Project on creating electronic keyboard-type of instruments using arduino Mega.
https://www.nutsvolts.com/magazine/article/making_waves https://circuitdigest.com/electronic-circuits/simple-square-wave-to-sine-wave-converter
//https://www.tutorialspoint.com/arduino/arduino_tone_library.htm
//http://harperjiangnew.blogspot.com/2013/05/arduino-port-manipulation-on-mega-2560.html //https://arduino.stackexchange.com/questions/36871/is-the-digitalwritefast-library-still-needed //https://www.arduino.cc/en/Hacking/PinMapping2560 //https://learn.sparkfun.com/tutorials/pull-up-resistors/all