PIC16F72

DS39588A-page 10

Preliminary

 2002 Microchip Technology Inc.

4.0

CODE PROTECTION

Once code protection is enabled, all program memory

locations read all ‘0’s; further programming of program

memory is disabled. ID locations and the configuration

word may still be read and programmed (1’s to 0’s

only).

4.1

Disabling Code Protection

The following procedure should be performed before

any other programming is attempted. This procedure

also turns off code protection (code protect bit = 1);

however, all program memory will be erased when

this procedure is executed and thus, the security of

the code is not compromised.

Procedure to disable code protection:

a)

Issue the ‘Chip Erase’ command.

b)

Wait for the erase cycle time (tera) to pass. The

program memory is erased, then the configura-

tion memory is erased.

4.2

Embedding Configuration Word and ID Information in the HEX File

4.3

Checksum Computation

The checksum is calculated by reading the contents of

the PIC16F72 memory locations and adding up the

opcodes, up to the maximum user addressable location

(i.e., 0x07FFh for the PIC16F72). Any carry bits exceed-

ing 16 bits are neglected. Finally, the configuration word

(appropriately masked) is added to the checksum.

The checksum is calculated by summing the following:

• The contents of all program memory locations

• The configuration word, appropriately masked

• Masked ID locations (when applicable)

The Least Significant 16 bits of this sum are the

checksum.

Table 4-1 describes how to calculate the checksum for

the PIC16F72. Note that the checksum calculation dif-

fers depending on the code protection setting. Since

the program memory locations read out differently

depending on the code protection setting, the table

describes how to manipulate the actual program mem-

ory values to simulate the values that would be read

from a protected device. When calculating a checksum

of a non-protected device, the entire program memory

can simply be read and summed. The configuration

word and ID locations can always be read.

TABLE 4-1:

CHECKSUM COMPUTATION

To allow portability of code, the programmer is required to read the configuration word and ID locations from the HEX

file, when loading the HEX file. If configuration word information was not present in the HEX file, then a simple warning

message may be issued. Similarly, while saving a HEX file, configuration word and ID information must be included.

An option to not include this information may be provided.

Microchip Technology Inc. feels strongly that this feature is important for the benefit of the end customer.

Device

Code

Protect

Checksum

Blank

Value

0x05E6 at

0x0000

and max

address

PIC16F72

OFF

SUM[0x000:0x07FF] + CFWD & 0x005F

0xF85F

0x842D

ALL

CFWD & 0x005F + SUM_ID

0x005E

0x005E

Legend: CFWD

= Configuration Word

SUM[a:b]

= [Sum of locations a to b inclusive]

SUM_ID

= ID locations masked by 0x0F, then concatenated into a 16-bit value with ID0 as the most significant nibble.

For example, ID0 = 0x01, ID2 = 0x02, ID3 = 0x03, ID4 = 0x04, then SUM_ID = 0x1234

Checksum

= [Sum of all the individual expressions] MODULO [0xFFFF]

+

= Addition

&

= Bitwise AND