October 1987
Revised January 1999
MM74C922 • MM74C923 16-Ke y Encoder • 20-Key Encoder
© 1999 Fairchild Semicond uctor Corpor ation DS006037.prf www.fairchildsemi .com
MM74C922 • MM74C923
16-Key Encoder • 20-Key Encoder
General Descript ion
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 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, ev en if another key is depressed. The Data
Available will retur n high to indi cate accep tance of th e new
key after a normal debounce per iod; this two-key roll-over
is provided between any two switches.
An internal register remembers the last key pressed even
after th e key is relea sed. The 3-STATE o utputs provid e for
easy expansion and bus operation and are LPTTL compat-
ible.
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
Ordering Code:
Device also available in Tape and Reel. Spec if y by app ending s uf f ix let t er “X” to the o rdering co de.
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 P ackage (PDIP), JEDEC MS-001, 0.300” Wide
