barcode reader in asp.net Figure 6.4 Basic PIC microcontroller processor architecture. in Software

Encoder Denso QR Bar Code in Software Figure 6.4 Basic PIC microcontroller processor architecture.

Figure 6.4 Basic PIC microcontroller processor architecture.
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These 128 register addresses are known as a bank. To expand the register space beyond 128 addresses for hardware and variable registers, Microchip has added the capability of accessing multiple banks of registers, each capable of registering 128 addresses in the mid-range PIC microcontrollers. The low-end PIC microcontrollers can access 32 registers per bank, also with the opportunity of having four banks accessible by processor for up to 128 register addresses in total. This will be explained later in this chapter, along with how register addressing is implemented for the PIC17C and PIC18C processors. The ALU shown in Fig. 6.4 is an acronym for the arithmetic/logic unit. This circuit is responsible for doing all the arithmetic and bitwise operations, as well as the conditional instruction skips implemented in the PIC microcontroller s instruction set. Every microprocessor available today has an ALU that integrates these functions into one block of circuits. The ALU will be discussed later in this chapter. The program counter maintains the current program instruction address in the program memory (which contains the instructions for the PIC microcontroller processor, each one of which is read out in sequence and stored in the instruction reg and then decoded by the instruction decode and control circuitry. The program memory contains the code that is executed as the PIC microcontroller application. The contents of the program memory consist of the full instruction at each address (which is 12 bits for the low-end, 14 bits for the mid-range and 16 bits for both the PIC17 and PIC18 devices). This differs from many other microcontrollers in which the program memory is only 8 bits wide, and instructions that are larger than 8 bits are read in subsequent reads. Providing the full instruction in program memory and reading it at the same time result in the PIC microcontroller being somewhat faster in instruction fetches than other microcontrollers. The block diagram in Fig. 6.4, while having 80 percent or more of the circuits needed for the PIC microcontroller s processor is not a viable processor design in itself. As drawn in Fig. 6.4, there is no way to pass data to the program memory for immediate addressing, and there is no way to modify the program counter. As I work through this chapter, I will be eshing out Fig. 6.4 until it is a complete processor that can execute PIC microcontroller instructions. To implement two-argument operations, a temporary holding register, often known as an accumulator, is required to save a temporary value while the instruction fetches data from another register or is passed a constant value from the instruction. In the PIC microcontroller, the accumulator is known as the working register or, more commonly, as the w register. The w register really cannot be accessed directly as a register address in itself in the low-end and mid-range PIC microcontrollers. Instead, the contents must be moved to other registers that can be accessed directly. The w register can be accessed as an addressed register in the PIC17 and PIC18 devices. Every arithmetic operation that takes place in the PIC microcontroller uses the w register. If you want to add the contents of two registers together, you would rst move the contents of one register into the w register and then add the contents of the second to it. The PIC microcontroller architecture is very powerful from the perspective that the result of this operation can be stored either in the w register or the source of the data.
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THE MICROCHIP PIC MCU PROCESSOR ARCHITECTURE
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Figure 6.5 PIC microcontroller processor architecture with the w register and le registers as source and destination for ALU operations.
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Storing the result back into the source effectively eliminates the need for an additional instruction for saving the result of the operation. There is a lot of exibility in how instructions are executed to provide arithmetic and bitwise operations for an application. Adding the w register changes how the ALU is wired in the PIC microcontroller processor block diagram, as shown in Fig. 6.5. Note that the ALU has changed to a device with two inputs (which is the case in the actual PIC microcontroller s ALU) and that the contents of the w register are used as one of the inputs. You also should note that when a result is passed from the ALU, it could either be stored into the w register or in one of the le registers. This is a bit of foreshadowing of one of the most important features of the PIC microcontroller architecture and how instructions execute. Figure 6.5 shows the PIC microcontroller at its simplest level. This simple circuit can execute well over half the PIC microcontroller s instructions.
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