Fairchild Semiconductor MM74C923WMX, MM74C923N, MM74C923CW, MM74C923WM Datasheet

October 1987 Revised January 1999

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 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 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

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

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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

Switch

Position

12 13 14 15 16 17 18 19

Y4,X1 Y4,X2 Y4,X3 Y4,X4 Y5(Note 1),X1Y5 (Note 1),X2Y5 (Note 1),X3 Y5 (Note 1),

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

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MM74C922 • MM74C923

Block Diagram

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