The MM74C922 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 an external capacitor. These encoders also have on-chip pull-up devices that permit switches with up to 50 kΩ 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.
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.
Features:
- 50 kΩ maximum switch on resistance
- On or off-chip clock
- On-chip row pull-up devices
- 2 key roll-over
- Key bounce elimination with a single capacitor
- The last key register at the outputs
- 3-STATE output LPTTL compatible
- Wide supply range: 3V to 15V
- Low power consumption
Absolute Maximum Ratings
- The voltage at Any Pin: VCC − 0.3V to V CC + 0.3V
- Operating Temperature Range MM74C923: −40°C to +85°C
- Storage Temperature Range: −65°C to +150°C
- Power Dissipation (P D)
- Dual-In-Line: 700 mW
- Small Outline: 500 mW
- Operating VCC Range: 3V to 15V
- VCC: 18V
