PIC16F631/677/685/687/689/690
When the data memory is code-protected, only the
CPU is able to read and write data to the data
10.2 Write Verify
Depending on the application, good programming
practice may dictate that the value written to the data
EEPROM should be verified (see Example 10-4) to the
desired value to be written.
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.
EXAMPLE 10-4:
WRITE VERIFY
BANKSEL EEDAT
;
MOVF
EEDAT, W
;EEDAT not changed
;from previous write
BANKSEL EECON1
;
BSF
EECON1, RD
;YES, Read the
;value written
;
BANKSEL EEDAT
XORWF
BTFSS
GOTO
:
EEDAT, W
STATUS, Z
WRITE_ERR
;
;Is data the same
;No, handle error
;Yes, continue
;Bank 0
BANKSEL 0x00
10.2.1
USING THE DATA EEPROM
The data EEPROM is
a 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.
10.3 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
10.4 Data EEPROM Operation During
Code-Protect
Data memory can be code-protected by programming
the CPD bit in the Configuration Word register
(Register 14-1) to ‘0’.
© 2007 Microchip Technology Inc.
DS41262D-page 125
MICROCHIP [ MICROCHIP ]
