MM74C922 • MM74C923

16-Key Encoder • 20-Key Encoder

General Description

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 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 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.

Features

■ 50 kΩ maximum switch on resistance

■ On or off chip clock

■ On-chip row pull-up devices

■ 2 key roll-over

■ Keybounce elimination with single capacitor

■ Last key register at outputs

■ 3-STATE output LPTTL compatible

■ Wide supply range: 3V to 15V

■ Low power consumption

Absolute Maximum Ratings(Note 2)

Voltage at Any Pin V_CC − 0.3V to V_CC + 0.3V

Operating Temperature Range

MM74C922, 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 V_CC Range 3V to 15V

V_CC 18V

Lead Temperature (Soldering, 10 seconds) 260°C

Note 2: “Absolute Maximum Ratings” are those values beyond which the safety of the device cannot be guaranteed. Except for “Operating Temperature Range” they are not meant to imply that the devices should be operated at these limits. The table of “Electrical Characteristics” provides conditions for actual device operation.

Block Diagram

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