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LOW-END AND MID-RANGE RTOSs
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allows code reuse or modi cation and allows multiple software developers to work on an application without having to understand how the other interfaces work. At rst, it may be unnatural for you to think in terms of simple, single devices, but once you get the hang of it, you ll be amazed at how easy it is to work with an RTOS. The last aspect of RTOSs that I want to present to you is the semaphore. The semaphore is a ag that is controlled by the operating system that can be used to restrict access to a resource (which can be a hardware device, data, or even another task) until the owning task has nished with it. Semaphores are used in situations such as controlling access to an operator console. In the SendModem task above, a semaphore could be used to indicate that the Modem hardware is available for a packet transmission or that the modem is involved in a data packet transfer.
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It is not possible to create a full-featured RTOS for low-end and mid-range PIC microcontroller architectures. The reason for making this sweeping statement is the two architectures inability to save and restore program counter stack values the PIC18 architecture has this ability and is able to have RTOSs developed for it. There are some tricks that you can use to create an application that mimics the operation of an RTOS, but they do not provide a truly satisfying RTOS and may make application development more complex rather than simpler, which is what is expected when development is carried out well in a multitasking operating system. The most likely method for implementing an RTOS in low-end or mid-range devices is to use a ring execution model that could be described in pseudocode as
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while (1 == 1) { for (i = 0; i < NumberOfThreads; i++) if (ThreadActive[i] || MessageWaiting[i}) Thread[i]; // Execute the next waiting thread INTCON.GIE = 1; NOP; INTCON.GIE = 0; } // elihw // // // Enable Interrupts Service waiting interrupt request Disable Interrupts
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In this code, each thread is polled to determine whether or not it can execute, and if it can, then control of the processor is passed to the thread until it nishes and then checks the next thread. There are a few issues with this model, not the least of which is the time required to poll each of the different threads, as well as the latency involved with going ahead and servicing the interrupt request. Another issue is that each thread is an autonomous subroutine with no initialization code (because it would be executed each time the thread executes). The best way to deal with initialization would be to create an initialization thread for each main thread, and on application startup, the initialization thread is set to execute, and at the end of the initialization thread, its ability to execute (ThreadActive == True) is terminated.
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If there is a need to execute threads asynchronously in order to respond to interrupts, you may want to resort to building up multiple polling routines. For example, in the TAB Electronics Build Your Own Robot Kit, I was faced with the need to perform six tasks while implementing the motor PWM loop: Time PWM duty cycle as required. Send out IR data modulated at 38 kHz for collision detection. Poll IR receivers and indicate when there is an object close by. Poll IR receivers and indicate when an infrared remote-control command was being received. 5 Poll the BS2 synchronous serial clock and data lines and receive data from the BS2. 6 Update real-time clock.
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Implementing these tasks simultaneously in an event-driven RTOS would be quite simple, but it was a signi cant challenge in the robot because these tasks were implemented in a PIC16C505, a low-end device with no interrupts. To do this, I spent many hours carefully architecting the PWM loop and ensuring that polling of the different interfaces was done at appropriate intervals and that no changes in the incoming data were lost. Once this was done, I spent 6 weeks (literally) on the simulator, working through every possible scenario, to ensure that there could be no situations where the application would not work as expected. The code was very successful, and no problems were reported in the eld. Unfortunately, this effort could have been minimized signi cantly by implementing this application in a device that could support an RTOS. The PIC16C505 was chosen for cost and not for functionality, and while it gave me a very good price for the manufacturing of the robots, it took signi cantly more effort than I would have expected for a similar application created for an RTOS. This is a reiteration of the one of the central tenets for developing application code for an RTOS it is signi cantly easier to produce high-quality code for an RTOS than it is using standard programming techniques.
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