RE: [OT] Some pic asm code help

["Ian Davidson" <ian@xxxxxxxxxxxxxxxx>]

Hi Frank

As a first test I would read the device and output the actual raw word
value you read. Then manually convert this. This will prove if you are
actually reading the correct value and converting wrong or if the raw
value is wrong in the first place. If you want to post the raw values I
can convert them here and let you know what I get the result to be.

Ok just reading your post again.

I can see where this is going wrong. You need to do the rotating first.
If you don't then when you add the 1 and later rotate you rotate it
straight off the end. In your case the bit 0 position starts at bit 3 so
instead of adding 1 you would have to add 8 but it is easier just to
rotate first then do the complementing. Also treat the two bytes as one
value so when you complement and add 1 only add 1 to the LSB. The only
other point to watch out for is after you complement you can mask off
the top nibble of the MSB as after rotating the first three are not used
and the next is just the sign bit which is not part of the 2's
complement.

I hope that made sense it far easier to talk it than write it!

Ian D


-----Original Message-----
From: ukha_d@xxxxxxx [mailto:ukha_d@xxxxxxx] On Behalf
Of Frank Mc Alinden
Sent: 09 September 2007 08:31
To: Ian
Subject: Re: [ukha_d] [OT] Some pic asm code help

Hi Ian

Firstly thanks for your feedback much appreciated...

What i have done is this firstly is test bit 7 of the MSB 0E and if it
is
set i compliment each byte and add 1  to each of them...??

I then rotate right the LSB register OF  3 times each time clearing the
carry flag........

I then shift bits 0,1,2 of the MSB into bits 5,6 +7 of the LSB and
rotate
the MSB 3 times.........???

With the  result im using existing sub routines that i use to convert
the
ds18s20 results into temperature readings....

The results im getting do not tally up with readings im getting using
another device (opnode) which also has moisture sensors attached to
it.........

The moisture sensors use an external resistor so the range should go
from
+64mV  to -64mV

Thanks again
Frank


----- Original Message -----
From: "Ian Davidson" <ian@xxxxxxx>
To: <ukha_d@xxxxxxx>
Sent: Sunday, September 09, 2007 12:11 PM
Subject: RE: [ukha_d] [OT] Some pic asm code help


> Hi Frank
>
> I've not used that ic before but looking at the data sheet it works
like
> this.
>
> The value you read from the ic will be a word value. It is left
> justified so you need to rotate the value right three times to get the
> LSB into the bit 0 position.
>
> Say you read
> 00000000 01010000
>
> By rotating it 3 times it will become
> 00000000 00001010
>
> Now you need to test bit 12, in the example it is a zero which means
the
> number is a positive number. You can therefore just use that number in
> this case decimal 10. If you are using the internal resistor then the
> resolution is 0.625mA. 10 x 0.625mA is 6.25mA which is the current the
> device is seeing.
>
> Now if bit 12 was a 1
>
> I.e.
> 00011111 11110110
> Then that is a negative number. In two complement you need to invert
all
> the bits and add one for all the bits below bit 12 so
>
> 00000000 00001001
>
> Add 1
>
> 00000000 00001010
>
> Which is 10 decimal times by 0.625mA equals -6.25mA
>
> I hope that made sense Frank.
>
> Ian D
>
>
> -----Original Message-----
> From: ukha_d@xxxxxxx [mailto:ukha_d@xxxxxxx] On Behalf
> Of Frank Mc Alinden
> Sent: 08 September 2007 23:45
> To: Ian
> Subject: [ukha_d] [OT] Some pic asm code help
>
> Hi Guys
>
> Sorry for the OT post but im hoping some of the pic coders can help
> me....Im
> trying to add support for the Hobby Boards moisture sensor to my Probe
> system and im stuck trying to write some code to convert the readings
> from
> the "current registers (OE +OF) " of the DS2760...(The
mositure sensor
> uses
> the current sense) ...I can read the DS2760 and get the values i just
> cant
> work out how to convert it to mVolts.......Any help much
> appreciated......If
> i get this sorted i will then have a crack at their Humidity
> sensor.......
> Heres a link to the hobby boards sensor and the DS2760 PDF
> http://datasheets.maxim-ic.com/en/ds/DS2760.pdf
>
>
http://www.hobby-boards.com/catalog/product_info.php?cPath=22&products_i
> d=70
>
> Thanks
> Frank
> www.armaghelectrical.com.au
>
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