16F54 datasheet(21/90 Pages) MICROCHIP

21 / 90 page

 2004 Microchip Technology Inc.

DS41213C-page 19

PIC16F5X

3.5

Program Counter

As a program instruction is executed, the Program

Counter (PC) will contain the address of the next

program instruction to be executed. The PC value is

increased by one, every instruction cycle, unless an

instruction changes the PC.

For a GOTO instruction, bits 8:0 of the PC are provided

by the GOTO instruction word. The PC Latch (PCL) is

mapped to PC<7:0> (Figure 3-6 and Figure 3-7).

For the PIC16F57 and PIC16F59, a page number must

be supplied as well. Bit 5 and bit 6 of the Status register

provide page information to bit 9 and bit 10 of the PC

(Figure 3-6 and Figure 3-7).

For a CALL instruction, or any instruction where the

PCL is the destination, bits 7:0 of the PC again are

provided by the instruction word. However, PC<8>

does not come from the instruction word, but is always

cleared (Figure 3-6 and Figure 3-7).

Instructions where the PCL is the destination or modify

PCL instructions, include MOVWF PCL, ADDWF PCL,

and BSF PCL,5.

For the PIC16F57 and PIC16F59, a page number

again must be supplied. Bit 5 and bit 6 of the Status

the PC (Figure 3-6 and Figure 3-7).

FIGURE 3-6:

LOADING OF PC BRANCH

INSTRUCTIONS – PIC16F54

FIGURE 3-7:

LOADING OF PC BRANCH

INSTRUCTIONS – PIC16F57

AND PIC16F59

3.5.1

PAGING CONSIDERATIONS

PIC16F57 AND PIC16F59

If the PC is pointing to the last address of a selected

memory page, when it increments, it will cause the pro-

gram to continue in the next higher page. However, the

page preselect bits in the Status register will not be

updated. Therefore, the next GOTO, CALL or MODIFY

PCL

instruction will send the program to the page

specified by the page preselect bits (PA0 or PA<1:0>).

For example, a NOP at location 1FFh (page 0)

increments the PC to 200h (page 1). A GOTO xxx at

200h will return the program to address xxh on page 0

(assuming that PA<1:0> are clear).

To prevent this, the page preselect bits must be

updated under program control.

3.5.2

EFFECTS OF RESET

The PC is set upon a Reset, which means that the PC

addresses the last location in the last page (i.e., the

Reset vector).

The Status register page preselect bits are cleared

upon a Reset, which means that page 0 is preselected.

Therefore, upon a Reset, a

GOTO

instruction at the

Reset vector location will automatically cause the

program to jump to page 0.

Note:

Because PC<8> is cleared in the CALL

instruction or any modified PCL instruc-

tion, all subroutine calls or computed

jumps are limited to the first 256 locations

of any program memory page (512 words

long).

PCL

Reset to '0'

Instruction Word

GOTO

Instruction

CALL

or Modify PCL Instruction

PA<1:0>

Status

PCL

PA<1:0>

Status

PCL

Instruction Word

Reset to ‘0’

Instruction Word

GOTO

Instruction

CALL

or Modify PCL Instruction

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