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clrf movf movwf bcf rlf rlf rlf rlf VarValue + varA + 1 ; VarValue.varA = VarValue.varB VarValue + varB, w VarValue + varA STATUS, C ; VarValue.varA = VarValue.varB*2 VarValue + varA, f VarValue + varA + 1, f VarValue + varA, f ; VarValue.varA = VarValue.varB*4 VarValue + varA + 1, f
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Pointers are variable types that can point to other variables anywhere within the application variable memory space. In the PIC microcontrollers, there are no devices that have more than 4,096 le registers, so the data size I normally use for PIC microcontroller pointers is a 16-bit variable. Depending on the PIC microcontroller architecture, pointers can be somewhat confusing and dif cult to work with, although after a few second s thought you probably can come up with a method for accessing registers in the different devices. For low-end devices, pointers are quite simple to implement with a single 8-bit register that can be stored in the FSR register. The general case for mid-range PIC microcontrollers is 9 bits to fully access registers in the four banks, which requires a 16-bit variable. For the PIC18, I would recommend that the 4-bit bank address of the selected register be included with the 8-bit register address as part of a 16-bit variable. Having a pointer address an 8-bit register is quite easy to implement, but pointers become much more complex if they are pointing to a structure. The last structure element
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in the VarStruct examples above is designed as a pointer to the next structure variable in a list of VarStructs. To implement a list of VarStructs, three structure variables can be de ned along with a pointer to VarStructs that will have structure variable strung along in a list. In C, the variables would be declared using the code
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struct struct struct struct VarStruct StructureA; VarStruct StructureB; VarStruct StructureC; * VarStruct StructurePtr;
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Note that StructureA, StructureB, and StructureC are each used to de ne the 5 bytes of VarStruct above. StructurePtr simply points to a structure and initially has no value behind it. In C, returning a pointer address is accomplished by using the splat (*) character, and the ampersand (&) is used to return the address of a value. To set up the list of StructureA pointing to StructureB pointing to StructureC and StructurePtr pointing to the start of the list (StructureA), the following C code is used:
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StructureA.NextVarStruct = &StructureB; StructureB.NextVarStruct = &StructureC; StructurePtr = &StructureA;
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To access a structure pointer in the list, the StructurePtr pointer is used with the > operator, which indicates that it is referring to the pointed to value. To set StructureB.varB to 0x012, the following code would be used:
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StructurePtr -> NextVarStruct -> varB = 0x012;
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As you would imagine, this is quite complex to do in the PIC microcontroller assembler and following the operation of the pointer. To give you an idea of how this would be done, the following code shows the assembler code for the preceding C statement using the SetPointer macro:
SetPointer macro Address if ((Address & 0x0100) == 0) bcf STATUS, IRP else bsf STATUS, IRP endif movlw Address 0x0FF movwf FSR endm
This macro was written for the mid-range PIC microcontroller, and to simplify operation of the pointer load, I made my pointer structure 9 bits (as discussed earlier) in size, with the value being the register address used in the Microchip device .inc le. The
STRUCTURES, POINTERS, AND ARRAYS
macro s Address parameter will be loaded into the FSR register and IRP bit of the STATUS register:
SetPointer StructurePtr ; Point to the Start of the List ; (StructureA) movlw NextVarStruct ; Point to the next List Element ; (StructureB) addwf FSR, f movf INDF, w ; Get the Low Byte of the Next List ; Element movwf PointerTemp incf FSR, f movf INDF, w ; Get the High Byte of the Next List ; Element movwf PointerTemp + 1 bcf STATUS, IRP ; Load the Next List Element into FSR btfsc PointerTemp + 1, 0 bsf STATUS, IRP ; AND the IRP movf PointerTemp, w movwf FSR movlw varB ; Point to the Element within the ; Structure addwf FSR, f movlw 0x012 ; Finally, do the Assignment movwf INDF
Despite the simpli cation of the macro, the PIC microcontroller code to implement the pointer statement is still quite complex and, more important, very confusing. Trying to follow the preceding code will be a bit dif cult which is characteristic of assemblylanguage pointer programming. Looking at the preceding code, one consideration about creating pointers and structures in the PIC microcontroller is not readily apparent, and that is the importance of never having a structure go over a bank boundary. This is probably obvious, but I just point it out as something to watch for if your application grows, and the latest structure you added to the application code seems to cause the PIC microcontroller to lock up or fail in strange ways. When a structure goes over a bank boundary, accesses to the structure will modify the PIC microcontroller s context registers (such as the STATUS, PCL, and OPTION registers) at the start of the bank, causing all kinds of chaos. If you are new to programming, you will nd working with pointers confusing. I m not new to programming, and I still nd pointers confusing. I realize that there are times when programming constructs such as the linked list example earlier is required for an application, but they should be avoided unless absolutely necessary in the PIC microcontroller. With a bit of thought about the application design, they should not be required at all. The last programming construct I want to introduce you to in this section is arrays. Elsewhere in the book I have discussed PIC microcontroller array programming, but I
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