HIT HD74HC149 Datasheet

HD74HC149

8-to-8-line Priority Encoder

Description

The HD74HC149 is priority encoder which has 8 input lines (0 - 7) and 8 output lies (Y0 - Y7).

It is the logical combination of a HD74HC148 8-3 line priority encoder driving a HD74HC138 3-8 line
decoder.

Only one request output can be low at a time. The output that is low is dependent on the highest priority
request that is low. The order of priority is 7 highest and 0 lowest.

When E input is high, all outputs are high.

When a output (Y0 - Y7) is low, P output is low and this indicates active condition.

Features

• High Speed Operation: tpd (0 - 7 to Y) = 16 ns typ (CL = 50 pF)

• High Output Current: Fanout of 10 LSTTL Loads

• Wide Operating Voltage: VCC = 2 to 6 V

• Low Input Current: 1 µA max

• Low Quiescent Supply Current: ICC (static) = 4 µA max (Ta = 25°C)

HD74HC149

Function Table

Inputs Outputs
01234567E Y

XXXXXXXXH HHHHHHHHH
HHHHHHHHL HHHHHHHHH
XXXXXXXLL HHHHHHHLL
XXXXXXLHL HHHHHHLHL
XXXXXLHHL HHHHHLHHL
XXXXLHHHL HHHHLHHHL
XXXL HHHHL HHHLHHHHL
XXLHHHHHL HHLHHHHHL
XLHHHHHHL HLHHHHHHL
LHHHHHHHL L HHHHHHHL

H : High level
L : Low level
X : Irrelevant

Y1Y2Y3Y4Y5Y6Y7P

0

Pin Arrangement

Inputs

GND

1

0

2

1

3

2

4

3

5

4

6

5

7

6

8

7

9

E

10

(Top view)

20

19

18

17

16

15

14

13

12

11

V

Y0

Y1

Y2

Y3

Y4

Y5

Y6

Y7

P

CC

Outputs

2

Logic Diagram

HD74HC149

7

6

5

4

3

2

1

0

Y7

Y6

Y5

Y4

Y3

Y2

Y1

Y

0

P

E

3