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PIC16C55XA

EPROM-Based 8-Bit CMOS Microcontroller

Microchip Technology 

3.0 ARCHITECTURAL OVERVIEW

The high performance of the PIC16C55X(A) family can

be attributed to a number of architectural features

commonly found in RISC microprocessors. To begin

with, the PIC16C55X(A) uses a Harvard architecture,

in which, program and data are accessed from sepa-

rate memories using separate busses. This improves

bandwidth over traditional von Neumann architecture

where program and data are fetched from the same

memory. Separating program and data memory further

allows instructions to be sized differently than 8-bit

wide data words. Instruction opcodes are 14-bits wide

making it possible to have all single word instructions.

A 14-bit wide program memory access bus fetches a

14-bit instruction in a single cycle. A two-stage pipeline

overlaps fetch and execution of instructions.

Consequently, all instructions (35) execute in a sin-

gle-cycle (200 ns @ 20 MHz) except for program

branches.

The PIC16C554(A) addresses 512 x 14 on-chip pro-

gram memory. The PIC16C556A addresses 1K x 14

program memory. The PIC16C558(A) addresses 2K x

14 program memory. All program memory is internal.

The PIC16C55X(A) can directly or indirectly address its

register files or data memory. All special function

registers including the program counter are mapped

into the data memory. The PIC16C55X(A) have 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 situations’ make program-

ming with the PIC16C55X(A) simple yet efficient. In

addition, the learning curve is reduced significantly.

PIC16C55X(A)

The PIC16C55X(A) devices contain an 8-bit ALU and

working register. The ALU is a general purpose

arithmetic 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,

subtraction, shift and logical operations. Unless

otherwise mentioned, arithmetic operations are two's

complement in nature. In two-operand instructions,

typically one operand is the working register

(W register). The other operand is a file register or an

immediate constant. In single operand instructions, the

operand is either the W register or a file register.

The W register is an 8-bit working register used for ALU

operations. It is not an addressable register.

Depending on the instruction executed, the ALU may

affect the values of the Carry (C), Digit Carry (DC), and

Zero (Z) bits in the STATUS register. The C and DC bits

operate as a Borrow and Digit Borrow out bit,

respectively, in subtraction. See the SUBLW and

SUBWF instructions for examples.

A simplified block diagram is shown in Figure 3-1, with

a description of the device pins in Table 3-1.

© 1997 Microchip Technology Inc.

Preliminary

DS40143B-page 9

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