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PIC17C75X Datasheet(PDF) 9 Page - Microchip Technology |
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PIC17C75X Datasheet(HTML) 9 Page - Microchip Technology |
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9 / 320 page  © 1997 Microchip Technology Inc. Preliminary DS30264A-page 9 PIC17C75X 3.0 ARCHITECTURAL OVERVIEW The high performance of the PIC17CXXX can be attrib- uted to a number of architectural features commonly found in RISC microprocessors. To begin with, the PIC17CXXX uses a modified Harvard architecture. This architecture has the program and data accessed from separate memories. So, the device has a program memory bus and a data memory bus. This improves bandwidth over traditional von Neumann architecture, where program and data are fetched from the same memory (accesses over the same bus). Separating program and data memory further allows instructions to be sized differently than the 8-bit wide data word. PIC17CXXX opcodes are 16-bits wide, enabling single word instructions. The full 16-bit wide program memory bus fetches a 16-bit instruction in a single cycle. A two-stage pipeline overlaps fetch and execution of instructions. Consequently, all instructions execute in a single cycle (121 ns @ 33 MHz), except for program branches and two special instructions that transfer data between program and data memory. The PIC17CXXX can address up to 64K x 16 of pro- gram memory space. The PIC17C752 integrates 8K x 16 of EPROM pro- gram memory on-chip. The PIC17C756 integrates 16K x 16 EPROM program memory. Program execution can be internal only (microcontrol- ler or protected microcontroller mode), external only (microprocessor mode) or both (extended microcon- troller mode). Extended microcontroller mode does not allow code protection. The PIC17CXXX can directly or indirectly address its register files or data memory. All special function regis- ters, including the Program Counter (PC) and Working Register (WREG), are mapped in the data memory. The PIC17CXXX has an orthogonal (symmetrical) instruction set that makes it possible to carry out any operation on any register using any addressing mode. This symmetrical nature and lack of ‘special optimal sit- uations’ make programming with the PIC17CXXX sim- ple yet efficient. In addition, the learning curve is reduced significantly. One of the PIC17CXXX family architectural enhance- ments from the PIC16CXX family allows two file regis- ters to be used in some two operand instructions. This allows data to be moved directly between two registers without going through the WREG register. Thus increasing performance and decreasing program memory usage. The PIC17CXXX devices contain an 8-bit ALU and working register. The ALU is a general purpose arith- metic unit. It performs arithmetic and Boolean functions between data in the working register and any register file. The ALU is 8-bits wide and capable of addition, sub- traction, shift, and logical operations. Unless otherwise mentioned, arithmetic operations are two's comple- ment in nature. The WREG register is an 8-bit working register used for ALU operations. All PIC17C75X devices have an 8 x 8 hardware multi- plier. This multiplier generates a 16-bit result in a single cycle. Depending on the instruction executed, the ALU may affect the values of the Carry (C), Digit Carry (DC), and Zero (Z) bits in the ALUSTA register. The C and DC bits operate as a borrow and digit borrow out bit, respec- tively, in subtraction. See the SUBLW and SUBWF instructions for examples. Although the ALU does not perform signed arithmetic, the Overflow bit (OV) can be used to implement signed math. Signed arithmetic is comprised of a magnitude and a sign bit. The overflow bit indicates if the magni- tude overflows and causes the sign bit to change state. That is if the result of the signed operation is greater then 128 (7Fh) or less then -127 (FFh). Signed math can have greater than 7-bit values (magnitude), if more than one byte is used. The use of the overflow bit only operates on bit6 (MSb of magnitude) and bit7 (sign bit) of the value in the ALU. That is, the overflow bit is not useful if trying to implement signed math where the magnitude, for example, is 11-bits. If the signed math values are greater than 7-bits (15-, 24- or 31-bit), the algorithm must ensure that the low order bytes ignore the overflow status bit. Care should be taken when adding and subtracting signed numbers to ensure that the correct operation is executed. Example 3-1 shows an item that must be taken into account when doing signed arithmetic on an ALU which operates as an unsigned machine. EXAMPLE 3-1: SIGNED MATH Signed math requires the result to be FEh (-126). This would be accomplished by subtracting one as opposed to adding one. A simplified block diagram is shown in Figure 3-1. The descriptions of the device pins are listed in Table 3-1. Hex Value Signed Value Math Unsigned Value Math FFh + 01h = ? -127 + 1 = -126 (FEh) 255 + 1 = 0 (00h); Carry bit = 1 |
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