PIC18F2525_08 Datasheet, PDF (266/412 Pages) Microchip Technology – 28/40/44-Pin Enhanced Flash Microcontrollers with 10-Bit A/D and nanoWatt Technology

PIC18F2525/2620/4525/4620
23.5.1 PROGRAM MEMORY
CODE PROTECTION
The program memory may be read to or written from
any location using the table read and table write
instructions. 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 normal execution mode, the CPx bits have no direct
effect. CPx bits inhibit external reads and writes. A
block of user memory may be protected from table
writes if the WRTx Configuration bit is ‘0’. The EBTRx
bits control table reads. For a block of user memory
with the EBTRx 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 23-6 through 23-8 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 operation or an
external programmer.
FIGURE 23-6:
TABLE WRITE (WRTx) DISALLOWED
Register Values
TBLPTR = 0008FFh
Program Memory
000000h
0007FFh
000800h
PC = 003FFEh
PC = 00BFFEh
TBLWT*
TBLWT*
003FFFh
004000h
007FFFh
008000h
00BFFFh
00C000h
00FFFFh
Results: All table writes disabled to Blockn whenever WRTx = 0.
Configuration Bit Settings
WRTB, EBTRB = 11
WRT0, EBTR0 = 01
WRT1, EBTR1 = 11
WRT2, EBTR2 = 11
WRT3, EBTR3 = 11
DS39626E-page 264
© 2008 Microchip Technology Inc.