Philips Semiconductors
Product specification
3-to-8 line decoder/demultiplexer; inverting
74HC/HCT138
The 74HC/HCT138 decoders accept three binary
FEATURES
weighted address inputs (A0, A1, A2) and when enabled,
provide 8 mutually exclusive active LOW outputs (Y0 to
Y7).
• Demultiplexing capability
• Multiple input enable for easy expansion
• Ideal for memory chip select decoding
• Active LOW mutually exclusive outputs
• Output capability: standard
• ICC category: MSI
The “138” features three enable inputs: two active LOW
(E1 and E2) and one active HIGH (E3). Every output will be
HIGH unless E1 and E2 are LOW and E3 is HIGH.
This multiple enable function allows easy parallel
expansion of the “138” to a 1-of-32 (5 lines to 32 lines)
decoder with just four “138” ICs and one inverter.
The ”138” can be used as an eight output demultiplexer by
using one of the active LOW enable inputs as the data
input and the remaining enable inputs as strobes. Unused
enable inputs must be permanently tied to their
appropriate active HIGH or LOW state.
GENERAL DESCRIPTION
The 74HC/HCT138 are high-speed Si-gate CMOS devices
and are pin compatible with low power Schottky TTL
(LSTTL). They are specified in compliance with JEDEC
standard no. 7A.
The ”138” is identical to the “238” but has inverting outputs.
QUICK REFERENCE DATA
GND = 0 V; Tamb = 25 °C; tr = tf = 6 ns
TYPICAL
SYMBOL
PARAMETER
propagation delay
CONDITIONS
UNIT
HC
HCT
CL = 15 pF; VCC = 5 V
t
PHL/ tPLH
An to Yn
12
14
17
19
ns
ns
tPHL/ tPLH
E3 to Yn
En to Yn
CI
input capacitance
3.5
67
3.5
67
pF
pF
CPD
power dissipation capacitance per package notes 1 and 2
Notes
1. CPD is used to determine the dynamic power dissipation (PD in µW):
PD = CPD × VCC2 × fi + ∑ (CL × VCC2 × fo) where:
fi = input frequency in MHz
fo = output frequency in MHz
∑ (CL × VCC2 × fo) = sum of outputs
CL = output load capacitance in pF
VCC = supply voltage in V
2. For HC the condition is VI = GND to VCC
For HCT the condition is VI = GND to VCC − 1.5 V
ORDERING INFORMATION
See “74HC/HCT/HCU/HCMOS Logic Package Information”.
September 1993
2
