Extending the MAX6959 LED Display-Driver Keyscan from 8-Keys to 12-Keys BAV70

The circuit is shown in the image. Each key requires a dual diode (such as the low-cost common-cathode BAV70 in SOT-23), which pulls both INPUT1 and INPUT2 low when the switch is pressed.

 Each of the four extension keys is wired to simulate a dual key press for the two keys on each of the four LED cathode drive outputs, DIG0/SEG0 through DIG3/SEG3. With this connection, each key pair is always scanned and debounced at the same time. Extra keys that simulate a dual key press of keys scanned by different LED cathode drive outputs will be unreliable. Because the keyscan is performed sequentially, two keys at a time, the extra key could miss the debounce cycle for one LED cathode drive, yet be correctly debounced by the other. This dual key press would then appear as two sequential key presses, not as a dual key press. This wouldn't happen with the recommended connection scheme because each key pair representing a dual key is debounced together.