PIC16F882-I datasheet(122/328 Pages) MICROCHIP

122 / 328 page

PIC16F882/883/884/886/887

DS41291F-page 120

10.3

Write Verify

Depending on the application, good programming

practice may dictate that the value written to the data

EEPROM should be verified (see Example 10-5) to the

desired value to be written.

EXAMPLE 10-5:

WRITE VERIFY

10.3.1

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 in 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.

10.4

Protection Against Spurious Write

There are conditions when the user 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, the

Power-up

Timer

(64 ms

duration)

prevents

EEPROM write.

The write initiate sequence and the WREN bit together

help prevent an accidental write during:

• Brown-out

•Power Glitch

• Software Malfunction

10.5

Data EEPROM Operation During

Code-Protect

Data memory can be code-protected by programming

the CPD bit in the Configuration Word Register 1

(Register 14-1) to ‘0’.

When the data memory is code-protected, only the

CPU is able to read and write data to the data

EEPROM. It is recommended to code-protect the pro-

gram memory when code-protecting data memory.

This prevents anyone from programming zeroes over

the existing code (which will execute as NOPs) to reach

an added routine, programmed in unused program

memory, which outputs the contents of data memory.

Programming unused locations in program memory to

‘0’ will also help prevent data memory code protection

from becoming breached.

BANKSEL EEDAT

;

MOVF

EEDAT, W

;EEDAT not changed

;from previous write

BANKSEL EECON1

;

BSF

EECON1, RD

;YES, Read the

;value written

BANKSEL EEDAT

;

XORWF

EEDAT, W

;

BTFSS

STATUS, Z

;Is data the same

GOTO

WRITE_ERR

;No, handle error

;Yes, continue

BCF

STATUS, RP1

;Bank 0

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