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OPTIMIZING PIC MICROCONTROLLER APPLICATIONS
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through to allow you to use the device that you have already chosen. This optimization of the application is not terribly hard to do, and I nd it fun to see how much more function I can cram into a PIC microcontroller. In this section I want to discuss some strategies that you can use. One of the most frustrating things that you can experience in an application is running out of le registers. None of the PIC microcontroller part numbers have a lot to begin with, and you probably will nd that you will run out of them when you plan complex applications in small (low-end) devices. There are a few things that you can look at to try to alleviate the need for le registers. The rst and most obvious action to take is to look for le registers that are used as ags. Flags should be implemented as individual bits. This can reduce the requirements from eight registers down to one. One of the reasons why people use a le register for a ag is to avoid having to remember the bit number for a speci c ag. This concern can be eliminated by using the #DEFINE directive to specify individual bits. For example, 8 bits of a ags register could be declared like
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#DEFINE RUNFLAG FLAGS, 0 #DEFINE STOPFLAG FLAGS, 1 #DEFINE REQUESTFLAG FLAGS, 2
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and to set or reset a ag, the bcf or bsf instruction is used like
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bsf RUNFLAG
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The #DEFINE actually puts two parameters to the label so that when RUNFLAG and the other #DEFINE labels are encountered, the instruction is loaded with the register and the bit number instead of having to put both parameters in the instruction. This optimization is interesting because not only does it reduce register requirements, but it also reduces the number of instructions required to test the state of a ag to one from the multiple instructions used in other processors and makes the code easier to read. For example, to jump if STOPFLAG is set, the instructions
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movf andlw btfss goto FLAGS, w ; Clear everything but the stop bit 1 << 1 STATUS, Z ; Jump to label if the bit is set LABEL
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could be used and typically are what would be used in other processors, but in the PIC microcontroller, using the de ne for a bit in a register, the same function can be implemented in
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btfss STOPFLAG goto Label
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which is much easier to read and understand than the previous code and requires fewer instructions. Some people will try to use hardware registers for temporary storage of data, and I would like to discourage this as much as possible. Writing random values to a register
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ASSEMBLY-LANGUAGE SOFTWARE TECHNIQUES
can result in unexpected and unwanted hardware operations. The only exception to this rule is using the FSR as a temporary register. For reducing code space, there are a number of things to look at that will not require large changes to the application. First off, check to see that all arrays are in bank 1 rather than in bank 0. The FSR register can access data in either bank, and code space will be saved if the single-byte variables are stored in the bank out of which the application mostly executes. If you have to switch banks in the application and access registers, use the multiple shared le register addresses available in the PIC microcontroller instead of the w register. This will allow the sharing of multiple bytes between the banks without having to toggle the RP0 or RP1 bits. Spending some time thinking about how variables are placed in the application will result in huge code savings. Ideally, you should try for never changing the bank register when accessing le registers. Inef cient array and stack accesses can be particularly wasteful in terms of instructions. When you are implementing arrays, look for built-in features of the PIC microcontroller architecture that will help you to simplify the accesses. One trick I like to use is to put array elements starting at a speci c offset instead of relying on CBLOCK to allocate the address for me. For example, if I were to put a 16-byte array at 0x040, to access an element, all I would have to do is load the index to the element and set bit 6 to create a correct address. This avoids the complications of adding the starting offset to the index, which may not seem like a major inconvenience, but it can add to the number of instructions required. This trick becomes very useful when working with circular buffers. In the preceding example, this 16-byte array is located in le register addresses 0x040 to 0x04F. To increment to the next address within the buffer, only the two instructions
incf ArrayIndex, f bcf ArrayIndex, 4
are required. These two instructions will increment the address and keep it within the correct range (which has bit 4 of its address always reset). If an arbitrary starting point for the circular buffer is used, then the required code becomes
incf xorlw btfsc movlw xorlw movwf ArrayIndex, w ArrayEnd +1 STATUS, Z ArrayStart ^ (ArrayEnd +1) ArrayEnd +1 ArrayIndex
These six instructions test for the array pointer to be past the end of the array after incrementing it and reset to the start of the array, if required. This code also takes up six instruction cycles and modi es the w register, which may require saving and loading the value in it, which the two-instruction solution does not. Jumping between pages can eat up a lot of instructions. To avoid this, try to keep functional blocks of code together on one page as much as possible. You may nd that if
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