OK, so it looks as though my method only works for a range values. Not
good. I will instead just add 128 to all numbers first (to pull them
into a 0-255 range) and then do the comparisons. Thanks everyone for
their input.
David...

Isaac Marino Bavaresco wrote:
> 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 =
than
>>> -128 ?
>>> Or -120 or +120 is less than -128?
>>>
>>> Tamas
>>>     =

>>>       =

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David Duffy        Audio Visual Devices P/L
Unit 8, 10 Hook St, Capalaba 4157 Australia
Ph: +61 7 38235717      Fax: +61 7 38234717
Our Web Site: www.audiovisualdevices.com.au
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