74HC138D vs 74HC123D

The 74HC123D is a dual retriggerable monostable multivibrator that generates a single output pulse of a specified width for each input trigger, with the pulse width set by an external resistor and capacitor. 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, commonly used for memory decoding and data routing.

No. They perform fundamentally different logic functions. The 74HC123D is a timing device (monostable multivibrator), while the 74HC138D is a combinational logic decoder/demultiplexer. Substitution requires a complete redesign of the surrounding circuit and cannot be done pin-for-pin or function-for-function.

Both devices are commonly offered in a 16-pin SOIC package (the "D" suffix in Nexperia naming denotes SOIC-16). However, their pinouts are different because their internal functions differ. The 74HC123D uses pins for trigger inputs, reset, external timing components, and outputs, while the 74HC138D uses pins for address inputs, enable inputs, and eight decoded outputs. They are not pin-compatible.

The 74HC123D operates from a supply voltage of 2.0 V to 6.0 V, features typical propagation delay of approximately 20 ns at 5 V, and provides standard CMOS output drive. The output pulse width is determined by an external resistor (R_ext) and capacitor (C_ext) with the relationship t_W ≈ 0.7 × R_ext × C_ext, and it supports both rising-edge and falling-edge trigger inputs with a reset function.

The 74HC138D operates from a supply voltage of 2.0 V to 6.0 V, has a typical propagation delay of approximately 12 ns at 5 V, and provides standard CMOS output drive. It includes three enable inputs (two active-LOW and one active-HIGH) that allow easy cascading for larger decoders, and its eight outputs are mutually exclusive, with the selected output going LOW while all others remain HIGH.

The 74HC123D is the correct choice for pulse generation and timing applications because it is specifically designed as a monostable multivibrator. The 74HC138D has no timing or pulse-generation capability and cannot perform this function.

The 74HC138D is the correct choice for address decoding and demultiplexing because it is a dedicated 3-to-8 line decoder/demultiplexer. The 74HC123D cannot perform decoding or demultiplexing functions.

Yes. Both devices are members of the 74HC family and operate over a supply voltage range of 2.0 V to 6.0 V, which includes the 3.3 V nominal supply. At 3.3 V, their input thresholds and output levels are compatible with standard 3.3 V CMOS logic, though timing parameters such as propagation delay will differ from the 5 V specifications and should be verified from the manufacturer datasheet.

Yes, they can be used together in the same system because they serve complementary roles. For example, the 74HC138D can decode address lines to select peripheral devices, while the 74HC123D can generate timing pulses for those peripherals. They are electrically compatible when operated from the same supply voltage within the 2.0 V to 6.0 V range.