SoftwareTone: an Arduino library to generate square wave tones on pins 2-13.
Written by Greg Friedland, November 20, 2011.

With hardware PWM, you could make square waves on up to 6 pins by using up all the hardware timers. This
can be limiting in some cases, such as when you want to use the timers for other things including some
of the built-in Arduino functionality. The motivation for this library is to allow creation of square wave 
tones on more pins than is to avoid these limitations and to generate sqaure waves on more pins than is possible
with hardware PWM alone, up to 12 pin excluding 0 and 1 (which are important for serial communication).

Since this library uses software instead of hardware to generate the tones, it has limitations at high frequencies.
The the pin states are updated every 50 microseconds but, which is not fast enough to create frequencies above
10kHz (since a square wave needs two pin state changes for each cycle). Also, higher frequencies will be rounded to 
a nearby frequency. For example, for 6kHz you would want a half period of 1000000us / 2 / 6000Hz = 83.3us which is 
obviously not a multiple of 50. The way this is handled is to take the floor of the number of the desired half period
divided by 50us. Thus, setting a frequncy of 6kHz will result in an output frequency of 10kHz. (Note: the library could
be altered to handle an arbitrary high frequency below 10kHz by changing the update time; however, this would break 
other frequencies.) If you have thoughts about how to improve this behavior, let me know.

To install this library, first download the code from https://github.com/gregfriedland/SoftwareTone/zipball/master.
Unzip the downloaded zip file and copy the SoftwareTone directory to <ArduinoDirectory>/libraries. On Mac OS X, 
this would be at ~/Documents/Arduino/libraries.

Usage is:

init(): 
	initialize the library before using it.

setFreq(pin, frequency):
	set the frequency of the given pin. to turn off, set the frequency to 0.


This library depends on the TimerOne library and you must include <TimerOne.h> in your sketch. (This should
be unnecessary because this file is included in the library also but it's an idiosyncrasy of the Arduino
environment AFAIK.) If you don't include this header, you will get a compile timer error such as:
	    SoftwareTone/SoftwareTone.cpp:8:22: error: TimerOne.h: No such file or directory
	    SoftwareTone/SoftwareTone.cpp: In member function 'void SoftwareToneClass::init()':
	    SoftwareTone/SoftwareTone.cpp:21: error: 'Timer1' was not declared in this scope

An example sketch is available in the examples directory.

Happy tone generation!