5.2 An Overview of Asynchronous Tools in FreeRTOS

Software Timers: For Application-Scheduled Events 

 A  FreeRTOS Software Timer  is a tool used to schedule the execution of a function at a future time. It's like setting an alarm clock within your software. When the timer expires, the RTOS automatically calls a predefined function, known as a  callback function . 

 Key Characteristics: 

 

 

 Managed by the RTOS:  Software timers are managed by a dedicated RTOS task (the "timer daemon"). This means they do not consume CPU time while they are waiting to expire. 

 

 

 Tied to the System Tick:  The resolution of a software timer is determined by the FreeRTOS system tick rate ( configTICK_RATE_HZ ). You cannot schedule a timer for a period shorter than one tick. 

 

 

 Use Case:  Ideal for repetitive, low-priority, or application-level timing. For example, you might use a software timer to: 

 

 

 Read a temperature sensor every five seconds. 

 

 

 Update a clock display once per minute. 

 

 

 Turn off an LED 500ms after it was turned on. 

 

 

 

 

 Types of Software Timers: 

 

 

 One-Shot Timer:  Executes its callback function only once after it is started. 

 

 

 Auto-Reload Timer:  Executes its callback function repeatedly at a fixed interval until it is explicitly stopped. 

 

 

 Hardware Interrupts: For Hardware-Triggered Events 

 A  Hardware Interrupt  is a mechanism for a hardware peripheral to signal the CPU that it needs immediate attention. Unlike a software timer, which is scheduled by your application, an interrupt is triggered by an external, physical event. 

 Key Characteristics: 

 

 

 High Priority:  An interrupt will immediately preempt the currently running code, regardless of the task's priority. The CPU will save its current state and jump to execute the ISR. 

 

 

 Hardware-Driven:  They are generated by peripherals like GPIO pins (e.g., a button press), hardware timers (for precise timing), or communication interfaces like UART/SPI (e.g., data has arrived). 

 

 

 Use Case:  Essential for time-critical operations and reacting to external events with minimal latency. For example, you would use a hardware interrupt to: 

 

 

 Count pulses from a motor encoder to measure its speed. 

 

 

 Immediately stop a machine when a safety limit switch is triggered. 

 

 

 Capture incoming data from a high-speed sensor before it is overwritten. 

 

 

 

 

 In summary, the choice between them is driven by the source of the event: 

 

 

 Use a  Software Timer  when the event is driven by the logic of your application (" I need to do X in 500 milliseconds "). 

 

 

 Use a  Hardware Interrupt  when the event is driven by an external hardware signal that requires an immediate response (" The hardware needs attention NOW ").