Need help with lookup table or direct calculation

[nverma]

Hi,

I am trying to allocate my A-D value which goes from 0-1023 to precalculated values. I tried this approach first because the calculation is pretty complex. So once I had my precalculated values, I tried storing them into the EEPROM like this:

DATA @1023,-782
DATA @1022,-696
DATA @1021,-633
DATA @1020,-594
.
.
.
DATA @3,3797
DATA @2,4126
DATA @1,4725
DATA @0,4730

Read these values from EEPROM:

FOR index0 = 0 to 1023
read index0,tem0[index0]
next index0

Then allocated precalulated stored values to A-D value:

ADCIN 2,pot_value
temperature = tem0[pot_value]


However, I think the EEPROM can only store bytes rather than words, thus I cannot store the value I wanted. I tried using lookup tables as well, but the maximum you can store in a lookup table are 256 values, I need 1023.
So, I thought about actually doing the math in the PIC itself. The equation I need to calculate is:

temperature = (80.8321*( (-1*LN( (2490*(Pot_value*(4096/1024)))/(40960-10000*(Pot_value*(4096/1024))))) + ( SQRT( (LN( (2490*pot_value)/(40960-10000*pot_value)) + 7.4063)^2 + 68.2415) -7.4063))) - 273.15

I have got an algorithm for the Ln but I will need to do all this in floating point math. Can someone please suggest what I can do?

[Darrel Taylor]

Hi nverma,

If you're using an 18F this might work for you ...

The EXT (external) modifier.
http://www.picbasic.co.uk/forum/showthread.php?t=3891

In particular, the section on Using EXT with Labels.

HTH,

[nverma]

Thanks Darrel,

I'm not exactly sure what the DW command does. And if you look at the example of the EXT with label, could I write code as:

POT_VALUE CON EXT

POT_VALUE

DW (basically enter my precalculated values in order from the one correposidng to 0 all the way to 1023 here)

And then reading:

READCODE(POT_VALUE + (OFFSET<<1)),Temperature

[Darrel Taylor]

DW is "Declare Data of One Word" in assembly language.

And, sounds like you've got the right idea for the rest of it.
<br>

[ErnieM]

That's a rather involved calculation to be sure. Honestly, I gave up trying to follow it.

You have just 1024 input values, I assume these lead to 1024 discrete output values else you wouldn't be looking for a look-up table solution.

However, can you solve this by some other simpler method? Piecewize linear, interpolation polynomial, or such? Do you have say an excel spreadsheet with the results calculated?

Also, you don't need EEPROM to save values, you can use RETLW (Return with literal in W) to get a byte look up table, that will take 2048 memory locations to get 1024 words (and yeah, its assembler, but not much).

[BigWumpus]

What type of thermometer do you use ?

I always make a table of maybe 30 points and make an interpolation between this points. It is good for me, but I don't know, what you need...

[nverma]

Ernie,

Yes I do have an excel spreadsheet with the values calculated. And I could possibly linearize the curve but loose accuracy. But about this RETLW command, how does it work and where can I find the syntax etc etc for it?