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TABLE 6.3 BANK RP1
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BANK ADDRESS TO RPX BIT SETTINGS RP0 ADDRESS RANGE XOR VALUE
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The index register (FSR), as I indicated earlier, is 8 bits in size, and its bit 7 is used to select between the odd and even banks (bank 0 and bank 2 versus bank 1 and bank 3). Put another way, if bit 7 of the FSR is set, then the register being pointed to is in the odd register bank. This straddling of the banks makes it very easy to access different banks without changing the RP0 bit. For the preceding example, if I were to use the FSR register to point to TRISB instead of accessing it directly, I could use the code
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PORTB.Bit0 = 1; FSR = TRISB; INDF.Bit0 = 0; // Load PORTB.Bit0 with a 1 // FSR Points to TRISB // Make PORTB.Bit0 Output
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This ability of the mid-range FSR register to access both banks 0 and 1 is why I recommend that for many applications array variables should be placed in odd banks, and single-element variables should be placed in even banks. Of course, this is only possible if the entire le register range is not shadowed across the banks as in the PIC16F84 and other simple mid-range PIC microcontrollers that are used in introductory applications. To select between the high and low banks with the FSR, the IRP bit of the STATUS register is used. This bit is analogous to the RP1 bit for direct addressing. Having separate bits for selecting between the high and low bank pairs means that data can be transferred between banks using direct and index addressing without having to change the bank-select bits for either case. There is one thing that I have to note with regard to the FSR register and indirect addressing. Even though the FSR register can access 256 different register addresses across two banks, it cannot be used to access more than 128 le registers contiguously (or all in a row). The reason for this is the control registers contained at the rst few addresses of each bank. If you try to wrap around a 128-byte bank, you will corrupt the PIC microcontroller s control registers with disastrous results.
ZERO REGISTERS
I don t really know if this quali es as a feature, but unused registers in a PIC microcontroller s register map will return 0 (0x00) when they are read. This capability can be useful in some applications. Zero registers (unde ned registers that return 0 when
THE MICROCHIP PIC MCU PROCESSOR ARCHITECTURE
read) are normally de ned in the Microchip documentation as shaded addresses in the device register map documentation. In Fig. 6.6, the PIC16F84 s register map is shown with addresses 7 (PORTC registers) in each bank shaded, indicating that they return 0 when read. Of course, when these registers are written to, their values are lost and not stored in the register. (One might say the information has gone to the great bit bucket in the sky. ) I am hesitant to recommend using the zero registers when programming. It is important to note that in different PIC microcontrollers, the zero registers are at different locations. Because of this, if code is transferred directly from one application to another and the zero register chosen is not available in the PIC MCU destination (e.g., a valid le or hardware register is at this location), then the code will not work correctly. Instead of using a hardware zero register, I would recommend that a le register be de ned and cleared for the purpose of always returning 0.
The PIC Microcontroller s ALU
The arithmetic/logic unit, which is labeled ALU in the PIC microcontroller block diagrams, performs arithmetic, bitwise, and shifting operations on 1 or 2 bytes of data at a time. These three simple functions have been optimized to maximize the performance of the PIC microcontroller and minimize the cost of building the MCUs. An in-depth understanding of the ALU s function is not critical to developing applications for the PIC microcontroller; however, having an idea of the tradeoffs that were made in designing the ALU will give you a better idea of how PIC microcontroller instructions execute and what is the best way to create your applications. In this discussion of how the PIC microcontroller s ALU operates and is designed, I have been able to encompass 27 of the 37 instructions available in the mid-range PIC microcontroller processor. Twenty- ve years ago, when the PIC microcontroller was rst developed, any savings in circuits used in the ALU (or anywhere else in the device) paid huge dividends in the nal cost of manufacturing the device. This philosophy has been embraced in the ALUs used in the different PIC microcontroller processor architectures. I tend to think of the ALU as a number of processor operations that execute in parallel with a single multiplexer that is used to select which result is to be used by the application. Graphically, this looks like the block diagram shown in Fig. 6.7. The STATUS register stores the results of the operations and will be described in more detail in the next section. The ALU is the primary modi er of the STATUS bits that are used to record the result of operations, as well as providing input to the data shift instructions. The circuit shown in the block diagram Fig. 6.7 certainly would work as drawn, but it would require a large number of redundant circuits. Many of these functions could be combined into a single circuit by looking for opportunities such as noting that an Increment is addition by one and combining the two functions. A list of arithmetic and bitwise functions available within the PIC microcontroller, along with the combinations necessary to provide the full range of arithmetic operations, can be found in Table 6.4.
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