## FST32253QSC: Overview and Technical Details The **FST32253QSC** is a high-speed CMOS bus switch manufactured by Fairchild Semiconductor (now part of **onsemi**). It is primarily used in digital circuits to connect or isolate signals with minimal propagation delay. --- ### 1. Key Specifications The following table outlines the fundamental electrical characteristics of the device: | Parameter | Specification | | :--- | :--- | | **Logic Type** | 8-Bit Bus Switch | | **Configuration** | Dual 4-Bit or Single 8-Bit | | **Supply Voltage (Vcc)** | 4.0V to 5.5V | | **Propagation Delay** | 0.25 ns (Max) | | **On-Resistance (R-on)** | 4Ω (Typical) | | **Package Type** | QSOP-24 (Quarter Size Outline Package) | | **Operating Temperature** | -40°C to +85°C | --- ### 2. Functional Description The FST32253 consists of two separate 4-bit switches with individual **Output Enable ($\overline{OE}$)** controls. * **Switch Mechanism:** It uses N-Channel FETs. When the switch is "ON," it provides a low-resistance path between ports. When "OFF," the ports are in a high-impedance state. * **Zero Propagation Delay:** Because it is a simple FET switch rather than a buffer, it does not add significant timing delays, making it ideal for high-speed memory or data bus applications. * **Level Shifting:** While primarily a 5V device, it can be used for voltage translation (e.g., 5V to 3.3V) if the $V_{cc}$ is biased correctly. --- ### 3. Pin Configuration and Logic The device is typically used to multiplex or demultiplex signals. | Pin Name | Function | | :--- | :--- | | **A0 - A7** | Port A Inputs/Outputs | | **B0 - B7** | Port B Inputs/Outputs | | **$\overline{OE}_1, \overline{OE}_2$** | Active Low Output Enables | | **Vcc** | Power Supply | | **GND** | Ground | **Truth Table:** | $\overline{OE}$ Input | State | | :--- | :--- | | LOW (L) | A Port = B Port (Connected) | | HIGH (H) | Disconnected (High Impedance) | --- ### 4. Typical Applications 1. **Bus Isolation:** Separating a heavily loaded bus into smaller segments to reduce capacitance. 2. **Hot-Plugging:** Protecting sensitive components when peripherals are plugged in while the system is powered. 3. **Memory Interleaving:** Switching between different banks of SDRAM or Flash memory. 4. **Signal Multiplexing:** Routing one data stream to multiple destinations. --- ### 5. Implementation Example (Verilog) If you were simulating the logic behavior of one 4-bit bank of this switch: ```verilog module fst32253_model ( input [3:0] A, inout [3:0] B, input OE_n ); // If OE_n is Low, A and B are connected (simplified) // If OE_n is High, B is High-Z assign B = (!OE_n) ? A : 4'bzzzz; endmodule ```