SysTick timer for RTOS tick in ARM Architecture - Time & Space Complexity

Time Complexity: SysTick timer for RTOS tick

O(n)

Understanding Time Complexity

We want to understand how the time taken by the SysTick timer interrupt grows as the system runs. This helps us see how the RTOS tick affects performance.

What happens to execution time when the SysTick interrupt fires repeatedly?

Scenario Under Consideration

Analyze the time complexity of the following SysTick interrupt handler code.

    SysTick_Handler:
        PUSH {LR}            ; Save return address
        INCREMENT tick_count ; Increase system tick
        BL RTOS_Scheduler    ; Decide next task
        POP {LR}             ; Restore return address
        BX LR                ; Return from interrupt
    

This code runs every SysTick interrupt to update the system tick and run the scheduler.

Identify Repeating Operations

Look at what repeats each time the SysTick interrupt occurs.

• Primary operation: The RTOS scheduler call inside the interrupt.
• How many times: Once per SysTick interrupt, which happens regularly (e.g., every 1 ms).

How Execution Grows With Input

As the number of tasks or system load increases, the scheduler work may grow.

Input Size (Number of Tasks)	Approx. Operations per Tick
10	10 operations
100	100 operations
1000	1000 operations

Pattern observation: The scheduler work grows roughly in direct proportion to the number of tasks.

Final Time Complexity

Time Complexity: O(n)

This means the time spent in the SysTick handler grows linearly with the number of tasks the scheduler manages.

Common Mistake

[X] Wrong: "The SysTick handler always takes the same time regardless of tasks."

[OK] Correct: The scheduler work depends on how many tasks it checks, so more tasks mean more time spent.

Interview Connect

Understanding how interrupt handlers scale with system load is key for embedded systems. It shows you can reason about real-time performance and system responsiveness.

Self-Check

"What if the scheduler used a more efficient data structure to pick the next task? How would the time complexity change?"

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