The **U74HCT138G-R16-R** is a high-speed CMOS device, specifically a **3-to-8 line decoder/demultiplexer**. It is designed to be pin-compatible with the standard 74 series logic family but utilizes TTL-compatible input thresholds. --- ### 1. General Specifications | Feature | Details | | :--- | :--- | | **Logic Type** | Decoder / Demultiplexer | | **Configuration** | 3-Line to 8-Line | | **Logic Family** | HCT (High-speed CMOS with TTL inputs) | | **Package Type** | SOP-16 / TSSOP-16 (Depending on manufacturer suffix) | | **Operating Voltage** | 4.5V to 5.5V (Standard TTL range) | | **Operating Temperature** | -40°C to +85°C | --- ### 2. Functional Description The U74HCT138 accepts three binary weighted address inputs ($A_0, A_1, A_2$) and, when enabled, provides eight mutually exclusive active-LOW outputs ($Y_0$ to $Y_7$). #### Enable Inputs To operate, the device uses a combination of three enable pins: * **Two Active-LOW enables ($E_1, E_2$)** * **One Active-HIGH enable ($E_3$)** 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**. This feature allows for easy expansion to a 1-of-24 or 1-of-32 decoder without external logic. --- ### 3. Pin Configuration (16-Pin) | Pin No. | Symbol | Description | | :--- | :--- | :--- | | 1, 2, 3 | A0, A1, A2 | Address Inputs | | 4, 5 | E1, E2 | Enable Inputs (Active Low) | | 6 | E3 | Enable Input (Active High) | | 7 | Y7 | Output 7 (Active Low) | | 8 | GND | Ground (0V) | | 9 - 15 | Y6 - Y0 | Outputs 6 through 0 (Active Low) | | 16 | VCC | Supply Voltage | --- ### 4. Key Applications 1. **Memory Address Decoding:** Used to select specific memory chips in a microprocessor system by decoding the upper bits of the address bus. 2. **Data Routing:** Acting as a demultiplexer to route data from a single source to one of several destinations. 3. **Digital Switching:** Enabling specific peripherals or sensors in embedded systems. --- ### 5. Implementation Example (Verilog) If you were simulating the logic of this hardware part in Verilog, it would look like this: ```verilog module u74hct138 ( input [2:0] A, // Address inputs input E1_n, E2_n, // Active-low enables input E3, // Active-high enable output reg [7:0] Y // Active-low outputs ); always @(*) begin if (!E1_n && !E2_n && E3) begin Y = ~(8'b1 << A); // Set the indexed bit to 0 end else begin Y = 8'hFF; // All outputs HIGH end end endmodule ```