16F874 datasheet(16/22 Pages) MICROCHIP

16 / 22 page

PIC16F87X

DS39025F-page 16

 2002 Microchip Technology Inc.

5.0

CHECKSUM COMPUTATION

Checksum is calculated by reading the contents of the

PIC16F87X memory locations and adding up the

opcodes, up to the maximum user addressable loca-

tion, e.g., 0x1FF for the PIC16F87X. Any carry bits

exceeding 16-bits are neglected. Finally, the configura-

tion word (appropriately masked) is added to the

checksum. Checksum computation for each member of

the PIC16F87X devices is shown in Table 5-1.

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.

The following table describes how to calculate the

checksum for each device. Note that the checksum cal-

culation differs depending on the code protect setting.

Since the program memory locations read out differ-

ently depending on the code protect 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

by reading a device, the entire program memory can

simply be read and summed. The configuration word

and ID locations can always be read.

Note that some older devices have an additional value

added in the checksum. This is to maintain compatibil-

ity with older device programmer checksums.

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