PIC16F84 datasheet(17/110 Pages) MICROCHIP

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PIC16C84

 1996-2013 Microchip Technology Inc.

DS30445D-page 17

4.3

Program Counter: PCL and PCLATH

The Program Counter (PC) is 13-bits wide. The low

byte is the PCL register, which is a readable and

writable register. The high byte of the PC (PC<12:8>) is

not directly readable nor writable and comes from the

PCLATH register. The PCLATH (PC latch high) register

is a holding register for PC<12:8>. The contents of

PCLATH are transferred to the upper byte of the

program counter when the PC is loaded with a new

value. This occurs during a CALL, GOTO or a write to

PCL. The high bits of PC are loaded from PCLATH as

shown in Figure 4-6.

FIGURE 4-6:

LOADING OF PC IN

DIFFERENT SITUATIONS

4.3.1

COMPUTED GOTO

A computed GOTO is accomplished by adding an offset

to the program counter (ADDWF

PCL

). When doing a

table read using a computed GOTO method, care should

be exercised if the table location crosses a PCL memory

boundary (each 256 word block). Refer to the application

note

“Implementing a Table Read” (AN556).

4.3.2

PROGRAM MEMORY PAGING

The PIC16C84 has 1K of program memory. The CALL

and GOTO instructions have an 11-bit address range.

This 11-bit address range allows a branch within a 2K

program memory page size. For future PIC16CXX

program memory expansion, there must be another

two bits to specify the program memory page. These

paging

bits

come

from

the

PCLATH<4:3>

bits

(Figure 4-6). When doing a CALL or a GOTO instruction,

the

user

must

ensure

that

these

page

bits

(PCLATH<4:3>) are programmed to the desired

program memory page. If a CALL instruction (or

interrupt) is executed, the entire 13-bit PC is “pushed”

onto the stack (see next section). Therefore, manipula-

tion of the PCLATH<4:3> is not required for the return

instructions (which “pops” the PC from the stack).

4.4

Stack

The PIC16C84 has an 8 deep x 13-bit wide hardware

stack (Figure 4-1). The stack space is not part of either

program or data space and the stack pointer is not

readable or writable.

The entire 13-bit PC is “pushed” onto the stack when a

CALL

instruction is executed or an interrupt is acknowl-

edged. The stack is “popped” in the event of a

RETURN,

RETLW

or a RETFIE instruction execution.

PCLATH is not affected by a push or a pop operation.

The stack operates as a circular buffer. That is, after the

stack has been pushed eight times, the ninth push over-

writes the value that was stored from the first push. The

tenth push overwrites the second push (and so on).

If the stack is effectively popped nine times, the PC

value is the same as the value from the first pop.

8 7

PCLATH<4:0>

PCLATH

INST with PCL

as dest

ALU result

GOTO, CALL

Opcode <10:0>

12 11 10

PCLATH<4:3>

PCH

PCL

PCLATH

PCH

PCL

Note:

The PIC16C84 ignores the PCLATH<4:3>

bits, which are used for program memory

pages 1, 2 and 3 (0800h - 1FFFh). The use

of PCLATH<4:3> as general purpose R/W

bits is not recommended since this may

affect upward compatibility with future

products.

Note:

There are no instruction mnemonics called

push or pop. These are actions that occur

from the execution of the CALL, RETURN,

RETLW,

and RETFIE instructions, or the

vectoring to an interrupt address.

Note:

There are no status bits to indicate stack

overflow or stack underflow conditions.

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