2. Key Pad and Number Encoding
Fig.2 16 key to Binary encoder
The
4 X 4 matrix keyboard forms the primary input to the RIVD. The matrix key pad is
connected to 74C922 16 key to binary encoder. This 16 key to binary encoder
provides debouncing of the keys, with a 4 bit binary output according to what
key is pressed.
The Data Available (DA) line on the 74C922 goes high when a key pressed and low once it has been released. The data available line is connected up to the UART’s transmitter buffer register load(nTBRL) via an inverter. This data available line is also acted as an interrupt to UART which consequently relay that information to computer via a level converter. When a key pressed, data is loaded into the UART’s transmitter buffer, and when the key is released, it is sent down the serial line.