PIC18F4520ESLASHMLQTP datasheet(266/390 Pages) MICROCHIP

266 / 390 page

PIC18F2420/2520/4420/4520

DS39631A-page 264

Preliminary

 2004 Microchip Technology Inc.

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 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 instruc-

tion 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.

FIGURE 23-6:

TABLE WRITE (WRTn) DISALLOWED

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 pro-

tection 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.

000000h

0007FFh

000800h

001FFFh

002000h

003FFFh

004000h

005FFFh

006000h

007FFFh

WRTB, EBTRB = 11

WRT0, EBTR0 = 01

WRT1, EBTR1 = 11

WRT2, EBTR2 = 11

WRT3, EBTR3 = 11

TBLWT*

TBLPTR = 0008FFh

PC = 001FFEh

TBLWT*

PC = 005FFEh

Program Memory

Configuration Bit Settings

Results: All table writes disabled to Blockn whenever WRTn = 0

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