Currently, this watch has four buttons; One is a limit switch used to enable the outputs of the shift register, and display the time. This is to save some power. The button should be flush with your arm, and be actuated by lifting your hand (this has not been tested yet.) because of this, the watch will be solely purposed for use on the left arm.
This may or may not be needed depending upon how bright the LEDs need to be to see them. I included a trimmer pot to adjust the brightness in the PCB, but will not allow access to it through the case.
Two of the other three buttons are purposed for incrementing hours and minutes. The last button is to enable the aforementioned two buttons.
The LEDs are arranged to display 24 hour time. They still can display 12 hour time with modification of the code, but the layout of the PCB is unituitive for this format.
formating base ten into BCD requires you to convert each digit in base ten to binary, four bits to each digit. This watch shorts some bits in the tens place of the hours and minutes, since they can only go up to 2 and 6, respectively.
Check out Wikipedia's article for more information on BCD.