PIC16LC71-04SLASHP datasheet(23/176 Pages) MICROCHIP

23 / 176 page

DS30272A-page 23

PIC16C71X

4.3

PCL and PCLATH

The program counter (PC) is 13-bits wide. The low byte

comes from the PCL register, which is a readable and

writable register. The upper bits (PC<12:8>) are not

readable, but are indirectly writable through the

PCLATH register. On any reset, the upper bits of the

PC will be cleared. Figure 4-14 shows the two situa-

tions for the loading of the PC. The upper example in

the ﬁgure shows how the PC is loaded on a write to

PCL (PCLATH<4:0>

→ PCH).The lower example in the

ﬁgure shows how the PC is loaded during a CALL or

GOTO

instruction (PCLATH<4:3>

→ PCH).

FIGURE 4-14: LOADING OF PC IN

DIFFERENT SITUATIONS

4.3.1

COMPUTED GOTO

A computed GOTO is accomplished by adding an off-

set 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).

PCLATH<4:0>

PCLATH

Instruction with

ALU

GOTO, CALL

Opcode <10:0>

11 10

PCLATH<4:3>

PCH

PCL

PCLATH

PCH

PCL

PCL as

Destination

4.3.2

STACK

The PIC16CXX family has an 8 level deep x 13-bit wide

hardware stack. The stack space is not part of either

program or data space and the stack pointer is not

readable or writable. The PC is PUSHed onto the stack

when a CALL instruction is executed or an interrupt

causes a branch. The stack is POPed in the event of a

RETURN,

RETLW

or a RETFIE instruction execution.

PCLATH is not affected by a PUSH or POP operation.

The stack operates as a circular buffer. This means that

after the stack has been PUSHed 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).

4.4

Program Memory Paging

The PIC16C71X devices ignore both paging bits

(PCLATH<4:3>, which are used to access program

memory when more than one page is available. The

use of PCLATH<4:3> as general purpose read/write

bits for the PIC16C71X is not recommended since this

may affect upward compatibility with future products.

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.

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