Frequency counter for Android using Arduino Nano or Uno.
The Arduino generates an accurate 1 second time base for the counter by cascading timer0 and timer2. The link between digital inputs 3 and 4 connects the output of timer2, 250 Hz, to input of timer0. The software waits for the output of timer0 to go positive to start the count of frequency input to timer1. Timer1 is a 16 bits timer, it overflows at the count of 2 power of 16, that in turn advances over-flow register. At the end of the 1 second the 16 bit register is recorded. The Arduino sends via USB cable and OTG cable or adaptor 3 bytes that contain the measured frequency. The specs recommend input up to 6 MHz, I tested it at 2 MHz.The Android app display 0 to 9 images of nixie tubes.
OTG (On The Go) is the cable that is used to connect USB devices (other than charger or PC) to phones or tablets.
The app is an APK file to transfer to the Android device and opening the file gives an option to install it. The library is needed if you wish to modify the app, it is in a B4A free program for PC.