PIC18LF4331 datasheet(81/392 Pages) MICROCHIP

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 2010 Microchip Technology Inc.

DS39616D-page 81

PIC18F2331/2431/4331/4431

7.3

Reading the Data EEPROM

Memory

To read a data memory location, the user must write the

address to the EEADR register, clear the EEPGD con-

trol bit (EECON1<7>) and then set control bit, RD

(EECON1<0>). The data is available for the very next

instruction cycle; therefore, the EEDATA register can

be read by the next instruction. EEDATA will hold this

value until another read operation, or until it is written to

by the user (during a write operation).The basic

process is shown in Example 7-1.

7.4

Writing to the Data EEPROM

Memory

To write an EEPROM data location, the address must

first be written to the EEADR register and the data writ-

ten to the EEDATA register. The sequence in

Example 7-2 must be followed to initiate the write cycle.

The write will not begin if this sequence is not exactly

followed (write 55h to EECON2, write 0AAh to

EECON2, then set WR bit) for each byte. It is strongly

recommended that interrupts be disabled during this

code segment.

Additionally, the WREN bit in EECON1 must be set to

enable writes. This mechanism prevents accidental

writes to data EEPROM due to unexpected code exe-

cution (i.e., runaway programs). The WREN bit should

be kept clear at all times, except when updating the

EEPROM. The WREN bit is not cleared by hardware.

After a write sequence has been initiated, EECON1,

EEADR and EEDATA cannot be modified. The WR bit

will be inhibited from being set unless the WREN bit is

set. The WREN bit must be set on a previous instruc-

tion. Both WR and WREN cannot be set with the same

instruction.

At the completion of the write cycle, the WR bit is

cleared in hardware and the EEPROM Interrupt Flag bit

(EEIF) is set. The user may either enable this interrupt

or poll this bit. EEIF must be cleared by software.

7.5

Write Verify

Depending on the application, good programming

practice may dictate that the value written to the mem-

ory should be verified against the original value. This

should be used in applications where excessive writes

can stress bits near the specification limit.

7.6

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 built-in. On power-up, the WREN bit is cleared.

Also, the Power-up Timer (72 ms duration) prevents

EEPROM write.

The write initiate sequence and the WREN bit together

help prevent an accidental write during brown-out,

power glitch, or software malfunction.

EXAMPLE 7-1:

DATA EEPROM READ

EXAMPLE 7-2:

DATA EEPROM WRITE

MOVLW

DATA_EE_ADDR

;

MOVWF

EEADR

; Data Memory Address to read

BCF

EECON1, EEPGD

; Point to DATA memory

BSF

EECON1, RD

; EEPROM Read

MOVF

EEDATA, W

; W = EEDATA

MOVLW

DATA_EE_ADDR

;

MOVWF

EEADR

; Data Memory Address to write

MOVLW

DATA_EE_DATA

;

MOVWF

EEDATA

; Data Memory Value to write

BCF

EECON1, EEPGD

; Point to DATA memory

BCF

EECON1, CFGS

; Access EEPROM

BSF

EECON1, WREN

; Enable writes

BCF

INTCON, GIE

; Disable Interrupts

MOVLW

55h

;

Required

MOVWF

EECON2

; Write 55h

Sequence

MOVLW

0AAh

;

MOVWF

EECON2

; Write 0AAh

BSF

EECON1, WR

; Set WR bit to begin write

BTFSC

EECON1, WR

; Wait for write to complete

GOTO

$-2

;

BSF

INTCON, GIE

; Enable interrupts

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