Part A
74HC138D
Toshiba Halbleiter und Speicher
- Lifecycle
- Active
- Stock
- 2131 Stückzahl
- Package
- SOIC-16
- Series
- Logic.Signal Schalter, Multiplexer, Decoder
Replacement verdict
Not a drop-in replacement
The 74HC138D and 74HC166D are not functionally interchangeable. The 74HC138D is a 3-to-8 line decoder/demultiplexer with active-low outputs, while the 74HC166D is an 8-bit parallel-in/serial-out shift register. Their core logic functions, pin configurations, and signal flow architectures are fundamentally different, making direct substitution impossible without complete circuit redesign.
Different logic function
Different input/output architecture
Different pinout configuration
Different signal flow direction
Parts at a glance
Part B
74HC166D
Toshiba Halbleiter und Speicher
- Lifecycle
- Active
- Stock
- 8001 Stückzahl
- Package
- SOIC-16
- Series
- Logic-Shift-Register
Key differences
Rows are prioritized by design impact. Highlighted values require attention during substitution.
Key electrical and mechanical differences between the two devices
| Parameter | 74HC138D | 74HC166D | Why it matters |
|---|---|---|---|
| Function | 3-to-8 line decoder/demultiplexer | 8-bit parallel-in/serial-out shift register | These devices perform fundamentally different logic functions, so they are not functionally substitutable in a circuit. |
| Logic Type | Decoder/Demultiplexer | Shift Register | The logic type determines the device's role in signal routing versus serial data shifting. |
| Number of Outputs | 8 | 1 (serial) | Output count affects how many loads can be driven and the device's interface capability. |
| Number of Inputs | 3 address inputs plus 3 enable inputs | 8 parallel data inputs plus serial input and control inputs | Input configuration determines how data is presented to the device and how it is controlled. |
| Operating Voltage Range | 2 V to 6 V | 2 V to 6 V | Both support the same HC-family supply range, so voltage compatibility is not a differentiator. |
| Propagation Delay | Typically around 15 ns at 4.5 V | Typically around 15 ns at 4.5 V | Similar propagation delay means timing performance is comparable, but function still differs. |
| Package | SOIC-16 | SOIC-16 | Matching package and pin count can allow board-level compatibility, but pin functions differ. |
| Pin Count | 16 | 16 | Same pin count does not imply pinout compatibility; the internal signal assignments are different. |
| Primary Application | Address decoding, memory selection, demultiplexing | Serial data shifting, parallel-to-serial conversion | Application fit is the key criterion for substitutability, and these parts serve different system roles. |
Need help validating a replacement?
Send your BOM and application constraints for technical and sourcing review.
Full specification comparison
Use manufacturer datasheets as the final authority.
| Specification | 74HC138D | 74HC166D |
|---|---|---|
| Supplier | - | Toshiba Semiconductor and Storage |
| Series | 74HC | 74HC |
| Part Status | Active | Active |
| Logic Type | - | Shift Register |
| Output Type | - | Push-Pull |
| Number of Elements | - | 1 |
| Number of Bits per Element | - | 8 |
| Function | - | Parallel or Serial to Serial |
| Voltage - Supply | 2V ~ 6V | 2V ~ 6V |
| Operating Temperature | -40°C ~ 125°C | -40°C ~ 125°C |
| Mounting Type | Surface Mount | Surface Mount |
| Type | Decoder/Demultiplexer | - |
| Circuit | 1 x 3:8 | - |
| Independent Circuits | 1 | - |
| Current - Output High Low | 5.2mA, 5.2mA | - |
| Voltage Supply Source | Single Supply | - |
Frequently asked questions
What are the primary functions of the 74HC138D and 74HC166D?
The 74HC138D is a 3-to-8 line decoder/demultiplexer that takes three binary select inputs and activates one of eight mutually exclusive outputs. The 74HC166D is an 8-bit parallel-in/serial-out shift register that can load eight bits in parallel and shift them out serially.
Can the 74HC138D and 74HC166D be used interchangeably in a circuit?
No. They perform fundamentally different logic functions. The 74HC138D decodes a 3-bit address into one of eight active outputs, while the 74HC166D stores and shifts 8-bit data. Substitution requires a complete redesign of the surrounding logic and timing.
Do the 74HC138D and 74HC166D have the same pin count and package?
Both devices are commonly offered in a 16-pin SOIC package (the "D" suffix typically denotes SOIC in Nexperia and similar manufacturer naming). However, pin assignments and functions differ completely, so they are not pin-compatible despite matching package and pin count.
What are the key control inputs for the 74HC138D?
The 74HC138D has three select inputs (A0, A1, A2) and three enable inputs: two active-LOW enables (E1, E2) and one active-HIGH enable (E3). All enable conditions must be satisfied for the selected output to go active LOW.
What are the key control inputs for the 74HC166D?
The 74HC166D has a parallel load input (PL), a clock input (CP), a serial data input (DS), eight parallel data inputs (D0–D7), a serial output (Q7), and an output enable (OE). When PL is LOW, data is loaded asynchronously; when PL is HIGH, data shifts on the rising edge of CP.
Are both the 74HC138D and 74HC166D compatible with 3.3 V and 5 V logic systems?
Yes. Both are 74HC-series CMOS devices typically specified for a supply voltage range of 2.0 V to 6.0 V, which covers standard 3.3 V and 5 V logic levels. Input thresholds and output drive should be verified against the specific manufacturer datasheet for the target supply voltage.
Which device is better for expanding microcontroller I/O?
They serve different expansion needs. The 74HC138D is used to decode address lines and select one of eight peripherals or memory banks. The 74HC166D is used to capture eight parallel inputs and shift them into a microcontroller serially, conserving I/O pins. The choice depends on whether the application requires address decoding or serial input capture.
Do the 74HC138D and 74HC166D have the same propagation delay and drive characteristics?
No. Propagation delays and output drive currents differ between the two functions and should be taken from the respective manufacturer datasheets. The 74HC138D decoder and 74HC166D shift register have different internal signal paths, so timing parameters are not interchangeable.