MM74C923WMX
October 1987
Revised January 1999
MM74C922 • MM74C923 16-Key Encoder • 20-Key Encoder
© 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 imp lemented
by either an external clock or external capacitor. These
encoders also have on-chip pull-up devices which pe rmit
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 m ade. 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 acceptan ce of the new
key after a normal debou nce period; this t wo-key roll-over
is provided between any two switches.
An internal regist er 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
■ 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. Specify by appendin g s uf f ix let t er “X” to the ordering co de.
Connection Diagrams
Pin Assignment for DIP
Top View
MM94C922
Pin Assignment for SOIC
Top Vi ew
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
www.fairchildsemi.com 2
MM74C922 • MM74C923
Connection Diagrams (Continued)
Pin Assignment f or
DIP and SOIC Package
Top View
MM74C923
Truth Tables
(Pins 0 through 11)
(Pins 12 through 19)
Note 1: Omit for MM74C 922
Switch
Position
01234 5678910 11
Y1,X1 Y1,X2 Y1,X3 Y1,X4 Y2,X1 Y2,X2 Y2,X3 Y2,X4 Y3,X1 Y3,X2 Y3,X3 Y3,X4
D
AA 010101010101
TB 001100110011
AC 000011110000
OD 000000001111
UE
(Note 1)000000000000
T
Switch
Position
12 13 14 15 16 17 18 19
Y4,X1 Y4,X2 Y4,X3 Y4,X4 Y5(Note 1),
X1
Y5 (Note 1),
X2
Y5 (Note 1),
X3
Y5 (Note 1),
X4
D
AA 0101 0 1 0 1
TB 0011 0 0 1 1
AC 1111 0 0 0 0
OD 1111 0 0 0 0
UE
(Note 1) 0 0 0 0 1 1 1 1
T
3 www.fairchildsemi.com
MM74C922 • MM74C923
Block Diagram
Loading...