STM8S103F keypad to UART

Matrix: row C3-6 x column D1-4

UART1 Tx: D5

Keypad format:

1 2 3 A
4 5 6 B
7 8 9 C
* 0 # D

Each keypress emits the corresponding ASCII to UART (ABCD uppercase). No repeat.

Only one key can be pressed at a time.