floating point subtraction

[roykyn]

can any one help me....a simple floating point subtraction is consuming 20% of rom...i am using pcm and 16f72
float a,b,c;
a=b-c;

[PCM programmer]

That PIC has only 2K ROM words available. Don't use floating point in
small PICs.

See this link:
http://www.ccsinfo.com/forum/viewtopic.php?t=26306&highlight=8K

[ckielstra]

All starting software engineers run into the same problems using floating point arithmetic when creating their first program on a processor with little memory. They discover one or more of the following:
A) Floating point arithmetic takes a lot of program space.
B) Floating point arithmetic is slow.
C) You get unexpected rounding errors, for example a value of 0.999999 when expecting 1.00000.


A common way to avoid these problems is to apply a technique called 'scaled integers'. This means that you are doing all the arithmetic in integers by multiplying the original data with a factor 10, 1000 or whatever number of digits accuracy you need. Only in displaying the data you add a decimal point.
For example when calculating the distance between two points you can say it is 1.45m + 12,23m = 13.68m
But calculating in centimeters it is all integers: 145cm + 1223cm = 1368cm = 13.68m
Working with integers like this is using less memory, is faster and you get predictable rounding errors.

Also have a look at http://www.ccsinfo.com/forum/viewtopic.php?t=28946&highlight=scaled+integer

[roykyn]

i know that the prob can be solved by increasing rom size....is there any other ideas....like this one
http://www.ccsinfo.com/forum/viewtopic.php?t=29034&highlight=

[Sherpa Doug]

Usually the best way is to avoid using floating point at all. Tell us more about your application and we can probably help you. Also read ckielstra's message carefully. Scaled integers are very powerful and very fast.

[roykyn]

i am building a LC meter......
http://www.aade.com/LCinst/lcm2b.htm

http://www.antrak.org.tr/gazete/111998/barbar.htm

http://f6bon.albert.free.fr/lcmetre.html

http://veron-meppel.atvrepeater.com/html/zelfbouw/lc-meter/lc_meter.htm

http://users.cableaz.com/~cappels/dproj/lgm/lgm.html

http://utenti.lycos.it/ik0gmm/resume.html

http://www.cqham.ru/lcmeter2.htm

http://www.hw.cz/constrc/lc_metr/

http://www.df3dcb.de/Elektronik/LC-Meter/lc-meter.html

http://www.sprut.de/electronic/pic/projekte/lcmeter/lcmeter.htm

http://ironbark.bendigo.latrobe.edu.au/~rice/lc/

http://www.oz5vf.dk/byggeprojekter/lc-meter/lc-meter.html

http://www.hw.cz/constrc/lc_metr/

http://xavier.fenard.free.fr/LCMeter.htm

http://www.qsl.net/iz7ath/web/02_brew/21_LCMeter01/

http://www.geocities.com/ttarsorg/contributors/homebrew-lc-meter-2.pdf

http://perso.wanadoo.fr/f6gog-web/files/brew.htm

http://www.touchinglittlelives.org/meter/

http://www.digital-measure.com

http://electronics-diy.com/lc_meter.php

[SherpaDoug]

That looks like an interesting little device. Basically an oscillator whose frequency is a function of the device being measured. So your measurement enters the software world as an integer count. And you want to end up with integer digits on a LCD display. Why not keep it in integers in the middle? Can you calculate inductance in nHy or uHy depending on the mode? How about calculating capacitance in centi-pF. 100,000 cpF = 1mFd. Then you just have to find out where to put the decimal point on the display.
_________________
The search for better is endless. Instead simply find very good and get the job done.

[roykyn]

i have 2 do all the calculations...to find the value of L or C it has only one subtraction....