12 bit A2D on 18F46K80, need a bit of help for figuring TAD and getting correct value

[HenrikOlsson]

Hi,

Need to figure out how to get from 3272 to 5.06!

Couple of ideas:

ADValue VAR WORD
Temp VAR WORD
Result VAR WORD

ADValue = 3272
Temp = ADValue ** 35809   ' Multiply by ~0.5464
Result = ADValue + Temp     ' Result now 5059

' Or, same as above but without using the Temp variable:
Result = ADValue + (ADValue ** 35809)

' Or, perhaps:
ADValue = 3272
Result = ADValue */ 396     ' Multiply by ~1.5469, Result now 5061

'Display the value
LCDOUT Result / 1000, ".", DEC3 Result // 1000

/Henrik.

[Aussie Barry]

Hi,

If you change your voltage divider to a 50%-50% configuration (say 4k7 and 4k7) and then set the reference voltage for the ADC to internal 4.096V, each step of the ADC will equate 0.5mV
Multiply your ADC reading by two and then manipulate the result to read "X.XXX Volts" per Henrik's LCDOUT routine.
Yes, I understand there may be some slight inaccuracies but I have found this approach to be quite effecting in recent projects.

Cheers
Barry
VK2XBP

[ecoli-557]

Henrik-
That looks like a cool way to do this, I have never used these operators, but I will try them all, just to see more about them.
It looks much more efficient than what I had done - multiple dividing......

Thanks! I will let you know how it turned out.
-Steve

[ecoli-557]

Barry-
I understand what you are saying, and yes, I am using the internal 4.1 reference - 1st time in fact to rely on an internal!. I was trying to use more of the available resolution on the ADC. If I do a 50% voltage divider, I have 2.5 volts against a 4.096 voltage reference. The divider I am using, uses more of the resolution of the 4.096 reference, 3.23 volts against 4.096 which gives me a little headroom above 5 volts.

I have done that in the past, but felt that I left some resolution on the table. This way does work, perhaps not straightforward, but I think it does give me more bits of resolution that can be used.

That being said, if I hadn't found a way (though it was more clunky than Henrik's) - I was sure thinking of doing just that!

Thanks and Regards to All,
Steve

[ecoli-557]

Henrik-
Sorry for the late reply, been out with a respiratory illness - geeesh.
In your example :
ADValue = 3272
Temp = ADValue ** 35809 ' Multiply by ~0.5464
Result = ADValue + Temp ' Result now 5059

Why are you using 'Top 16 bits' for multiplying? and how did you arrive at '35809' as the multiplier?

I really want to learn from this, Thanks.
-Steve

[HenrikOlsson]

Hi,
We want a "raw" display value of 5060 when the ADC returns 3272. That's a factor of 1.5464, right?
The ** operator does a 16*16bit multiplication and then returns the 16 top bits of the intermediate 32bit result which is the same as dividing that intermediate result by 65536.

So, on a calculator, 3272 * 35809 / 65536 = 1787 (truncated). Add in the "original" 3272 and we get 5059 which is pretty close to 5060.

Just think of the ** operators as multiplying by units of 1/65536.
65536*0.5464 = 35809...
35809 * (1/65536) = 0.5464...
And so on.

/Henrik.

[ecoli-557]

Henrik-
So, I kinda get the top 16 bits thing, but I still don't know where you got the number 35809?
65536 * 0.5464 ----> shouldn't this be 65535 * 1.5464?
-Steve

[HenrikOlsson]

Hi,

65536 * 0.5464 ----> shouldn't this be 65535 * 1.5464?

Idealy, yes. But that would result in a value larger than 65535 which won't work without resorting to LONGs. So I treat what's on the right hand side of the decimal point separately from what's on the left hand side which is why I first multiply ADValue by 0.5464 and then ADD the ADValue to that result to get the 1.5464. See if this makes more sense:

Result = ADValue + (ADValue ** 35809)   ' ~Same as ADValue * 1.5464

Since you're using an 18F part you atleast have the option to use LONGs so if you ARE then by all means go ahead and try Result = ADValue ** 101345, it should work.

Hope I don't make it more confusing....

/Henrik.

[ecoli-557]

Is a bit more confusing........ BUT I will try what you had said. I just can't quite wrap my head about using sections of numbers....... I'll draw it out on paper and see if that helps....
I'll get it though, may take me a bit.
This forum is an amazing central clearing for knowledge and the fact that others are not only willing but patient is refreshing...
Thanks,
Steve

[HenrikOlsson]

Hi Steve,

On paper, Result = X * 3 is the same as Result = X+X+X, right?
If the above is correct then Result = X * 1.5 is the same as Result = X + (X/2), right?
And if we agree that the above is correct then I think we'll agree that Result = X + (X*0.5) is the same as Result = X * 1.5 too?

Same thing with the formula in the previous message but the number is 1.5464 instead of 1.5.

Again, ideally, we would want to do Result = ADValue ** 101345 (where 101345 is 65536*1.5464) but that will result in a compile time error if LONGs aren't enabled (because 101345 is larger than 65536 so there's a risk of overflowing the 32bit intermediate result thus returning the wrong value) so we resort to first calculating what 0.5464 is and then add that to the original value, exactly as we did in the example above.

Don't worry, I'm sure you'll get it!

/Henrik.