The scan_keys() function starts by recording which of the
columns contains the pressed button. Each column is assigned a value which is stored in
the key_pressed variable as shown in the figure below. When the column
has been recorded, the row and column pins are inverted in the sense that the rows
become inputs and the columns become outputs.
The key_pressed variable is incremented by a value
depending on which row is detected to contain the pushed button. If, for example, button
B is pressed, COLUMN 3 would be recorded first, with a value of 3 being
stored into the key_pressed variable. Then ROW 1 would be recorded,
which would add 4 to the variable, giving the variable a total of 7. Counting the
buttons of the keypad left to right and top to bottom, button B (with a
zero-indexed numbering scheme) is button number 7.
After recording the button press, the pins are reset to the initial state with columns as inputs and rows as outputs.