Timer prescaler and clock division in Embedded C - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is a timer prescaler in embedded systems?

A timer prescaler is a hardware feature that divides the input clock frequency by a set value to slow down the timer counting speed. It helps to measure longer time intervals without changing the main clock.

beginner

How does clock division affect timer operation?

Clock division reduces the frequency of the clock signal fed to the timer by dividing it by a fixed number. This makes the timer count slower, allowing it to measure longer durations or lower frequencies.

intermediate

Why do we use prescalers instead of changing the main clock frequency?

Changing the main clock frequency affects the whole system and can cause instability. Using a prescaler only slows down the timer clock, keeping the rest of the system stable and running at the desired speed.

intermediate

In embedded C, how do you typically set a timer prescaler?

You set a timer prescaler by writing a value to a specific register of the timer peripheral. For example, setting bits in a TIMx_PSC register to divide the clock by a chosen factor.

beginner

If a timer input clock is 16 MHz and the prescaler is set to 7999, what is the timer counting frequency?

The timer frequency = 16 MHz / (7999 + 1) = 16 MHz / 8000 = 2000 Hz. The timer counts 2000 times per second.

What does a timer prescaler do?

A Divides the input clock frequency to slow down the timer

B Increases the timer counting speed

C Changes the main system clock frequency

D Resets the timer counter

A prescaler divides the input clock frequency, slowing down the timer counting speed.

Why is using a prescaler better than changing the main clock frequency?

A Changing main clock is easier

B Prescaler increases system speed

C Prescaler resets the timer

D Prescaler only affects the timer, keeping system stable

Using a prescaler affects only the timer clock, so the rest of the system remains stable.

If the main clock is 8 MHz and prescaler is 7999, what is the timer frequency?

A 8 kHz

B 1 kHz

C 1000 Hz

D 8000 Hz

Timer frequency = 8 MHz / (7999 + 1) = 8 MHz / 8000 = 1 kHz.

Which register is commonly used to set the prescaler in embedded C timers?

A TIMx_PSC

B TIMx_CNT

C TIMx_ARR

D TIMx_CR1

TIMx_PSC register holds the prescaler value for the timer.

What happens if you set the prescaler to 0?

A Timer counts twice as fast

B Timer stops counting

C Timer counts at full input clock speed

D Timer resets automatically

Prescaler 0 means divide by 1, so timer counts at full input clock speed.

Explain what a timer prescaler is and why it is useful in embedded systems.

Think about how slowing the timer clock helps measure time without changing the whole system speed.

Describe how you would calculate the timer counting frequency given the main clock and prescaler value.

Remember the prescaler divides the clock by (value + 1).

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