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MidGoto: MID-RANGE JUMPING BETWEEN PROGRAM MEMORY PAGES
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In mid-range PIC microcontrollers, jumping between the pages is accomplished by setting the PCLATH register before executing a goto or call instruction or changing the PIC microcontroller s program counter via the PCL register. This feature may
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JUMPING AROUND
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seem intimidating when you rst start working with it, but as you gain experience with the PIC microcontroller, it actually will be quite easy to work through. To show how jumping between program memory pages is accomplished, I created the MidGoto application, which can be found in the code\MidGoto folder:
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title MidGoto - Low-End Jumping Around. ; ; In this Application, Jumps Between Device Pages is ; Demonstrated. ; ; ; 99.12.25 - Created for the Second Edition ; ; Myke Predko ; LIST R=DEC INCLUDE p16c73b.inc ; < Note the Changed Processor ; Registers
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; Macros MyLGoto MACRO Label movlw HIGH Label movwf PCLATH goto Label & 0x07FF Selections endm
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__CONFIG _CP_OFF & _WDT_OFF & _XT_OSC & _BODEN_OFF & _PWRTE_ON PAGE Mainline of LowGoto 0 Page0Label ; Goto an Address Within the Same Page
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Page0Label: MyLGoto Page1Label org 0x0800 Page1Label: MyLGoto Page0Label
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Label in Page 0
Label in Page 1
PIC MICROCONTROLLER APPLICATION BASICS
This application rst jumps to a label within the current page (0) and then does an interpage jump using the MyLGoto macro. This macro sets up the PCLATH register before executing the goto instruction. To demonstrate the interpage jumps, I wanted to use a PIC microcontroller with more than one code page. One of the most basic devices for doing this is the PIC16C73B, which is a 28-pin mid-range PIC microcontroller with a number of ADC inputs. In mid-range (and higher-end) PIC microcontrollers, the lower 8 bits of the program counter are available in PCL, whereas the upper bits use a PCLATH (and optionally a PCLATU in the PIC18 microcontrollers) register. When a goto or call instruction is executed or PCL is updated, the contents of PCLATH are loaded into the high bits of the PIC microcontroller s program counter along with the new address. To demonstrate how this works, after you have simulated the application and watched the PCLATH register change in the 16C73B Watch window, comment out the line that updates PCLATH (movwf PCLATH) in the MyLGoto macro, rebuild, and then step through the code. This macro differs from the one demonstrated in the preceding section in that it changes the value of the w register a similar, conditionally executing macro that just changes the appropriate bits of PCLATH also could be created. The MyLGoto macro will allow the jump between pages to execute correctly, but there are a few caveats that you should be aware of. If you want to have interpage call instructions, check in the experiments for my comments on this. Second, this macro never should be used with conditional skip instructions. As I will note in the next experiment, only nonconditional jumps should be done between pages as you are learning the PIC microcontroller. The third issue is that you will get a message if you are jumping from a page in which a PCLATH page selection bit is set to an address where the page selection bit is reset. This isn t a big problem by changing the macro goto instruction to
goto (Label & 0x07FF) | ($ & 0x01800)
you will add the appropriate high level bits for the current page to the goto instruction, eliminating the message.
CondJump: CONDITIONAL JUMPING
Conditional jumping in the PIC microcontroller is something that probably won t seem all that intuitive, but once you understand how it can be used not only for executing according to the state of the STATUS register bits but also for all the register bits in the processor, you will begin to see how you can write applications for the PIC microcontroller that are a lot more ef cient than in other chips. Instead of having instructions that are based on the state of one of the execution STATUS bits (zero, carry, and digit carry), the conditional instructions are based on the state of any bit accessible to the processor. This is quite a profound method of operation and one that you really have to sit down and think about because it can allow you to create applications that are startling in their ef ciency and their ability to work through complex comparisons. Elsewhere in this book I have described the skip on bit condition instructions, and I want to take this opportunity to demonstrate their use in a few cases, along with discussing
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