18F452 datasheet(61/332 Pages) MICROCHIP

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 2002 Microchip Technology Inc.

DS39564B-page 59

PIC18FXX2

5.3

Reading the FLASH Program

Memory

The TBLRD instruction is used to retrieve data from pro-

gram memory and place into data RAM. Table Reads

from program memory are performed one byte at a

time.

TBLPTR points to a byte address in program space.

Executing TBLRD places the byte pointed to into

TABLAT. In addition, TBLPTR can be modified

automatically for the next Table Read operation.

The internal program memory is typically organized by

words. The Least Significant bit of the address selects

between the high and low bytes of the word. Figure 5-4

shows the interface between the internal program

memory and the TABLAT.

FIGURE 5-4:

READS FROM FLASH PROGRAM MEMORY

EXAMPLE 5-1:

READING A FLASH PROGRAM MEMORY WORD

(Even Byte Address)

Program Memory

(Odd Byte Address)

TBLRD

TABLAT

TBLPTR = xxxxx1

FETCH

Instruction Register

(IR)

Read Register

TBLPTR = xxxxx0

MOVLW CODE_ADDR_UPPER

; Load TBLPTR with the base

MOVWF TBLPTRU

; address of the word

MOVLW CODE_ADDR_HIGH

MOVWF TBLPTRH

MOVLW CODE_ADDR_LOW

MOVWF TBLPTRL

READ_WORD

TBLRD*+

; read into TABLAT and increment

MOVF TABLAT, W

; get data

MOVWF WORD_EVEN

TBLRD*+

; read into TABLAT and increment

MOVF TABLAT, W

; get data

MOVWF WORD_ODD

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