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ifdef USARTPres else : endif ; ; If USART Present, don t add any code Put in Non-USART Code
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It can be somewhat dif cult to understand what is happening. Instead of using else to provide conditional execution in the case of the label not being present, the absence of the de ne should be checked for using the ifndef directive. There are a number of tricks that you can use with ifdef and ifndef conditional assembly statements that can make your code development easier and more exible. The rst is conditionally deleting code. As you work through an application, often you will want to remove some code to test out different aspects of it. Elsewhere in the book I talk about the idea of commenting out the code simply by putting the comment directive (the ; or semicolon character) before the statement such as
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; addlw 0x0FF ; #### - Instruction not needed, but kept
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For single instructions, this is easy to do and easy to keep track of. For many instructions, it can be dif cult to keep track of everything that has to be removed (but kept). An easy way of doing this is to put an ifdef and endif statement before and after the code, as in
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ifdef Used : endif ; ; ; #### - Ignore following Block of Code NOT to be part of application #### - Ignore above
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It takes literally just a few seconds to remove the code and can be disabled just as quickly (by de ning Used or deleting the ifdef and endif statements). The second trick is to allow the logic of an application to be used in multiple PIC microcontrollers that may have different built-in hardware features. In the preceding example, by using the ifdef directive, I can have the code that takes advantage of the built-in serial port hardware of a speci c PIC microcontroller or insert bit banging code in its place if the selected PIC MCU does not have a built-in serial port. I should point out that when an MPLAB IDE application is built, a de ne label is created indicating the PIC MCU part number. For example if you were assembling an application for the the PIC16F84, the __16F84 label is available for testing by the ifdef and ifndef directives. I have taken advantage of this feature in some applications where the code can run in different devices. For example, the following code will allow an application to run
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on either PIC16F84 or PIC16F877 MCUs or generate an error indicating that a supported PIC microcontroller has not been selected for the application:
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ifdef __16F84A include p16f84a.inc else ifdef __16F877 include p16f877.inc else error Code is Not Designed for Speci ed Processor endif endif
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In this example, note that I have put in nested if statements. Up to eight levels of nesting are possible in MPASM, although this can be very confusing to read. I would recommend that no more than two levels be used, as in the preceding example. Along with conditional assembly based on the presence or absence of labels, constant and variable condition testing also can be done with conditional assembly statements. For example, tests against addresses could be performed for interpage jumping in midrange PIC MCUs:
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if ((($ movlw movwf endif goto & 0x01800) ^ (Label & 0x01800)) != 0) HIGH Label ; Different Pages - Update PCLATH PCLATH Label & 0x07FF ; Jump to Label
In this example, if the destination is in a different page from the current location (which is returned by the $ directive in MPLAB), then PCLATH is updated before the goto statement. The preceding example is suboptimal for four reasons. The rst is that whether or not PCLATH has to be updated is variably based on the address of the goto statement. A more accurate way of doing this would be
if (((($ + 2) & 0x01800) ^ (Label & 0x01800)) != 0) movlw HIGH Label ; Different Pages - Update PCLATH movwf PCLATH endif goto Label & 0x07FF ; Jump to Label
In this case, the possible address of the goto is checked rather than the current address. There is the possibility that the current address will be in a different page than the goto, and PCLATH may or may not be updated correctly. The second problem is that this code takes up a different amount of space depending on which path is taken. Doing this can result in a address phase error that indicates that during the different passes in the assembler, required addresses change in a way that makes correct assembly impossible. These different addresses are caused when
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