PIC18FXX8 Datasheet, PDF (279/402 Pages) Microchip Technology – 28/40-Pin High-Performance, Enhanced Flash Microcontrollers with CAN Module

PIC18FXX8
24.4.1 PROGRAM MEMORY
CODE PROTECTION
The user memory may be read to or written from any
location using the table read and table write instruc-
tions. The device ID may be read with table reads. The
Configuration registers may be read and written with
the table read and table write instructions.
In user mode, the CPn bits have no direct effect. CPn
bits inhibit external reads and writes. A block of user
memory may be protected from table writes if the
WRTn configuration bit is ‘0’. The EBTRn bits control
table reads. For a block of user memory with the
EBTRn bit set to ‘0’, a table read instruction that
executes from within that block is allowed to read. A
table read instruction that executes from a location
outside of that block is not allowed to read and will
result in reading ‘0’s. Figures 24-4 through 24-6
illustrate table write and table read protection.
Note:
Code protection bits may only be written to
a ‘0’ from a ‘1’ state. It is not possible to
write a ‘1’ to a bit in the ‘0’ state. Code
protection bits are only set to ‘1’ by a full
chip erase or block erase function. The full
chip erase and block erase functions can
only be initiated via ICSP or an external
programmer.
FIGURE 24-4:
TABLE WRITE (WRTn) DISALLOWED
Register Values
TBLPTR = 000FFF
Program Memory
000000h
0001FFh
000200h
PC = 001FFE
PC = 004FFE
TBLWT *
001FFFh
002000h
TBLWT *
003FFFh
004000h
005FFFh
006000h
007FFFh
Results: All table writes disabled to Blockn whenever WRTn = 0.
Configuration Bit Settings
WRTB, EBTRB = 11
WRT0, EBTR0 = 01
WRT1, EBTR1 = 11
WRT2, EBTR2 = 11
WRT3, EBTR3 = 11
 2004 Microchip Technology Inc.
DS41159D-page 277