The MM74C922 and MM74C923 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 pull-up devices which permit switches with up to 50 kohm 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 roll-over is provided between any two switches.
An internal Register remembers the last key pressed even after the key is released. The 3-STATE outputs provide for easy expansion and bus operation and are LPTTL compatible.
By Fairchild Semiconductor
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