fixed-point math

Projects Programming
1

I've heard people mention fixed-point math with regards to micro controllers with limited precision. Is there a description of how to implement this on the Arduino?

Thanks,
memotick

2

Just for example, you could use an int (which can go up to 32767) to hold "tenths" of something, so therefore 20000 in the int could represent 2000.0.

3

Fixed point math doesn't do tenths very well but it works well on halfs, quarters, eigths, sixteenths plus a few others.

4

Fixed point math doesn't do tenths very well but it works well on halfs, quarters, eigths, sixteenths plus a few others

do you have a reference for that?

imho there are multiple differenet FPM models,

  1. integer FPM ("nicks" model)
  2. binary FPM where a bit pattern is representing whole and decimal part (like the DS18B20 uses)
  3. BCD format (does not use every possible bitpattern)
  4. probably there are others too
5

memotick:
I've heard people mention fixed-point math with regards to micro controllers with limited precision. Is there a description of how to implement this on the Arduino?

Perhaps we should ask the objective here. Is it to get more precision? More speed? Save space?

6

More speed

7

This is starting to sound like one of these projects where very detailed questions are being asked, with no description of the overall intention. Perhaps if you describe a bit more what you are attempting? There are lots of ways of saving time, for example using bytes instead of ints, or ints instead of longs.

8

Fair enough. I am working on an FFT (fast Fourier Transform) using Arduino. This is a complex algorithm and I have read many times that using fixed point rather than floating point numbers helps greatly in the speed of the processing.

I'm planning on doing between 64-256 point FFTs (depending on the results and clock speeds).

Hope that helps, if not please let me know what information would.

Thanks,
memotick

9

Why reinvent the wheel?
This appears to do 16-bit fixed point.
http://www.arduino.cc/cgi-bin/yabb2/YaBB.pl?num=1286718155
and if you need floating point try this one:
http://code.google.com/p/makefurt/source/browse/trunk/arduino-libraries/PlainFFT/?r=11

Pete