PIC18F4685-ISLASHSP datasheet(110/484 Pages) MICROCHIP

110 / 484 page

PIC18F2682/2685/4682/4685

DS39761B-page 108

Preliminary

7.6

Operation During Code-Protect

Data EEPROM memory has its own code-protect bits in

Configuration

Words.

External

read

and

write

operations are disabled if code protection is enabled.

The microcontroller itself can both read and write to the

internal Data EEPROM, regardless of the state of the

code-protect Configuration bit. Refer to Section 24.0

“Special Features of the CPU” for additional

information.

7.7

Protection Against Spurious Write

There are conditions when the device may not want to

write to the data EEPROM memory. To protect against

spurious EEPROM writes, various mechanisms have

been implemented. On power-up, the WREN bit is

cleared. In addition, writes to the EEPROM are blocked

during

the

Power-up

Timer

period

(TPWRT,

parameter 33).

The write initiate sequence and the WREN bit together

help prevent an accidental write during brown-out,

power glitch or software malfunction.

7.8

Using the Data EEPROM

The data EEPROM is a high-endurance, byte address-

able array that has been optimized for the storage of

frequently

changing

information

(e.g.,

program

variables or other data that are updated often).

Frequently changing values will typically be updated

more often than specification D124. If this is not the

case, an array refresh must be performed. For this

reason, variables that change infrequently (such as

constants, IDs, calibration, etc.) should be stored in

Flash program memory.

A simple data EEPROM refresh routine is shown in

Example 7-3.

EXAMPLE 7-3:

DATA EEPROM REFRESH ROUTINE

Note:

If data EEPROM is only used to store

constants and/or data that changes rarely,

an array refresh is likely not required. See

specification D124.

CLRF

EEADR

; Start at address 0

CLRF

EEADRH

;

BCF

EECON1, CFGS

; Set for memory

BCF

EECON1, EEPGD

; Set for Data EEPROM

BCF

INTCON, GIE

; Disable interrupts

BSF

EECON1, WREN

; Enable writes

Loop

; Loop to refresh array

BSF

EECON1, RD

; Read current address

MOVLW

55h

;

MOVWF

EECON2

; Write 55h

MOVLW

0AAh

;

MOVWF

EECON2

; Write 0AAh

BSF

EECON1, WR

; Set WR bit to begin write

BTFSC

EECON1, WR

; Wait for write to complete

BRA

$-2

INCFSZ

EEADR, F

; Increment address

BRA

LOOP

; Not zero, do it again

INCFSZ

EEADRH, F

; Increment the high address

BRA

LOOP

; Not zero, do it again

BCF

EECON1, WREN

; Disable writes

BSF

INTCON, GIE

; Enable interrupts

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