Question

For Arduino UNO, the keypad library requires the constant scanning of keypads. A lower power design would scan the keypad only when some keys have been pressed down.

(2) Using the HelloKeypad source code as an example, explain conceptually how your modified library can be used.

Answer 1

Set the column pins to INPUT_PULLUP. Set the row pins to INPUT.
For each row:
Set that row pin to OUTPUT and write LOW.
For each column:
If the column pin reads LOW the button that connects row and column is pressed.
Set the row pin back to INPUT.