Part A
74HC138D
Toshiba Semiconductor and Storage
- Lifecycle
- Active
- Stock
- 2131 PCS
- Package
- SOIC-16
- Series
- Lógica - Conmutadores de sinais, Multiplexers, Decodificadores
Replacement verdict
Not a drop-in replacement
The 74HC157D quad 2-to-1 multiplexer and the 74HC138D 3-to-8 line decoder serve fundamentally different logic functions, with incompatible input/output architectures, channel configurations, and pin assignments. They are not interchangeable in existing designs.
Different logic function
Different channel count
Different pinout
Different output polarity
Parts at a glance
Part B
74HC157D
Toshiba Semiconductor and Storage
- Lifecycle
- Active
- Stock
- 7197 PCS
- Package
- SOIC-16
- Series
- Lógica - Conmutadores de sinais, Multiplexers, Decodificadores
Key differences
Rows are prioritized by design impact. Highlighted values require attention during substitution.
Key electrical and mechanical differences between the two devices
| Parameter | 74HC157D | 74HC138D | Why it matters |
|---|---|---|---|
| Logic Function | Quad 2-input multiplexer | 3-to-8 line decoder/demultiplexer | Defines the core function; these devices are not functionally interchangeable. |
| Number of Inputs | 8 data inputs (2 per channel) | 3 address inputs plus 3 enable inputs | Determines how many signals the device can accept and how it is wired into a system. |
| Number of Outputs | 4 outputs | 8 outputs | Affects fan-out and the number of downstream loads that can be driven. |
| Output Type | Non-inverting (Y = selected input) | Inverting (active-LOW selected output) | Polarity mismatch can invert logic levels and break circuit behavior. |
| Select/Address Lines | 1 common select input | 3 address inputs (A0, A1, A2) | Determines how many distinct channels or outputs can be addressed. |
| Enable/Strobe Inputs | 1 active-LOW enable (G) | 3 enable inputs (two active-LOW, one active-HIGH) | Enable configuration affects cascading, gating, and power-up behavior. |
| Supply Voltage Range | 2.0 V to 6.0 V | 2.0 V to 6.0 V | Both support the same HC-family supply window, so voltage compatibility is not a differentiator. |
| Logic Family | 74HC CMOS | 74HC CMOS | Same family means comparable input thresholds, noise margins, and drive characteristics. |
| Package | SO16 (D suffix) | SO16 (D suffix) | Identical package and pin count simplify PCB footprint and assembly considerations. |
| Propagation Delay (typical) | Approximately 11 ns at 4.5 V | Approximately 15 ns at 4.5 V | Timing differences can affect setup/hold margins in high-speed designs. |
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Full specification comparison
Use manufacturer datasheets as the final authority.
| Specification | 74HC138D | 74HC157D |
|---|---|---|
| Series | 74HC | 74HC |
| Part Status | Active | Active |
| Type | Decoder/Demultiplexer | Multiplexer |
| Circuit | 1 x 3:8 | 4 x 2:1 |
| Independent Circuits | 1 | 1 |
| Current - Output High Low | 5.2mA, 5.2mA | 5.2mA, 5.2mA |
| Voltage Supply Source | Single Supply | Single Supply |
| Voltage - Supply | 2V ~ 6V | 2V ~ 6V |
| Operating Temperature | -40°C ~ 125°C | -40°C ~ 85°C |
| Mounting Type | Surface Mount | Surface Mount |
Frequently asked questions
What are the primary functions of the 74HC157D and 74HC138D?
The 74HC157D is a quad 2-input multiplexer that selects one of two 4-bit data sources based on a common select input. The 74HC138D is a 3-to-8 line decoder/demultiplexer that accepts a 3-bit binary address and activates one of eight mutually exclusive outputs.
Can the 74HC157D and 74HC138D be used interchangeably in a design?
No. They perform fundamentally different logic functions. The 74HC157D routes data from two sources to four outputs, while the 74HC138D decodes a 3-bit input into eight mutually exclusive low outputs. Substitution requires a complete redesign of the surrounding logic.
Do the 74HC157D and 74HC138D share the same package and pinout?
Both devices are commonly offered in a 16-pin SOIC package (the D suffix in 74HC157D and 74HC138D typically denotes SOIC). However, their pinouts differ because their internal logic and input/output counts are different. They are not pin-to-pin compatible.
What are the key differences in input and output counts between the 74HC157D and 74HC138D?
The 74HC157D has two 4-bit data inputs (8 data pins total), one common select input, one enable input, and four 4-bit outputs. The 74HC138D has three address inputs, three enable inputs (two active-low, one active-high), and eight active-low outputs.
Are the 74HC157D and 74HC138D both compatible with standard HC-family logic levels?
Yes. Both are 74HC-series high-speed CMOS devices. They operate over a typical supply voltage range of 2 V to 6 V and are compatible with standard CMOS and TTL logic levels when used within the recommended operating conditions specified in their respective manufacturer datasheets.
Which device is better for expanding a microcontroller's I/O for address decoding?
The 74HC138D is intended for address decoding because it converts a 3-bit binary code into eight mutually exclusive outputs. The 74HC157D is intended for data selection or multiplexing, not for generating decoded chip-select or enable signals.
Do the 74HC157D and 74HC138D have the same propagation delay and drive characteristics?
No. Propagation delay and output drive depend on the specific manufacturer, supply voltage, and load conditions. The 74HC157D and 74HC138D have different internal logic paths, so their timing parameters are not identical. Always consult the relevant manufacturer datasheet for verified timing and drive specifications.
Can both devices operate from the same power supply in a mixed design?
Yes, provided both are operated within their specified supply voltage range and the system logic levels are compatible. Both are 74HC-family devices, so they can typically share a common VCC rail when the design respects the recommended operating conditions and decoupling requirements.