PIC16C84A datasheet(18/117 Pages) MICROCHIP

18 / 117 page

PIC16C8X

DS30081F-page 18

4.3

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 byte block). Refer to the

application note

“Implementing a Table Read” (AN556).

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

4.3.2

STACK

The PIC16CXX 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 PUSH'ed

onto the stack when a CALL instruction is executed or

an interrupt is acknowledged. The stack is POP'ed 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 PUSH'ed eight times, the ninth push

overwrites the value that was stored from the ﬁrst push.

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

If the stack is effectively POP'ed nine times, the PC

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

4.3.3

PROGRAM MEMORY PAGING

The PIC16C83 and PIC16CR83 have 512 words of

program memory. The PIC16C84, PIC16C84A, and

PIC16CR84 have 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 PIC16C8X

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. Therefore,

manipulation

the

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

(which POPs the PC from the stack).

Note 1: There are no status bits to indicate stack

overﬂow or stack underﬂow conditions.

Note 2: There are no instructions mnemonics

called PUSH or POP. These are actions

that occur from the execution of the CALL,

RETURN,

RETLW,

and RETFIE instruc-

tions, or the vectoring to an interrupt

address

Note:

The PIC16C8X 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.

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