You are forgetting the mathematical overflow.

Suppose you set the limits at +25 (lower) and +100 (upper ).

Value read: +10

+10 - ( +25 ) =3D -15
+10 - ( +100 ) =3D -90
So it is below both limits

Value read: +30

+30 - ( +25 ) =3D +5
+30 - ( +100 ) =3D -70
So it is between the lower and the upper limits.

Value read: +120

+120 - ( +25 ) =3D +95
+120 - ( +100 ) =3D +20
So it is above both limits.

But suppose you read -50

-50 - ( +25 ) =3D -75
-50 - ( +100 ) =3D -150 <=3D problem here. A 8-bit two's complement number
can not go below -128

-50 in two's complement is 206 and -100 is 156, so the above subtraction
becomes:

206 + 156 =3D 362. This number gets truncated to 106, which is positive.
That=B4s right: 206 + 156 =3D 106 !!!

You must test the OVERFLOW bit also, but the 12-bit and 14-bit
instruction PICs do not have it. The PIC18 has.

You will need to do more tests and calculations to get the correct result.

David Duffy escreveu:
> It's signed (two's complement) format. The MSB is the sign. (set when =

> negative)
> David...
>
> Tamas Rudnai wrote:
>   =

>> Sorry, I do not get it, how do you make difference if +5 or -5 is less t=
han
>> -128 ?
>> Or -120 or +120 is less than -128?
>>
>> Tamas
>>     =

__________________________________________________
Fa=E7a liga=E7=F5es para outros computadores com o novo Yahoo! Messenger =

http://br.beta.messenger.yahoo.com/ =


-- =

http://www.piclist.com PIC/SX FAQ & list archive
View/change your membership options at
http://mailman.mit.edu/mailman/listinfo/piclist