Nuts and Volts - February 2011

FIGURE 10. RTOS structure overview.

systems like Windows and Linux for PCs and workstations, and these are now making an impact in high-performance microcontrollers like the PIC32. In fact, with the computational power and memory of the PIC32, an RTOS is needed to efficiently process lots of simultaneous complex multi-task software operations at the same time. The best way to visualize this is to appreciate the types of applications that these advanced microcontrollers are handling today. In this demo, you will be introduced to a free open source RTOS for the PIC32 developed by Richard Barry (see www.freertos.org).

FreeRTOS allows PIC32 applications to be organized as a collection of independent tasks operating under control of a real time scheduler (see Figure 10). The scheduler decides which task should be executing by examining the priority assigned to each task by the application designer. Tasks can move from running to block or to ready or suspended, depending on the priority and how you design them. In this demo, we will cover task creation, scheduling, removal, and reinstatement, as well as interrupt handling. We will use FreeRTOS to perform our “Hello World” function. It’s a modest start but highlights the powerful capabilities of organizing your code around an RTOS. To make things interesting, we will create the ability to simultaneously handle independent LED blinking tasks — each task assigned to blink an individual LED. This demo requires use of the LED matrix and the hook-up diagram mentioned earlier. As you push and release SW1, an individual LED blinking task will be dynamically eliminated until the max of eight tasks is reached. At this point, as you continue to depress SW1; tasks that were previously eliminated are restored in reverse order. The main code is shown in Figure 11. The functions used are described in the on-line documentation associated with FreeRTOS. Here’s a quick overview:

• vSetupEnvironment() — Encapsulates the code to set up the hardware (in this case, eight LEDs and a switch).

• x TaskCreate( v Task1,”Task 1”,240,(void*)image,1,&x Task1Handle) — Creates a “task 1” and passes it an image (digital port value of the row/column for a specific LED in the matrix). This task can be referenced later by its handle. The task is written as an LED blink function.

• vSemaphoreCreateBinary (xBinarySemaphore) — Creates a semaphore or binary flag/control that will be used uniquely by the switch interrupt service to invoke its task handler.

• x TaskCreate(vHandler Task, “Handler”, 240, NULL, 3, NULL) — Creates a task to handle the switch once an interrupt occurs. The interrupt invokes this task using the above semaphore; the handler adds and removes tasks by their handles.

• v TaskStartScheduler() — Starts the RTOS scheduler so that the tasks created so far start executing.

FIGURE 11. RTOS ‘Hello World’ code.

It is interesting to watch the LED activity as the FreeRTOS scheduler spreads and de-spreads the PIC32 processing power across the blinking tasks. This demo is included in the folder named RTOS example. Open this folder and navigate to examples, and then open RTOSEXAMPLE.MCP. This demo is also available for download from the NV website.