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I ve written a lot about interrupts and interrupt handlers in this book. While they are not terribly hard to create, there are some rules and conventions that you should follow when writing the software handlers:
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1 Use the standard header information provided in the earlier section of this chapter. 2 Keep them as short as possible with interrupt controller hardware reset taking place
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as early in the interrupt handler as possible.
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3 Avoid nested interrupts. 4 Do not call subroutines from an interrupt handler, or if this is not possible, avoid reen-
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trant subroutines. When you rst start writing interrupt handlers on your own, by following these four rules, you should minimize the problems with the interrupt handler code that will make debugging your application easier.
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CONTEXT SAVING
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When working with interrupts, the standard interrupt handler saves the w, STATUS, PCLATH, and any other context registers that are used by both an interrupt handler and a mainline application. The FSR register often ts into this de nition and should be saved along with the other three registers as well. The standard assembly-language context saving and restoring code is
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Int movwf _w movf STATUS, w bcf STATUS, RP0 movwf _status movf PCLATH, w ; Save w Contents ; Save STATUS in Bank 0
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ASSEMBLY-LANGUAGE SOFTWARE TECHNIQUES
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movwf _pclath clrf PCLATH : movf _pclath, w movwf PCLATH movf _status, w movwf STATUS swapf _w, f swapf _w, w ret e
; Make Sure Execution out of Page 0
The rst time you see this code, it will seem somewhat strange and dif cult to understand what is happening. The rst movwf w seems straightforward enough as saving the contents of the w register into a temporary register. There is one thing you should beware of, and that is that the bank bits in the STATUS register (RP0 and RP1) can be any value. To ensure that the contents of the w register can be saved safely, the variable _w should be placed either at the same address in each page or in a common register that is shadowed across all pages. Personally, I prefer the latter method because it means that only one declaration is registered for the variable, whereas the other method requires one declaration per page. Next, the STATUS register is placed in the w register, and the STATUS register s RP bits are loaded with the bank used to store the context registers. In the example code, I used bank 0, but any bank can be used. The important point to remember is that the RP bits are set after the STATUS register s contents are saved in the w register. Finally, the PCLATH register is saved and reset to zero (the interrupt handler starts executing in page 0 of the PIC microcontroller). These three lines can be avoided if the PIC microcontroller you are using has less than one page (2048 instructions for the mid-range device) of PIC microcontroller, and the interrupt handler doesn t change PCLATH. Other context registers (such as the FSR register, as mentioned earlier) can be saved using the process of loading them in the w register and saving them in a variable. With the mainline context register values stored, the interrupt handler now can load the registers with any values required as it responds to the interrupt request. Before completing, the requesting IF ag will have to be reset. The process of restoring the registers is the reverse of saving them, except that no instructions are used to set the registers to speci c values. The only surprising aspect of the context registration restore is the two swapf instructions before the ret e instruction. These instructions address the issue of how to load the w register without changing the STATUS register s zero ag. If you look at the instruction de nitions, you ll see that the swapf instruction does not change any STATUS register ags. Using the movf instruction will modify the zero ag but by rst swapping the two nybbles in place and then swapping them again as the value is loaded into the w register. This operation will load the w register with the correct mainline value without changing the STATUS register s zero ag.
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