74LS922 16-Key Encoder IC

The CMOS key encoders 74LS922 provide all of the logic required to fully encode an array of SPST switches. An external clock or an external capacitor can be used to implement the keyboard scan. These encoders also contain on-chip pull-up mechanisms, allowing them to be used with switches with up to 50 k on resistance. To avoid ghost switches, no diodes in the switch array are required. The internal debounce circuit only requires a single external capacitor, which can be bypassed by omitting it.

When a valid keyboard entry is made, the Data Available output goes to a high level. When the entered key is released, the Data Available output drops to a low level, even if another key is pushed. After a standard debounce interval, the Data Available will return to high to signify acceptance of the new key; this two-key roll-over is supplied between any two switches. Even when the key is released, an internal register recalls the last key pressed. The 3-STATE outputs are LPTTL compliant and allow for easy expansion and bus operation.