PIC16LC554 datasheet(21/96 Pages) MICROCHIP

21 / 96 page

Preliminary

DS40143B-page 21

PIC16C55X(A)

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 high bits (PC<12:8>) are not directly

readable or writable and come from PCLATH. On any

reset, the PC is cleared. Figure 4-10 shows the two

situations 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 4-10: 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).

PCLATH<4:0>

PCLATH

Instruction with

ALU result

GOTO, CALL

Opcode <10:0>

11 10

PCLATH<4:3>

PCH

PCL

PCLATH

PCH

PCL

PCL as

Destination

4.3.2

STACK

The PIC16C55X(A) family has an 8 level deep x 13-bit

wide hardware stack (Figure 4-1, Figure 4-2 and

Figure 4-3). The stack space is not part of either pro-

gram or data space and the stack pointer is not read-

able 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).

Note 1: There are no STATUS bits to indicate

stack

overﬂow

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

instructions, or vectoring to an interrupt

address.

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