SysTick timer for RTOS tick in ARM Architecture - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to enable the SysTick timer interrupt.

ARM Architecture

SysTick->CTRL |= [1];

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

ASysTick_CTRL_COUNTFLAG_Msk

BSysTick_CTRL_CLKSOURCE_Msk

CSysTick_CTRL_ENABLE_Msk

DSysTick_CTRL_TICKINT_Msk

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing the ENABLE bit with the interrupt enable bit.

Using the clock source bit instead of the interrupt enable bit.

The SysTick interrupt is enabled by setting the TICKINT bit in the CTRL register.

2fill in blank

medium

Complete the code to set the SysTick reload value for a 1ms tick assuming a 48 MHz clock.

ARM Architecture

SysTick->LOAD = [1] - 1;

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

A48000

B480000

C4800

D480

Attempts:

3 left

💡 Hint

Common Mistakes

Using 4800 instead of 48000, confusing kHz with MHz.

Forgetting to subtract 1 from the reload value.

For a 48 MHz clock, 1 ms corresponds to 48000 clock cycles (48,000,000 / 1000).

3fill in blank

hard

Fix the error in the code to start the SysTick timer with processor clock as source.

ARM Architecture

SysTick->CTRL = [1] | SysTick_CTRL_TICKINT_Msk | SysTick_CTRL_ENABLE_Msk;

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

ASysTick_CTRL_COUNTFLAG_Msk

BSysTick_CTRL_ENABLE_Msk

CSysTick_CTRL_CLKSOURCE_Msk

DSysTick_CTRL_TICKINT_Msk

Attempts:

3 left

💡 Hint

Common Mistakes

Using ENABLE or TICKINT bits instead of CLKSOURCE for clock selection.

Using COUNTFLAG which is a status bit, not a control bit.

The CLKSOURCE bit selects the processor clock as the SysTick clock source.

4fill in blank

hard

Fill both blanks to create a SysTick handler function that increments the RTOS tick count and clears the interrupt flag.

ARM Architecture

void SysTick_Handler(void) {
    [1]();
    [2] = 0;
}

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ARTOS_Tick_Handler

BSysTick->CTRL

CSysTick->VAL

DRTOS_Tick_Count

Attempts:

3 left

💡 Hint

Common Mistakes

Trying to clear the CTRL register instead of VAL.

Calling a wrong function name or missing the function call.

The RTOS tick handler function is called to update the OS tick count. Clearing the SysTick current value register (VAL) resets the timer.

5fill in blank

hard

Fill all three blanks to configure SysTick for a 1ms RTOS tick with interrupt enabled and processor clock source.

ARM Architecture

SysTick->LOAD = [1] - 1;
SysTick->VAL = 0;
SysTick->CTRL = [2] | [3] | SysTick_CTRL_ENABLE_Msk;

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

A48000

BSysTick_CTRL_TICKINT_Msk

CSysTick_CTRL_CLKSOURCE_Msk

DSysTick_CTRL_COUNTFLAG_Msk

Attempts:

3 left

💡 Hint

Common Mistakes

Using COUNTFLAG bit which is read-only status.

Forgetting to clear VAL before starting.

The reload value is set for 1ms ticks at 48 MHz. The interrupt and clock source bits are set to enable SysTick interrupt and select processor clock.

Practice

1. What is the main purpose of the SysTick timer in an RTOS?

easy

A. To generate regular interrupts for task switching

B. To increase CPU clock speed

C. To store data temporarily

D. To manage memory allocation

Solution

1. Step 1: Understand SysTick timer role

   The SysTick timer is designed to create periodic interrupts.

2. Step 2: Connect interrupts to RTOS function

   These interrupts allow the RTOS to switch tasks regularly.

