Implementing a linear function...

**Darrel Taylor** · - 25th July 2007, 09:43

"Almost" is a relative term, and I guess it depends on how much "Almost" your application can tolerate.

From this excel graph you can see there's quite a bit of "curve" to it. But it may not be that bad.

<img src="http://www.picbasic.co.uk/forum/attachment.php?attachmentid=1880&stc=1&d=118535245 1">

Converting the formula excel came up with to this...

Code:

TempC  VAR WORD
ADval  VAR WORD

TempC  = ADval * 396
TempC  = DIV32 1000
TempC  = 67 - TempC  

LCDOUT "Temp C = ", SDEC TempC,"  "

You should get these results

Code:

       8-bit
Volts	A/D	Temp	PIC reads
-----	---	-----	---------
3.45	175	0	-2
2.84	144	10	10
2.3	117	20	21
1.81	92	30	31
1.36	69	40	40
1.02	52	49	47

It's off by 2 degrees at the top and bottom of the scale, 1 degree in the middle, and is very close in the areas of 10 and 40 deg.

If that's not within the "Almost" for your program, then Sayzer's idea of a Stepped Linear Interpolation using Select or Lookup/down, will get you closer.

HTH,