8-to-8-line Priority Encoder
Oct 11, 2005
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.
• 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)
• Ordering Information
HD74HC149RPEL SOP-20 pin (JEDEC)
EL (1,000 pcs/reel)
0 1 2 3 4 5 6 7 E Y0 Y1 Y2 Y3 Y4 Y5 Y6 Y7 P
HHHHHHHH L HHHHHHHHH
XXXXXXX L LHHHHHHHL L
H : High level
L : Low level
X : Irrelevant
Rev.2.00, Oct 11, 2005 page 1 of 5