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:

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

2. 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").