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 74HC164D and 74HC138D are functionally unrelated logic ICs and cannot serve as substitutes for one another. The 74HC164D is an 8-bit serial-in/parallel-out shift register, while the 74HC138D is a 3-to-8 line decoder/demultiplexer. Their internal architectures, pin functions, and logic operations differ fundamentally, making direct replacement impossible without complete circuit redesign.
Different logic function
Different pinout
Different input type
Different output type
Parts at a glance
Part B
74HC164D
Toshiba Halbleiter und Speicher
- Lifecycle
- Active
- Stock
- 2726 Stückzahl
- Package
- SOIC-8
- 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 | 74HC164D | 74HC138D | Why it matters |
|---|---|---|---|
| Logic Function | 8-bit serial-in/parallel-out shift register | 3-to-8 line decoder/demultiplexer | Defines the core operation; these devices perform fundamentally different logic functions and are not functionally interchangeable. |
| Number of Outputs | 8 parallel outputs | 8 outputs (one active per address) | Both provide 8 outputs, but the activation behavior differs: shift register outputs hold shifted data, while decoder activates only one output at a time. |
| Input Type | Serial data inputs (2) with clock | 3 binary address inputs plus enable inputs | Input interface determines how data is loaded; serial loading versus address selection changes system wiring and control logic. |
| Output Polarity | Non-inverting (true data outputs) | Active-LOW outputs | Polarity affects how downstream circuits interpret signals; active-LOW decoder outputs require different interfacing than non-inverting shift register outputs. |
| Enable/Control Pins | Clock and clear (reset) inputs | Three enable inputs (two active-LOW, one active-HIGH) | Control pins govern when the device operates; different enable schemes affect cascading and system timing design. |
| Typical Propagation Delay | Approximately 20 ns at 4.5 V | Approximately 20 ns at 4.5 V | Similar delay supports comparable timing budgets, but functional differences still prevent direct substitution. |
| Supply Voltage Range | 2 V to 6 V | 2 V to 6 V | Identical supply range allows both to operate in the same HC-family logic systems without level shifting. |
| Package | SOIC-14 (D suffix) | SOIC-16 (D suffix) | Different pin counts and package sizes affect PCB footprint and layout compatibility. |
| Logic Family | 74HC high-speed CMOS | 74HC high-speed CMOS | Same logic family ensures compatible voltage thresholds and noise margins when used together. |
| Primary Application | Serial-to-parallel data conversion, LED driving, I/O expansion | Address decoding, memory chip selection, demultiplexing | Intended use cases differ significantly; selecting the wrong device would not fulfill the required system function. |
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Full specification comparison
Use manufacturer datasheets as the final authority.
| Specification | 74HC138D | 74HC164D |
|---|---|---|
| Series | 74HC | 74HC |
| Part Status | Active | Active |
| Type | Decoder/Demultiplexer | - |
| Circuit | 1 x 3:8 | - |
| Independent Circuits | 1 | - |
| Current - Output High Low | 5.2mA, 5.2mA | - |
| Voltage Supply Source | Single Supply | - |
| Voltage - Supply | 2V ~ 6V | 2V ~ 6V |
| Operating Temperature | -40°C ~ 125°C | -40°C ~ 125°C |
| Mounting Type | Surface Mount | Surface Mount |
| Logic Type | - | Shift Register |
| Output Type | - | Push-Pull |
| Number of Elements | - | 1 |
| Number of Bits per Element | - | 8 |
| Function | - | Serial to Parallel |
Frequently asked questions
What are the primary functional differences between the 74HC164D and the 74HC138D?
The 74HC164D is an 8-bit serial-in/parallel-out shift register, while the 74HC138D is a 3-to-8 line decoder/demultiplexer. The 74HC164D shifts serial data through eight internal flip-flops and presents it on eight parallel outputs, whereas the 74HC138D accepts a 3-bit binary address and activates one of eight mutually exclusive outputs.
Are the 74HC164D and 74HC138D pin-compatible?
No. Both devices are typically supplied in a 14-pin SOIC package (D suffix), but their pin functions differ. The 74HC164D uses pins for serial inputs (A and B), clock (CP), clear (MR), and eight parallel outputs (Q0–Q7). The 74HC138D uses pins for three address inputs (A0–A2), three enable inputs (E1, E2, E3), and eight active-LOW outputs (Y0–Y7). They cannot be interchanged without redesigning the circuit.
Can the 74HC164D and 74HC138D be used together in the same design?
Yes. A common configuration uses the 74HC164D to expand the number of control lines, with its parallel outputs driving the address or enable inputs of one or more 74HC138D decoders. This allows a small number of microcontroller pins to control a larger number of decoded outputs.
What are the key electrical characteristics of the 74HC164D and 74HC138D?
Both are 74HC-series CMOS devices typically specified for a supply voltage range of 2.0 V to 6.0 V, with standard CMOS input thresholds and low power consumption. The 74HC164D provides eight parallel outputs capable of driving standard CMOS loads, while the 74HC138D provides eight active-LOW outputs. Exact propagation delays, output drive currents, and input capacitances should be verified from the specific manufacturer datasheet, as they can vary slightly between suppliers.
What is the function of the MR pin on the 74HC164D and the E1/E2/E3 pins on the 74HC138D?
On the 74HC164D, MR (Master Reset) is an asynchronous active-LOW input that clears all eight internal shift register stages and forces the parallel outputs LOW when asserted. On the 74HC138D, E1 and E2 are active-LOW enable inputs and E3 is an active-HIGH enable input; all three must be in their active states for the decoder to operate, and any inactive enable forces all eight outputs HIGH.
Which device is better for expanding I/O in a microcontroller system?
It depends on the requirement. The 74HC164D is used when serial-to-parallel conversion is needed, such as driving multiple LEDs or relays from a few microcontroller pins. The 74HC138D is used when a small binary address must select one of eight mutually exclusive outputs, such as chip-select decoding or memory-mapped I/O expansion. They serve different logical functions and are not direct substitutes.
Do the 74HC164D and 74HC138D have the same output drive capability?
Both are standard 74HC CMOS logic devices and typically provide similar output drive characteristics, but the exact source/sink current ratings and voltage drop specifications must be confirmed from the manufacturer datasheet for the specific part number and package. The 74HC164D outputs are standard push-pull CMOS outputs, while the 74HC138D outputs are also standard CMOS outputs but are active-LOW decoded outputs.
Are the 74HC164D and 74HC138D available in the same package options?
Both are commonly available in a 14-pin SOIC package, which is indicated by the "D" suffix in many manufacturer part numbering systems. However, package availability can vary by manufacturer and distributor, and other package options such as TSSOP or DIP may exist for one or both devices. Always verify the specific orderable part number and package drawing from the manufacturer or authorized distributor.