PIC12F635/PIC16F636/639
9.5
Protection Against Spurious Write
9.6
Data EEPROM Operation During
Code Protection
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 (nominal 64 ms duration) prevents
EEPROM write.
Data memory can be code-protected by programming
the CPD bit in the Configuration Word (Register 12-1)
to ‘0’.
When the data memory is code-protected, the CPU is
able to read and write data to the data EEPROM. It is
recommended to code-protect the program 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.
The write initiate sequence and the WREN bit together
help prevent an accidental write during:
• Brown-out
• Power Glitch
• Software Malfunction
TABLE 9-1:
SUMMARY OF REGISTERS ASSOCIATED WITH DATA EEPROM
Value on
Value on
POR, BOR
Name
Bit 7
Bit 6
Bit 5
Bit 4
Bit 3
Bit 2
Bit 1
Bit 0
all other
Resets
INTCON
PIR1
GIE
EEIF
EEIE
PEIE
LVDIF
LVDIE
EEDAT6
EEADR6
—
T0IE
CRIF
INTE
C2IF(1)
C2IE(1)
EEDAT4
EEADR4
—
RAIE
C1IF
T0IF
OSFIF
INTF
—
RAIF
TMR1IF
TMR1IE
EEDAT0
EEADR0
RD
0000 000x
0000 00-0
0000 00-0
0000 0000
0000 0000
---- x000
---- ----
0000 000x
0000 00-0
0000 00-0
0000 0000
0000 0000
---- q000
---- ----
PIE1
CRIE
C1IE
OSFIE
EEDAT2
EEADR2
WREN
—
EEDAT
EEDAT7
EEADR7(1)
—
EEDAT5
EEADR5
—
EEDAT3
EEADR3
WRERR
EEDAT1
EEADR1
WR
EEADR
EECON1
EECON2
Legend:
EEPROM Control Register 2 (not a physical register)
x= unknown, u= unchanged, – = unimplemented read as ‘0’, q= value depends upon condition.
Shaded cells are not used by the data EEPROM module.
Note 1: PIC16F636/639 only.
DS41232D-page 94
© 2007 Microchip Technology Inc.
MICROCHIP [ MICROCHIP ]
