PIC12F635ESS datasheet(95/234 Pages) MICROCHIP

95 / 234 page

DS41232D-page 93

PIC12F635/PIC16F636/639

9.2

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 of the EECON1 register, as shown in Example 9-1.

The data is available, in the very next cycle, in the

EEDAT register. Therefore, it can be read in the next

instruction. EEDAT holds this value until another read, or

until it is written to by the user (during a write operation).

EXAMPLE 9-1:

DATA EEPROM READ

9.3

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 EEDAT register. Then the user must follow a

specific sequence to initiate the write for each byte, as

shown in Example 9-2.

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 that is not equal to the

required cycles to execute the required sequence will

prevent the data from being 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 of the

PIR1 register must be cleared by software.

EXAMPLE 9-2:

DATA EEPROM WRITE

9.4

Write Verify

Depending on the application, good programming

practice may dictate that the value written to the data

EEPROM should be verified (see Example 9-3) to the

desired value to be written.

EXAMPLE 9-3:

WRITE VERIFY

9.4.1

USING THE DATA EEPROM

The

data

EEPROM

high-endurance,

byte

addressable array that has been optimized for the

storage of frequently changing information (e.g.,

program variables or other data that are updated

often).

When

variables

one

section

change

frequently, while variables in another section do not

change, it is possible to exceed the total number of

write cycles to the EEPROM (specification D124)

without exceeding the total number of write cycles to a

single byte (specifications D120 and D120A). If this is

the case, then a refresh of the array must be

performed. For this reason, variables that change

infrequently (such as constants, IDs, calibration, etc.)

should be stored in Flash program memory.

BANKSEL

EEADR

;

MOVLW

CONFIG_ADDR

;

MOVWF

EEADR

;Address to read

BSF

EECON1,RD

;EE Read

MOVF

EEDAT,W

;Move data to W

BANKSEL EEADR

;

BSF

EECON1,WREN

;Enable write

BCF

INTCON,GIE

;Disable INTs

MOVLW

55h

;Unlock write

MOVWF

EECON2

;

MOVLW

AAh

;

MOVWF

EECON2

;

BSF

EECON1,WR

;Start the write

BSF

INTCON,GIE

;Enable INTS

BANKSEL

EEDAT

;

MOVF

EEDAT,W

;EEDAT not changed

;from previous write

BSF

EECON1,RD

;YES, Read the

;value written

XORWF

EEDAT,W

;

BTFSS

STATUS,Z

;Is data the same

GOTO

WRITE_ERR

;No, handle error

;Yes, continue

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