Jinx wrote:
> What I really want to do is make a universal system so that any
> ASCII string can be compared to any sensor voltage, so that the
> user receives not only the alert but also a message in appropriate
> terms and units

Since you're dealing with verbose ASCII strings over a serial port, I'll
assume that compute cycles aren't critical.  If so, this sounds like a
natural application for floating point.  That way you don't have to worry
about the max range on one end and the min precision on the other.  For each
A/D channel, you keep two numbers that can be pre-computed at assembly time.
These are the scale factor and offset to go from the user values to the A/D
reading.  Using the same two numbers you do a subtract and a divide to go
the other way.

The remaining issue is converting a floating point value to its ASCII
representation.  Again I'm assuming cycles are available, so brute force it.
One general routine will be easier to write and maintain, even if it takes a
few more cycles.  By brute force it, I mean using multiple of 10 scale
factors, probably again using floating point because of the unknown range.

I use a 24 bit floating point format for this, which leaves 16 bits of
precision.  That's plenty good enough considering you're ultimately dealing
with 10 or 12 bit A/D readings, even allowing a few bits for roundoff
errors.  As for pre-computing the fixed scale factors and offsets at
assembly time, use my preprocessor.  It adds floating point as a native data
type to MPASM and does the assembly time calculations in the host floating
point format.  It also includes conversion functions to emit the hex values
in MPASM syntax for the resulting values in my PIC 24 bit floating point
format.  If you're not already using my PIC development environment and
relocatable mode and the preprocessor, maybe now's a good time to See the
Light and jump in.  I won't ask you to drink any grape coolaid.  It's really
not that hard and in a few weeks you'll wonder how you ever got along with
out it.

If you do that, I'll send you my 24 bit floating point routines privately.
I don't give these away on the web, but I don't mind you using them as long
as you don't give them out publicly.


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