PIC16F877-20SLASHP datasheet(47/218 Pages) MICROCHIP

47 / 218 page

 2001 Microchip Technology Inc.

DS30292C-page 45

PIC16F87X

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

cleared and the EEIF interrupt flag bit is set. (EEIF

must be cleared by firmware.) Since the microcontroller

does not execute instructions during the write cycle, the

firmware does not necessarily have to check either

EEIF, or WR, to determine if the write had finished.

EXAMPLE 4-4:

FLASH PROGRAM WRITE

4.6

Write Verify

The PIC16F87X devices do not automatically verify the

value written during a write operation. Depending on

the application, good programming practice may dic-

tate that the value written to memory be verified against

the original value. This should be used in applications

where excessive writes can stress bits near the speci-

fied endurance limits.

4.7

Protection Against Spurious

Writes

There are conditions when the device may not want to

write to the EEPROM data memory or FLASH program

memory. To protect against these spurious write condi-

tions, various mechanisms have been built into the

PIC16F87X devices. On power-up, the WREN bit is

cleared and the Power-up Timer (if enabled) prevents

writes.

The write initiate sequence, and the WREN bit

together, help prevent any accidental writes during

brown-out, power glitches, or firmware malfunction.

4.8

Operation While Code Protected

The PIC16F87X devices have two code protect mecha-

nisms, one bit for EEPROM data memory and two bits for

FLASH program memory. Data can be read and written

to the EEPROM data memory, regardless of the state of

the code protection bit, CPD. When code protection is

enabled and CPD cleared, external access via ICSP is

disabled, regardless of the state of the program memory

code protect bits. This prevents the contents of EEPROM

data memory from being read out of the device.

The state of the program memory code protect bits,

CP0 and CP1, do not affect the execution of instruc-

tions out of program memory. The PIC16F87X devices

can always read the values in program memory,

regardless of the state of the code protect bits. How-

ever, the state of the code protect bits and the WRT bit

will have different effects on writing to program mem-

ory. Table 4-1 shows the effect of the code protect bits

and the WRT bit on program memory.

Once code protection has been enabled for either

EEPROM data memory or FLASH program memory,

only a full erase of the entire device will disable code

protection.

BSF

STATUS, RP1

;

BCF

STATUS, RP0

;Bank 2

MOVF

ADDRL, W

;Write address

MOVWF

EEADR

;of desired

MOVF

ADDRH, W

;program memory

MOVWF

EEADRH

;location

MOVF

VALUEL, W

;Write value to

MOVWF

EEDATA

;program at

MOVF

VALUEH, W

;desired memory

MOVWF

EEDATH

;location

BSF

STATUS, RP0

;Bank 3

BSF

EECON1, EEPGD ;Point to Program memory

BSF

EECON1, WREN

;Enable writes

;Only disable interrupts

BCF

INTCON, GIE

;if already enabled,

;otherwise discard

MOVLW

0x55

;Write 55h to

MOVWF

EECON2

;EECON2

MOVLW

0xAA

;Write AAh to

MOVWF

EECON2

;EECON2

BSF

EECON1, WR

;Start write operation

NOP

;Two NOPs to allow micro

NOP

;to setup for write

;Only enable interrupts

BSF

INTCON, GIE

;if using interrupts,

;otherwise discard

BCF

EECON1, WREN

;Disable writes

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