The **U74HC138G-R16-R** is a high-speed CMOS device, specifically a **3-to-8 line decoder/demultiplexer**. It is widely used in digital electronics to expand the number of outputs from a microcontroller or to select specific memory banks or peripherals. --- ### 1. Key Technical Specifications | Feature | Description | | :--- | :--- | | **Logic Family** | HC (High-speed CMOS) | | **Function** | 3-to-8 Decoder / Demultiplexer | | **Voltage Range (Vcc)** | 2.0V to 6.0V | | **Input Type** | Standard CMOS | | **Output Type** | 8 Mutually Exclusive Outputs (Active LOW) | | **Package Type** | SOP-16 / TSSOP-16 (Depending on exact suffix) | | **Operating Temperature** | -40°C to +85°C | --- ### 2. Functional Description The device takes three binary weighted address inputs ($A_0, A_1, A_2$) and, when enabled, activates one of the eight outputs ($Y_0$ through $Y_7$). #### Enable Pins The device features three enable pins to allow for easy "daisy-chaining" or memory expansion: * **E1 & E2:** Active LOW inputs. * **E3:** Active HIGH input. * **Condition:** All three enable conditions must be met for the outputs to function. If any enable pin is in its inactive state, all eight outputs remain HIGH. --- ### 3. Pin Configuration The U74HC138 typically follows this 16-pin layout: | Pin Number | Symbol | Function | | :--- | :--- | :--- | | 1, 2, 3 | A0, A1, A2 | Binary Address Inputs | | 4, 5 | E1, E2 | Enable Inputs (Active LOW) | | 6 | E3 | Enable Input (Active HIGH) | | 8 | GND | Ground (0V) | | 7, 9-15 | Y0 - Y7 | Decoded Outputs (Active LOW) | | 16 | VCC | Supply Voltage | --- ### 4. Logic Truth Table (Simplified) When the device is enabled ($E3=H, E1=L, E2=L$): | Address A2 | Address A1 | Address A0 | Active Output (LOW) | | :---: | :---: | :---: | :---: | | L | L | L | **Y0** | | L | L | H | **Y1** | | L | H | L | **Y2** | | ... | ... | ... | ... | | H | H | H | **Y7** | --- ### 5. Common Applications * **Memory Address Decoding:** Selecting specific RAM or ROM chips in a microprocessor system. * **Data Routing:** Acting as a demultiplexer to send data to one of eight different channels. * **I/O Expansion:** Using 3 pins of a microcontroller to control 8 separate digital lines (e.g., controlling 8 LEDs or relays). ---