3. Final Answer:

   To generate regular interrupts for task switching -> Option A

4. Quick Check:

   SysTick timer = periodic interrupts for RTOS [OK]

Hint: SysTick timer triggers interrupts for switching tasks [OK]

Common Mistakes:

• Confusing SysTick with CPU clock speed control
• Thinking SysTick stores data
• Assuming SysTick manages memory

2. Which of the following is the correct way to enable the SysTick timer interrupt in ARM Cortex-M?

easy

A. Write zero to the RELOAD register

B. Set the TICKINT bit in the SysTick CTRL register

C. Clear the ENABLE bit in the SysTick CTRL register

D. Disable the NVIC interrupt controller

Solution

1. Step 1: Identify the control register bits

   The SysTick CTRL register controls timer and interrupt enabling.

2. Step 2: Enable interrupt by setting TICKINT bit

   Setting the TICKINT bit enables SysTick interrupts.

3. Final Answer:

   Set the TICKINT bit in the SysTick CTRL register -> Option B

4. Quick Check:

   Enable interrupt = set TICKINT bit [OK]

Hint: Enable SysTick interrupt by setting TICKINT bit [OK]

Common Mistakes:

• Clearing ENABLE disables timer, not interrupt
• Writing zero to RELOAD stops timer from counting
• Disabling NVIC stops all interrupts, not just SysTick

3. Given a CPU clock of 48 MHz and a desired RTOS tick interval of 1 ms, what reload value should be set in the SysTick timer?

medium

A. 47999

B. 480

C. 48000

D. 47

Solution

1. Step 1: Calculate ticks per 1 ms interval

   CPU clock = 48,000,000 Hz means 48,000 ticks per 1 ms (since 1 ms = 0.001 s).

2. Step 2: Determine reload value

   Reload value = ticks per interval - 1 = 48000 - 1 = 47999.

3. Final Answer:

   47999 -> Option A

4. Quick Check:

   Reload = (CPU clock * tick interval) - 1 = 47999 [OK]

Hint: Reload = (CPU clock x interval) - 1 [OK]

Common Mistakes:

• Using CPU clock directly without subtracting 1
• Confusing microseconds with milliseconds
• Using too small reload value causing faster ticks

4. A developer sets the SysTick reload value to 0 and enables the timer and interrupt. What issue will occur?

medium

A. System will reset immediately

B. SysTick will never generate interrupts

C. CPU clock speed will increase

D. SysTick will generate interrupts continuously without delay

Solution

1. Step 1: Understand reload value effect

   A reload value of 0 means the timer counts from 0 to 0, triggering interrupt immediately.

2. Step 2: Resulting behavior

   This causes continuous interrupts with no delay, overwhelming the system.

3. Final Answer:

   SysTick will generate interrupts continuously without delay -> Option D

4. Quick Check:

   Reload=0 causes continuous interrupts [OK]

Hint: Reload 0 means no delay, interrupts fire nonstop [OK]

Common Mistakes:

• Assuming no interrupts occur with reload 0
• Thinking CPU clock changes due to SysTick
• Expecting system reset from this setting

5. In an RTOS using SysTick for task switching, if the CPU clock changes dynamically (e.g., due to power saving), what must be done to maintain correct tick timing?

hard

A. Disable SysTick and rely on software delays

B. Keep the reload value fixed regardless of clock changes

C. Update the SysTick reload value to match the new CPU clock frequency

D. Increase the interrupt priority of SysTick

Solution

1. Step 1: Recognize effect of CPU clock changes

   SysTick timer counts CPU clock cycles, so changing clock speed affects tick timing.

2. Step 2: Adjust reload value accordingly

   To keep tick interval constant, reload value must be recalculated based on new CPU clock.

3. Final Answer:

   Update the SysTick reload value to match the new CPU clock frequency -> Option C

4. Quick Check:

   Adjust reload for clock changes to keep tick timing [OK]

Hint: Reload value must match CPU clock for correct ticks [OK]

Common Mistakes:

• Ignoring clock changes and keeping reload fixed
• Disabling SysTick breaks RTOS timing
• Changing interrupt priority doesn't fix timing