 2002 Microchip Technology Inc.

DS39607B-page 17

PIC16F87/88

5.0

EMBEDDING CONFIGURATION WORD AND ID INFORMATION IN HEX FILE

6.0

CHECKSUM COMPUTATION

Checksum is calculated by reading the contents of the

PIC16F87/88 memory locations and totaling the

opcodes, up to the maximum user-addressable

location (e.g., 0xFFF for the PIC16F87/88). Any carry

bits exceeding 16 bits are neglected. Finally, the

configuration word (appropriately masked) is added to

the checksum. Checksum computation for each

member of the PIC16F87/88 devices is shown in

Table 6-1.

The checksum is calculated by summing the following:

• The contents of all program memory locations

• The configuration words, 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

calculation differs depending on the code protect

setting. Since the program memory locations read out

differently depending on the code protect setting, the

table describes how to manipulate the actual program

memory 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 words and ID locations can always be

read.

Note that some older devices have an additional value

added in the checksum. This is to maintain compatibility

with older device programmer checksums.

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, 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.

Specifically for the PIC16F87/88, the EEPROM data memory should also be embedded in the HEX file (see

Section 3.2 “Data EEPROM Memory”).

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

TABLE 6-1:

CHECKSUM COMPUTATION

Device

Code-

Protect

Checksum*

Blank

Value

0x25E6 at 0

and Max

Address

PIC16F87

OFF

SUM(0000:0FFF) + (CONFIG0 & 3FFF) + (CONFIG1 & 0003)

3002

FBD0

ON

(CONFIG0 & 3FFF) + (CONFIG1 & 0003) + SUM_ID

5004

IBD2

PIC16F88

OFF

SUM(0000:0FFF) + (CONFIG0 & 3FFF) + (CONFIG1 & 0003)

3002

FBD0

ON

(CONFIG0 & 3FFF) + (CONFIG1 & 0003) + SUM_ID

5004

IBD2

Legend: CFGW

= Configuration Word

SUM[a:b] = [Sum of locations a to b inclusive]

SUM_ID

= ID locations masked by 0xF, then made into a 16-bit value with ID0 as the Most Significant

nibble.

For example, ID0 = 0x1, ID1 = 0x2, ID3 = 0x3, ID4 = 0x4, then SUM_ID = 0x1234.

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

+

= Addition

&

= Bitwise AND