### FST32253MTC: Electronic Components & Functional Overview The **FST32253MTC** is a high-speed CMOS 8-bit Bus Switch manufactured by Fairchild (now part of ON Semiconductor). It is primarily used in digital circuits to connect or isolate signal paths with minimal propagation delay. --- ### 1. Key Technical Specifications | Parameter | Value/Description | | :--- | :--- | | **Logic Family** | FST (Fairchild Switch Technology) | | **Configuration** | 8-Bit (Dual 4-Bit) | | **Package Type** | TSSOP-24 (MTC suffix) | | **Supply Voltage ($V_{CC}$)** | 4.0V to 5.5V | | **Propagation Delay** | 0.25 ns (max) | | **On-Resistance ($R_{ON}$)** | 4Ω (typical) | | **Output Current** | 128 mA | --- ### 2. Internal Architecture The device consists of two sets of 4-bit switches. Internally, these are **N-Channel MOSFETs** that act as high-speed gates. * **Switching Mechanism:** When the Enable ($\overline{OE}$) pin is LOW, the switch is closed (low resistance), allowing signals to flow between Port A and Port B. * **High Impedance:** When $\overline{OE}$ is HIGH, the switch is open (High-Z), effectively disconnecting the two buses. * **Minimal Delay:** Because it acts as a physical wire (via a transistor) rather than a logic gate with propagation stages, the delay is near zero ($t_{pd} \approx 0.25ns$). --- ### 3. Pinout Description The FST32253 is typically housed in a 24-pin TSSOP package. | Pin Names | Function | | :--- | :--- | | **$1\overline{OE}, 2\overline{OE}$** | Output Enable (Active Low) | | **S0, S1** | Select Inputs (for multiplexing versions) | | **A1 - A8** | Data Port A | | **B1 - B8** | Data Port B | | **$V_{CC}$ / GND** | Power Supply and Ground | --- ### 4. Primary Applications 1. **Bus Isolation:** Used to prevent noise or capacitive loading by disconnecting unused sections of a data bus. 2. **Level Shifting:** Can be used for 5V to 3.3V translation when a diode is added or $V_{CC}$ is adjusted. 3. **Hot-Plugging:** Protects sensitive components during live insertion of peripheral cards. 4. **Signal Multiplexing:** Routing signals from one source to multiple destinations. --- ### 5. Circuit Design Considerations * **$V_{CC}$ Range:** It is optimized for 5V systems. If using for 3.3V logic, ensure the input voltage levels meet the threshold requirements. * **No Drive Strength:** Unlike a buffer, this device **does not** regenerate or amplify signals. It only passes what is provided. * **Undershoot Protection:** The FST series includes circuitry to handle negative voltage undershoot common in high-speed switching.