Overview

Product details

    The XD74C922 CMOS key encoder provides all the necessary logic to fully encode the SPST switch array. Keypad scanning can be accomplished with an external clock or external capacitors. These encoders also have on-chip pull-up devices that allow switches up to 50KΩ to be used with resistors. No diodes are required in the switch array to eliminate ghost switching. The internal debounce circuit requires only one external capacitor and can be eliminated by omitting the capacitor. When a valid keyboard input is entered, an Available Data output goes high. When the entered key is released, the Available Data output returns to a low level even if another key is pressed. Available Data will return high to indicate acceptance of a new key after the normal debounce period; rollover of the two keys is provided between any two switches. An internal register remembers the last key pressed, even after the key has been released. The three-state outputs provide convenient expansion and bus operation and are LPTTL compatible.