Understanding System Tick Timers
System tick timers, commonly referred to as SysTick, are vital for enabling timed operations within a microcontroller environment. They provide a regular timed interrupt source, which is crucial for task scheduling in embedded systems using a bare-metal approach. With SysTick, you can create a time base, trigger periodic tasks, and manage timeouts.
Configuring SysTick Timer
Before using the SysTick timer for task scheduling, a configuration process is necessary. Here is a step-by-step method on how to configure a SysTick timer in C for a generic ARM Cortex-M microcontroller.
#include "stm32f4xx.h" // Include the appropriate header file for your MCU
#define SYSTICK_LOAD_VAL 16000 // For a 1ms time base at 16 MHz CPU frequency
void SysTick_Init(void) {
SysTick->LOAD = SYSTICK_LOAD_VAL - 1; // Set reload register
SysTick->VAL = 0; // Reset the SysTick counter value
SysTick->CTRL |= 0x7; // Enable SysTick with processor clock, enable interrupt
}
// SysTick interrupt service routine
void SysTick_Handler(void) {
// This function will be called every 1ms
// Insert task scheduler logic here
}
Implementing Task Scheduling
Once the SysTick timer is configured and enabled, it can be used to schedule tasks. One common approach is creating a simple scheduler that increments a counter at each tick and executes tasks based on the counter value.
#include <stdint.h>
typedef void (*task_t)(void);
typedef struct {
uint32_t period; // Period of the task in milliseconds
uint32_t elapsed; // Time elapsed since last run
task_t task; // Function pointer to task
} TaskScheduler;
#define NUM_TASKS 3
TaskScheduler taskList[NUM_TASKS];
// Dummy task functions
void Task1(void) {
// Implement task-specific logic
}
void Task2(void) {
// Implement task-specific logic
}
void Task3(void) {
// Implement task-specific logic
}
void Scheduler_Init(void) {
taskList[0] = (TaskScheduler){ .period = 100, .elapsed = 0, .task = Task1 };
taskList[1] = (TaskScheduler){ .period = 200, .elapsed = 0, .task = Task2 };
taskList[2] = (TaskScheduler){ .period = 500, .elapsed = 0, .task = Task3 };
}
void Scheduler_Update(void) {
for (uint8_t i = 0; i < NUM_TASKS; ++i) {
taskList[i].elapsed++;
if (taskList[i].elapsed >= taskList[i].period) {
taskList[i].task();
taskList[i].elapsed = 0; // Reset elapsed time
}
}
}
Integrating Scheduler with SysTick
Modify the SysTick_Handler to call the scheduler update function, which will verify if any tasks need to be executed.
void SysTick_Handler(void) {
Scheduler_Update();
}
Optimization and Considerations
Interrupt Priority: Since SysTick is a periodic interrupt, ensure its priority is appropriately set relative to other interrupts in the system to prevent jitter or starving the SysTick interrupt.
Resolving Overflows: It’s important to handle counter overflow in tasks that depend on timers, especially in long-running systems.
Power Consumption: Continuously running a SysTick timer might affect power consumption in battery-operated systems. Consider stopping SysTick when the system does not require task scheduling.
- Concurrency: In a real-world scenario, tasks might need concurrency control, especially when accessing shared resources. Use mechanisms like mutexes if needed.
By employing these steps and considerations, the system tick timer can effectively manage the scheduling of various tasks in a bare-metal embedded application, offering a balance between simplicity and functionality.
