PIC16 datasheet(17/418 Pages) MICROCHIP

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DS40001609C-page 17

PIC16(L)F1508/9

3.1.1

READING PROGRAM MEMORY AS

DATA

There are two methods of accessing constants in pro-

gram 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 sim-

ple 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 set-

ting bit 7 of the FSRxH register and reading the match-

ing INDFx register. The MOVIW instruction will place the

lower eight bits of the addressed word in the W register.

Writes to the program memory cannot be performed via

the INDF registers. Instructions that access the pro-

gram memory via the FSR require one extra instruction

cycle to complete. Example 3-2 demonstrates access-

ing the program memory via an FSR.

The HIGH operator 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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