16F1788 datasheet(21/450 Pages) MICROCHIP

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Preliminary

DS41579D-page 21

PIC16(L)F1782/3

3.1.1

READING PROGRAM MEMORY AS

DATA

There are two methods of accessing constants in

program memory. The first method is to use tables of

RETLW instructions. The second method is to set an

FSR to point to the program memory.

3.1.1.1

RETLW Instruction

The RETLW instruction can be used to provide access

to tables of constants. The recommended way to create

such a table is shown in Example 3-1.

EXAMPLE 3-1:

RETLW INSTRUCTION

The BRW instruction makes this type of table very

simple to implement. If your code must remain portable

with previous generations of microcontrollers, then the

BRW instruction is not available so the older table read

method must be used.

3.1.1.2

Indirect Read with FSR

The program memory can be accessed as data by

setting bit 7 of the FSRxH register and reading the

matching INDFx register. The MOVIW instruction will

place the lower 8 bits of the addressed word in the W

performed via the INDF registers. Instructions that

access the program memory via the FSR require one

extra instruction cycle to complete. Example 3-2

demonstrates accessing the program memory via an

FSR.

The high directive will set bit<7> if a label points to a

location in program memory.

EXAMPLE 3-2:

ACCESSING PROGRAM

MEMORY VIA FSR

constants

BRW

;Add Index in W to

;program counter to

;select data

RETLW DATA0

;Index0 data

RETLW DATA1

;Index1 data

RETLW DATA2

RETLW DATA3

my_function

;… LOTS OF CODE…

MOVLW

DATA_INDEX

call constants

;… THE CONSTANT IS IN W

constants

RETLW DATA0

;Index0 data

RETLW DATA1

;Index1 data

RETLW DATA2

RETLW DATA3

my_function

;… LOTS OF CODE…

MOVLW

LOW constants

MOVWF

FSR1L

MOVLW

HIGH constants

MOVWF

FSR1H

MOVIW

0[FSR1]

;THE PROGRAM MEMORY IS IN W

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