SysTick timer for RTOS tick in ARM Architecture - Mini Project: Build & Apply

SysTick Timer Setup for RTOS Tick

📖 Scenario: You are working on an embedded system using an ARM Cortex-M microcontroller. To run a Real-Time Operating System (RTOS), you need to configure the SysTick timer to generate periodic interrupts that act as the system tick. This tick helps the RTOS manage task switching and timing.

🎯 Goal: Set up the SysTick timer to generate interrupts at a 1 millisecond interval, which will serve as the RTOS system tick.

📋 What You'll Learn

Create a variable for the system clock frequency in Hz.

Create a variable for the SysTick reload value to achieve a 1 ms tick.

Write the code to configure the SysTick timer with the reload value.

Enable the SysTick timer and its interrupt.

💡 Why This Matters

🌍 Real World

SysTick timer configuration is essential in embedded systems running RTOS to provide a consistent time base for task scheduling and time management.

💼 Career

Embedded software engineers and firmware developers frequently configure SysTick timers to implement RTOS ticks, ensuring reliable multitasking and timing in microcontroller applications.

1

Define the system clock frequency

Create a variable called SystemCoreClock and set it to 48000000 to represent a 48 MHz system clock frequency.

ARM Architecture

# Define the system clock frequency in Hz
# Your code here

Hint

The system clock frequency is usually given in Hertz (Hz). For example, 48 MHz is 48000000 Hz.

2

Calculate the SysTick reload value

Create a variable called SysTickReloadValue and set it to the value of SystemCoreClock divided by 1000 to get the reload value for a 1 millisecond tick.

ARM Architecture

SystemCoreClock = 48000000
# Calculate the SysTick reload value for 1 ms tick
# Your code here

Hint

Since the clock is in Hz, dividing by 1000 gives the number of clock cycles in 1 millisecond.

3

Configure the SysTick timer

Write code to configure the SysTick timer by setting the LOAD register to SysTickReloadValue - 1, clear the current value register VAL to 0, and set the control register CTRL to enable the timer, enable the interrupt, and select the processor clock.

ARM Architecture

SystemCoreClock = 48000000
SysTickReloadValue = SystemCoreClock // 1000
# Configure SysTick registers
# Your code here

Hint

The SysTick control register bits: bit 0 enables the counter, bit 1 enables the interrupt, bit 2 selects the processor clock.

4

Enable the SysTick timer and interrupt

Ensure the SysTick timer and its interrupt are enabled by confirming the SysTick_CTRL register is set to 0x07.

ARM Architecture

SystemCoreClock = 48000000
SysTickReloadValue = SystemCoreClock // 1000

SysTick_LOAD = SysTickReloadValue - 1
SysTick_VAL = 0
SysTick_CTRL = 0x07  # Confirm timer and interrupt enabled
# Your code here

Hint

Setting SysTick_CTRL to 0x07 enables the timer, interrupt, and selects the 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