October 1987

Revised January 1999

© 1999 Fairchild Semiconductor Corporation

DS006037.prf

www.fairchildsemi.com

MM74C922 • MM74C923

16-Key Encoder • 20-Key Encoder

General Description

The MM74C922 and MM74C923 CMOS key encoders pro-

vide 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 k

Ω 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 compat-

ible.

Features

s 50 k

Ω maximum switch on resistance

s On or off chip clock

s On-chip row pull-up devices

s 2 key roll-over

s Keybounce elimination with single capacitor

s Last key register at outputs

s 3-STATE output LPTTL compatible

s Wide supply range:

3V to 15V

s Low power consumption

Ordering Code:

Device also available in Tape and Reel. Specify by appending suffix letter “X” to the ordering code.

Connection Diagrams

Pin Assignment for DIP

Top View

MM94C922

Pin Assignment for SOIC

Top View

MM74C922

Order Number

Package Number

Package Description

MM74C922N

N18A

18-Lead Plastic Dual-In-Line Package (PDIP), JEDEC MS-001, 0.300” Wide

MM74C922WM

M20B

20-Lead Small Outline Integrated Circuit (SOIC), JEDEC MS-013, 0.300” Wide

MM74C923WM

M20B

20-Lead Small Outline Integrated Circuit (SOIC), JEDEC MS-013, 0.300” Wide

MM74C923N

N20A

20-Lead Plastic Dual-In-Line Package (PDIP), JEDEC MS-001, 0.300” Wide