PIC16F720-ESLASHML datasheet(24/244 Pages) MICROCHIP

24 / 244 page

PIC16F/LF720/721

DS41430A-page 24

 2010 Microchip Technology Inc.

2.3

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 byte (PC<12:8>) is not

directly

readable

writable

and

comes

from

PCLATH. On any Reset, the PC is cleared. Figure 2-5

shows the two situations for the loading of the PC. The

upper example in Figure 2-5 shows how the PC is

loaded on a write to PCL (PCLATH<4:0>

 PCH).

The lower example in Figure 2-5 shows how the PC is

loaded

during

CALL

GOTO

instruction

(PCLATH<4:3>

 PCH).

FIGURE 2-5:

LOADING OF PC IN

DIFFERENT SITUATIONS

2.3.1

COMPUTED GOTO

A computed GOTO is accomplished by adding an offset

to the program counter (ADDWF PCL). When perform-

ing 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 AN556, “Implementing a Table Read”

(DS00556).

2.3.2

STACK

All devices have an 8-level x 13-bit wide hardware

stack (refer to Figures 2-1 and 2-2). 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).

2.4

Program Memory Paging

All devices are capable of addressing a continuous 8K

word block of program memory. The CALL and GOTO

instructions provide only 11 bits of address to allow

branching within any 2K program memory page. When

doing a CALL or GOTO instruction, the upper 2 bits of

the address are provided by PCLATH<4:3>. When

doing a CALL or GOTO instruction, the user must ensure

that the page select bits are programmed so that the

desired program memory page is addressed. If a return

from a CALL instruction (or interrupt) is executed, the

entire 13-bit PC is POPed off the stack. Therefore,

manipulation of the PCLATH<4:3> bits is not required

for the RETURN instructions (which POPs the address

from the stack).

Example 2-1 shows the calling of a subroutine in

page 1 of the program memory. This example assumes

that PCLATH is saved and restored by the Interrupt

Service Routine (if interrupts are used).

EXAMPLE 2-1:

CALL OF A SUBROUTINE

IN PAGE 1 FROM PAGE 0

8 7

PCLATH<4:0>

PCLATH

Instruction with

ALU Result

GOTO

, CALL

Opcode<10:0>

12 11 10

PCLATH<4:3>

PCH

PCL

PCLATH

PCH

PCL

PCL as

Destination

Note 1: There are no Status bits to indicate stack

overflow or stack underflow conditions.

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

the

vectoring

interrupt address.

Note:

The contents of the PCLATH register are

unchanged after a RETURN or RETFIE

instruction is executed. The user must

rewrite the contents of the PCLATH regis-

ter for any subsequent subroutine calls or

GOTO

instructions.

ORG 500h

PAGESEL SUB_P1

;Select page 1

;(800h-FFFh)

CALL

SUB1_P1 ;Call subroutine in

;page 1 (800h-FFFh)

ORG

900h

;page 1 (800h-FFFh)

SUB1_P1

;called subroutine

;page 1 (800h-FFFh)

RETURN

;return to

;Call subroutine

;in page 0

;(000h-7FFh)

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