PIC16F87X Datasheet, PDF (47/218 Pages) Microchip Technology – 28/40-pin 8-Bit CMOS FLASH Microcontrollers

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
BSF
BCF
MOVF
MOVWF
MOVF
MOVWF
MOVF
MOVWF
MOVF
MOVWF
BSF
BSF
BSF
BCF
MOVLW
MOVWF
MOVLW
MOVWF
BSF
NOP
NOP
BSF
STATUS, RP1 ;
STATUS, RP0 ;Bank 2
ADDRL, W
;Write address
EEADR
;of desired
ADDRH, W
;program memory
EEADRH
;location
VALUEL, W
;Write value to
EEDATA
;program at
VALUEH, W
;desired memory
EEDATH
;location
STATUS, RP0 ;Bank 3
EECON1, EEPGD ;Point to Program memory
EECON1, WREN ;Enable writes
;Only disable interrupts
INTCON, GIE ;if already enabled,
;otherwise discard
0x55
;Write 55h to
EECON2
;EECON2
0xAA
;Write AAh to
EECON2
;EECON2
EECON1, WR ;Start write operation
;Two NOPs to allow micro
;to setup for write
;Only enable interrupts
INTCON, GIE ;if using interrupts,
;otherwise discard
BCF EECON1, WREN ;Disable writes
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.
PIC16F87X
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.
 2001 Microchip Technology Inc.
DS30292C-page 45