PendSV and SysTick exceptions in ARM Architecture

Introduction

PendSV and SysTick are special signals in ARM processors that help manage time and switching tasks smoothly.

When you want to switch between different tasks in a program without losing data.

When you need a timer to trigger actions regularly, like blinking an LED every second.

When building an operating system that runs many programs at once.

When you want to handle background tasks without stopping the main program.

When you need precise timing for events in embedded devices.

Core Concept

ARM Architecture

PendSV_Handler(void) {
    // Code to handle task switching
}

SysTick_Handler(void) {
    // Code to handle timer tick
}

Both PendSV and SysTick are exception handlers, special functions called automatically by the processor.

PendSV is usually used for switching tasks, while SysTick is used as a timer interrupt.

Key Points

This example shows PendSV used to switch tasks by saving and restoring their states.

ARM Architecture

void PendSV_Handler(void) {
    // Save current task state
    // Switch to next task
    // Restore next task state
}

This example shows SysTick used as a timer to keep track of time and trigger actions.

ARM Architecture

void SysTick_Handler(void) {
    // Increment system tick count
    // Perform periodic actions
}

Detailed Explanation

This simple program counts 5 timer ticks using SysTick, then calls PendSV to simulate a task switch.

ARM Architecture

volatile unsigned int system_ticks = 0;

void SysTick_Handler(void) {
    system_ticks++; // Increase tick count every time SysTick fires
}

void PendSV_Handler(void) {
    // Imagine switching tasks here
    // For simplicity, just print a message
    // (In real ARM code, printing is not typical in handlers)
}

int main(void) {
    // Setup SysTick to fire every 1ms (not shown here)
    // Main loop
    while (system_ticks < 5) {
        // Wait until 5 ticks have passed
    }
    // After 5 ticks, simulate PendSV call
    PendSV_Handler();
    return 0;
}

Important Notes

PendSV is a low priority interrupt, so it runs after other interrupts finish.

SysTick is a built-in timer that helps keep track of time in embedded systems.

These exceptions help build multitasking systems on ARM processors.

Summary

PendSV is used for switching tasks smoothly in ARM systems.

SysTick is a timer interrupt that helps keep track of time.

Together, they help create responsive and multitasking embedded programs.