PIC16F628A-ISLASHML datasheet(93/168 Pages) MICROCHIP

93 / 168 page

 2002 Microchip Technology Inc.

Preliminary

DS40044A-page 91

PIC16F627A/628A/648A

13.3

READING THE EEPROM DATA

MEMORY

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

address to the EEADR register and then set control bit

RD (EECON1<0>). The data is available, in the very

next cycle, in the EEDATA register; therefore it can be

read in the next instruction. EEDATA will hold this value

until another read or until it is written to by the user

(during a write operation).

EXAMPLE 13-1:

DATA EEPROM READ

13.4

WRITING TO THE EEPROM DATA

MEMORY

To write an EEPROM data location, the user must first

write the address to the EEADR register and the data

to the EEDATA register. Then the user must follow a

specific sequence to initiate the write for each byte.

EXAMPLE 13-2:

DATA EEPROM WRITE

The write will not initiate if the above sequence is not

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

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

recommend that interrupts be disabled during this

code segment. A cycle count is executed during the

required sequence. Any number what is not equal to

the required cycles to execute the required sequence

will cause the data not to be written into the EEPROM.

Additionally, the WREN bit in EECON1 must be set to

enable write. This mechanism prevents accidental

writes to data EEPROM due to errant (unexpected)

code execution (i.e., lost programs). The user should

keep the WREN bit clear at all times, except when

updating EEPROM. The WREN bit is not cleared

by hardware.

After a write sequence has been initiated, clearing the

WREN bit will not affect this write cycle. The WR bit will

be inhibited from being set unless the WREN bit is set.

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

cleared in hardware and the EE Write Complete

Interrupt Flag bit (EEIF) is set. The user can either

enable this interrupt or poll this bit. The EEIF bit in the

PIR1 registers must be cleared by software.

13.5

WRITE VERIFY

Depending on the application, good programming

practice may dictate that the value written to the Data

EEPROM should be verified (Example 13-3) to the

desired value to be written. This should be used in

applications where an EEPROM bit will be stressed

near the specification limit.

EXAMPLE 13-3:

WRITE VERIFY

13.6

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, WREN is cleared. Also

when enabled, 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.

BSF

STATUS, RP0

;Bank 1

MOVLW

CONFIG_ADDR

;

MOVWF

EEADR

;Address to read

BSF

EECON1, RD

;EE Read

MOVF

EEDATA, W

;W = EEDATA

BCF

STATUS, RP0

;Bank 0

BSF

STATUS, RP0

;Bank 1

BSF

EECON1, WREN

;Enable write

BCF

INTCON, GIE

;Disable INTs.

MOVLW

55h

;

MOVWF

EECON2

;Write 55h

MOVLW

AAh

;

MOVWF

EECON2

;Write AAh

BSF

EECON1,WR

;Set WR bit

;begin write

BSF INTCON, GIE

;Enable INTs.

BSF

STATUS, RP0 ;Bank 1

MOVF

EEDATA, W

BSF

EECON1, RD ;Read the

;value written

;

;Is the value written (in W reg) and

;read (in EEDATA) the same?

;

SUBWF EEDATA, W

;

BTFSS STATUS, Z

;Is difference 0?

GOTO

WRITE_ERR

;NO, Write error

;YES, Good write

;Continue program

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