• Assuming prescaler triggers interrupts

2. Which of the following is the correct way to set a prescaler value of 8 for a timer in embedded C?

easy

A. TCCR0 |= (1 << CS01);

B. TCCR0 |= (1 << CS00);

C. TCCR0 |= (1 << CS02);

D. TCCR0 |= (1 << CS01) | (1 << CS00);

Solution

1. Step 1: Recall prescaler bits for timer0

   For prescaler 8, only bit CS01 must be set.

2. Step 2: Match bits to options

   TCCR0 |= (1 << CS01); sets CS01 bit, correctly configuring prescaler 8.

3. Final Answer:

   TCCR0 |= (1 << CS01); -> Option A

4. Quick Check:

   Prescaler 8 = CS01 set only [OK]

Hint: Check datasheet bits for prescaler values [OK]

Common Mistakes:

• Setting multiple bits for prescaler 8
• Confusing bit positions for prescaler values
• Using wrong register for timer control

3. Given a microcontroller clock of 16 MHz and a timer prescaler set to 64, what is the timer tick frequency?

medium

A. 1,000,000 Hz

B. 250,000 Hz

C. 16,000,000 Hz

D. 64 Hz

Solution

1. Step 1: Calculate timer tick frequency

   Timer tick frequency = CPU clock / prescaler = 16,000,000 / 64 = 250,000 Hz.

2. Step 2: Compare with options

   250,000 Hz matches the calculated frequency exactly.

3. Final Answer:

   250,000 Hz -> Option B

4. Quick Check:

   16MHz/64 = 250kHz [OK]

Hint: Divide clock by prescaler for tick frequency [OK]

Common Mistakes:

• Multiplying instead of dividing clock by prescaler
• Using wrong prescaler value
• Confusing frequency units

4. Identify the error in this code snippet setting a prescaler of 256 for Timer1:

  TCCR1B = (1 << CS12) | (1 << CS10);

medium

A. CS12 bit is missing

B. No error, code is correct

C. TCCR1B register is incorrect for prescaler

D. CS10 should not be set for prescaler 256

Solution

1. Step 1: Check prescaler bits for 256 on Timer1

   Prescaler 256 requires only CS12 bit set; CS10 must be cleared.

2. Step 2: Analyze given code

   Code sets both CS12 and CS10 bits, which corresponds to prescaler 1024, not 256.

3. Final Answer:

   CS10 should not be set for prescaler 256 -> Option D

4. Quick Check:

   Prescaler 256 = only CS12 set [OK]

Hint: Only set bits matching prescaler exactly [OK]

Common Mistakes:

• Setting extra bits causing wrong prescaler
• Confusing prescaler bit combinations
• Assuming code is correct without checking datasheet

5. You want to measure a 1-second delay using a 16-bit timer with a 16 MHz clock. Which prescaler value should you choose to make the timer count up to 1 second without overflow? (Timer max count = 65535)

hard

A. Prescaler 1024

B. Prescaler 64

C. Prescaler 128

D. Prescaler 8

Solution

1. Step 1: Calculate timer tick frequency for each prescaler

   Tick frequency = 16,000,000 / prescaler. For 1024: 16,000,000 / 1024 ≈ 15,625 Hz.

2. Step 2: Calculate max timer duration

   Max duration = max count / tick frequency = 65535 / 15,625 ≈ 4.19 seconds, which covers 1 second.

3. Step 3: Check smaller prescalers

   Prescaler 128 gives 125,000 ticks per second, 64 gives 250,000, and 8 gives 2,000,000--all exceeding 65,535, causing overflow before 1 second.

4. Final Answer:

   Prescaler 1024 -> Option A

5. Quick Check:

   Prescaler 1024 allows 1s count without overflow [OK]

Hint: Choose prescaler so max count ≥ ticks in 1 second [OK]

Common Mistakes:

• Ignoring timer max count limit
• Choosing prescaler too small causing overflow
• Not calculating ticks per second correctly