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RTOS01: SIMPLE PIC18 RTOS
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When I originally set out to develop an RTOS for the PIC18, I had a number of ideas in mind. These were based on the RTOSs that I had written for other devices with the philosophy that what I came up with would have to be quite simple. The goals I gave for myself were
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Required program memory for the application shoud be less than 256 instructions. There should be fast task switch capabilities (less than 0.5 ms when running at 4 MHz). Sixteen bytes or less of overhead le registers should be required. Eight tasks could be accomplished with an initial AllTask. Tasks would execute as if they were the only application in the PIC microcontroller. Each task could have at least 32 bytes of variable space. The RTOS would support at least one subroutine call. Interrupts would be supported. Application tasks would be assembled with the RTOS code. There would be a single operating system entry point.
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The result can be found in the rtos subdirectory of the downloaded PIC microcontroller folder. This is a reasonably full-function RTOS with a simple application that increments a series of light-emitting diodes (LEDs) on a PIC18C452 s PORTB. The application code was developed to test out the different functions of the PIC microcontroller. I will discuss the application in more detail in this section and present a more complex application at the end of this chapter. To achieve a very small program memory requirement for the RTOS, I wanted to make sure that I had optimized the application as much as possible for the PIC18. I was able to write and debug the code over the space of about 12 hours, but the design of the RTOS was done over the space of a couple of weeks. This time spent in design and doodling resulted in fairly small code but what I feel is a reasonably elegant RTOS that is well suited to the PIC18 and offers good protection for the application developer. Since the RTOS was the rst full application I implemented on the PIC18, this was to be a learning experience for me. It probably sounds risky to do an entire operating system as a rst application, but I actually used this occasion to make sure that I understood how the PIC18 worked and how the instructions execute. Making this more of an adventure was my use of an interim upgrade to MPLAB and engineering sample PIC18 when I rst wrote the code. This was actually a right-sized application to learn on because it challenged me to learn the processor architecture completely but didn t require an extremely large application. In terms of measuring against requirements, the nal RTOS weighs in at 748 instructions, requires only 8 bytes of overhead to run eight tasks, and executes a task switch in about 750 s with the PIC microcontroller running at 4 MHz. It may sound like I blew the RTOS size speci cation, but owing to the relatively large amount of program memory available to the PIC18 (I used a PIC18C452, which has 16,384 instructions) and the ability to multiply up the clock, these overages are really not that signi cant to the total memory available, and the speeding up brings the task switching well within the target time.
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PIC18 RTOS DESIGN
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When you write RTOS applications, you will discover that the actual application code is quite small. The test application that is included with the RTOS requires only 26 instructions to create an application that starts up TMR0 to interrupt once every 16.384 ms and uses a counter to count down 64 TMR0 interrupts and increment the LEDs. When I rst learned about RTOSs, I was told that the maximum task-switching time should be 500 s. In this RTOS, I ended up taking 750 s at 4 MHz. I don t consider this to be a major concern because a 4-MHz clock is actually quite modest, and to meet the 500- s requirement, I could simply turn on the four times clock PLL built into the PIC18 s oscillator circuit or add a faster clock. This may sound somewhat facetious, but I have found over the years with reasonably simple microcontroller RTOSs and properly designed and written applications that switching times of 2 or 3 ms can be tolerated without any problems by most applications. When tasks are created for this RTOS, they execute as if they are the only task running in the PIC microcontroller. To facilitate this, I made the assumptions listed below about how the application tasks would work. This results in a somewhat simpli ed view of the PIC18 but one that is more than offset by the capabilities offered by the RTOS. The basic assumption I made was that the application would only access the rst 63 le registers of the access bank. When I created the RTOS, I wanted to avoid having global variables as much as possible and instead to take advantage of the ability of the RTOS to send messages back and forth. Sixty-three le registers is actually quite generous and will allow you to develop tasks for most applications with very few problems. The number of le registers is speci ed when the task is started. For many applications, you will discover that quite a few tasks require only one or two le registers, which will allow you to run the RTOS quite easily in PIC18 devices with 512 le registers. At the top of the memory space in the PIC18s with 1,536 le registers, you could put in some global variables, but I suggest that you place them starting at address 1,024 to make sure that none of the tasks can access them. When the RTOS executes, you will notice that the le registers dedicated to the task are placed at the rst beginning of the access bank and even the PIC microcontroller s le register memory itself. This was done to further simulate the perception that the task was the only code running in the PIC microcontroller. The 23 special function registers that are speci c to each task are located in the second 128 bytes of the access bank and are listed in Table 13.1. The limitations are the lack of a third index register (FSR2) and the fact that restricting the stack depth to only three pushes could not be used by the application because it is changed during the RTOS s execution. I will discuss the stack in more detail below. When a task is not executing, these 24 registers are read out of the PIC microcontroller s special-function register area and are copied into what I call the task information block (TIB). This data structure, which is provided for each task, consists of the 24 specialfunction registers listed above along with the variable le registers discussed earlier. To make moving data back and forth between the TIB simpler in the RTOS, I copy the 24 bytes of tasks of the special-function registers into address 0x060 to 0x077 of
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