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Porting Code Between PIC Microcontroller Device Architectures
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Historically, I design my PIC microcontroller assembly-language code in terms of the mid-range architecture; if the application is destined for another PIC microcontroller family, I then change the methods used for the code by the rules I present in the next two sections. When it comes right down to it, the changes are quite subtle, and application code can be created that can be ported between PIC microcontroller architectures very easily. This was illustrated in Chap. 7 when I talked about the different types of instruction elements and showed how they had analogies in each of the three architectures. Designing your application on the mid-range PIC microcontroller devices rst is not a bad approach because these devices have many of the same capabilities as lowend and PIC18 architectures, and the availability of MPLAB ICD-enabled parts will allow you to cost-effectively develop and debug your applications before porting them to the nal device.
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PORTING MID-RANGE APPLICATIONS TO THE LOW END
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All the low-end PIC microcontrollers instructions are available in the mid-range device and work exactly the same way. The lack of some instructions requires some workarounds, but I nd that by writing mid-range code with the low-end device in mind, the code can be ported remarkably easily with few changes. There are seven primary differences to be aware of between the mid-range and low-end PIC microcontroller instruction sets. Some of these differences are a result of the 12-bit instruction word of the low-end devices, and others are due to the low-end device s architecture.
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PORTING CODE BETWEEN PIC MICROCONTROLLER DEVICE ARCHITECTURES
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These seven differences are
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Smaller instruction page size Four fewer instructions Subroutine call instruction differences Reduced program counter stack Subroutine return instruction differences No interrupt capability No register bank select bits
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The low-end PIC microcontroller s page size is 512 instructions and requires 9 address bits. This will seem like a lot smaller than the mid-range device, which has a page size of 2,048 instructions (11 address bits), and will seem to be dif cult to create applications for. With this restriction, along with others discussed in this section, I tend to look at the low-end devices as being better suited for applications that require no more than 512 instructions with limited text/data output. Five hundred and twelve instructions may not seem like a lot. To put this in perspective, I always thought that the 512 bytes used for booting in the PC was frightfully restrictive. When the PC boots, a single 512-byte sector is loaded into memory from disk, and this code is used to load and start up the operating system in the PC. Some years ago I wrote my own code for an operating systemless boot from a diskette in a PC and found that I only required 107 bytes. Knowing that each 8086 instruction averages 3 bytes in size, you can see that a lot can be done in the 512 bytes of a low-end PIC microcontroller page. The mid-range instructions that are not available in the low-end devices are listed in Table 8.7. The lack of addlw and sublw can make some applications awkward where these instructions are made on immediate values stored in the w register. To counter the lack of the addlw and sublw instructions, I rst try to arrange the statements to avoid the need for these instructions. For example, instead of
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movf addlw andlw movf A, w 3 0x3F PORTB
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TABLE 8.7 THE MID-RANGE PIC MICROCONTROLLER INSTRUCTIONS NOT AVAILABLE IN LOW-END DEVICES
addlw sublw return ret e
ASSEMBLY-LANGUAGE SOFTWARE TECHNIQUES
I would write the code function as
movlw addlw andlw movwf 3 A, w 0x3F PORTB
which can execute on both mid-range and low-end PIC microcontroller processors. Things can get a bit tricky when you have speci c requirements for the code. When I wrote the preceding example for the second edition of this book, I wrote it as
movf andlw addlw movf A, w 0x3F 3 PORTB
which is a subtle change to the preceding and provides a different function the sum of the least signi cant 6 bits of A have 3 added to them, and the sum placed in PORTB. In this situation, it seems like there is a need for setting bit 6 of PORTB if the least signi cant 6 bits of A are 0x3D or greater. To implement this as ef ciently in the low-end PIC microcontroller architecture is not possible. After some deliberation, the best conversion that I could come up with for the four mid-range instructions is the low-end sixinstruction snippet that requires a temporary value:
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