PIC16C55X(A)
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.
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
MICROCHIP [ MICROCHIP TECHNOLOGY ]
