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'How can I calculate checksum of code protected PIC'
1998\09\09@133111 by

Can someone explain the SUM_XNOR7 method?

I have read capter 4.3 in the "EEPROM Memory Programming Specification" documentation,
but I don't understand it...

What does the text in the upper right cell mean? (0x25E6 at 0 and max address)

I have tried to calculate theire example but I don't get the right answer.

location_a = 0x123 = 00 0001 0010 0011
location_b = 0x456 = 00 0100 0101 0110

SUM_XNOR7[location_a:location_b] = 0x001F      How?!?!?!

XNOR of location_a:
upper 7 bits: 0000010
lower 7 bits: 0100011
XNOR: 1011110

XNOR of location_b:
upper 7 bits: 0001000
lower 7 bits: 1010110
XNOR: 0100001

1011110 + 0100001 = 0x007F But it suppose to be 0x001F..?!?!?

and I would like to implement errorchecking for code protected chips.
Is that possible??

Yours
Tomas Franzon

>Sender:       pic microcontroller discussion list <PICLISTMITVMA.MIT.EDU>
>From:         Tomas Franzon <tomfr343STUDENT.LIU.SE>
>Subject:      How can I calculate checksum of code protected PIC16C84?
>To:           PICLISTMITVMA.MIT.EDU
>
>Can someone explain the SUM_XNOR7 method?
>
>I have read capter 4.3 in the "EEPROM Memory Programming Specification"
documentation,
>but I don't understand it...
>
>What does the text in the upper right cell mean? (0x25E6 at 0 and max
to test your function, fill 0x25E6 at address 0 and max, then compare your
result.

{Quote hidden}

I think it should be 0xFF7F
use this function:
for (i=start; i<end; i++)
{
high = (TempWord[i] >> 7) & 0x007F;
low = TempWord[i] & 0x007F;
hml = ~(high ^ low);
SUM += hml;
}

goodluck
http://www.poboxes.com/f
f

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