These CMOS key encoders provide all the necessary logic
to fully encode an array of SPST switches. The keyboard
scan can be implemented by either an external clock or
external capacitor. These encoders also have on-chip pullup
devices which permit switches with up to 50 kX on resistance
to be used. No diodes in the switch array are needed
to eliminate ghost switches. The internal debounce circuit
needs only a single external capacitor and can be defeated
by omitting the capacitor. A Data Available output goes to a
high level when a valid keyboard entry has been made. The
Data Available output returns to a low level when the entered
key is released, even if another key is depressed. The
Data Available will return high to indicate acceptance of the
new key after a normal debounce period; this two-key rollover
is provided between any two switches.