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16C554

EPROM-Based 8-Bit CMOS Microcontrollers

Microchip Technology 

3.0 ARCHITECTURAL OVERVIEW

The high performance of the PIC16C55X family can be

attributed to a number of architectural features

commonly found in RISC microprocessors. To begin

with, the PIC16C55X uses a Harvard architecture in

which program and data are accessed from separate

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 from 8-bit

wide data words. Instruction opcodes are 14-bit 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 single-

cycle (200 ns @ 20 MHz) except for program branches.

The table below lists the memory (EPROM and RAM).

Device

PIC16C554

PIC16C557

PIC16C558

Program

Memory

(EPROM)

512

2K

2K

Data

Memor

(RAM)

80

128

128

The PIC16C554 addresses 512 x 14 on-chip program

memory. The PIC16C557 and PIC16C558 addresses

2 K x 14 program memory. All program memory is inter-

nal.

The PIC16C55X 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 has an orthog-

onal (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 programming with the

PIC16C55X simple yet efficient. In addition, the

learning curve is reduced significantly.

The PIC16C55X 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.

PIC16C55X

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.

 1996-2013 Microchip Technology Inc.

Preliminary

DS40143E-page 9

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