## Technical Overview: U74HCT138L-R16-R The **U74HCT138L-R16-R** is a high-speed CMOS device, specifically a **3-to-8 line decoder/demultiplexer**. It is designed to be pin-compatible with standard 74 series logic (specifically the 74138 family) but utilizes HCT (High-speed CMOS with TTL-compatible inputs) technology. --- ### 1. Key Technical Specifications | Parameter | Specification | | :--- | :--- | | **Logic Family** | HCT (High-speed CMOS, TTL Input Compatible) | | **Logic Type** | Decoder / Demultiplexer | | **Number of Lines** | 3-to-8 | | **Operating Voltage ($V_{CC}$)** | 4.5V to 5.5V | | **Input Thresholds** | TTL Level (Compatible with 5V TTL logic) | | **Package Type** | SOP-16 / TSSOP-16 (Depending on suffix) | | **Output Type** | Active Low (Inverted) | --- ### 2. Functional Description The device 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 utilizes three enable pins: * **E1, E2:** Active Low enable inputs. * **E3:** Active High enable input. * *Condition:* The device only decodes when $E_1$ and $E_2$ are LOW and $E_3$ is HIGH. If any enable condition is not met, all outputs ($Y_n$) remain HIGH. --- ### 3. Pin Configuration | Pin Number | Symbol | Description | | :--- | :--- | :--- | | 1, 2, 3 | $A_0, A_1, A_2$ | Address Inputs | | 4, 5 | $\bar{E}_1, \bar{E}_2$ | Enable Inputs (Active Low) | | 6 | $E_3$ | Enable Input (Active High) | | 7 | $Y_7$ | Output 7 (Active Low) | | 8 | GND | Ground (0V) | | 9 - 15 | $Y_6 - Y_0$ | Outputs 6 to 0 (Active Low) | | 16 | $V_{CC}$ | Supply Voltage | --- ### 4. Part Number Breakdown: U74HCT138L-R16-R * **U:** Often indicates a specific manufacturer prefix (e.g., Unisonic Technologies - UTC). * **74HCT:** Logic Family (High-speed CMOS, TTL compatible). * **138:** Function code (3-to-8 Decoder). * **L:** Often stands for Lead-free / RoHS compliant. * **R16-R:** Refers to the packaging (likely SOP-16 or TSSOP-16) and "Tape & Reel" delivery format. --- ### 5. Common Applications * **Memory Address Decoding:** Selecting specific memory chips in a microprocessor system. * **Data Routing:** Functioning as a demultiplexer by using the enable lines as data inputs. * **I/O Expansion:** Increasing the number of output lines controlled by a microcontroller using only 3 GPIO pins.