## 74FST3245: 8-Bit Bus Switch The **74FST3245** is a high-speed CMOS 8-bit bus switch. It is primarily used in digital circuits to connect or disconnect two buses with minimal propagation delay and no added noise. Unlike a buffer, it acts more like a mechanical relay or a physical wire that can be turned on or off. --- ### 1. Key Technical Specifications | Parameter | Value/Description | | :--- | :--- | | **Logic Family** | FST (Bus Switch) | | **Number of Channels** | 8-Bit | | **Supply Voltage ($V_{CC}$)** | 4.0V to 5.5V | | **Propagation Delay** | < 0.25 ns (nearly zero) | | **On-Resistance ($R_{ON}$)** | ~4Ω to 8Ω (Typical) | | **Off-State Leakage** | Low ($1 \mu A$ max) | | **Pin Count** | 20 Pins | --- ### 2. Pin Configuration and Logic The device features an **Output Enable ($\overline{OE}$)** pin which controls the connection between the 'A' bus and the 'B' bus. | $\overline{OE}$ Input | A and B Bus Status | Description | | :---: | :--- | :--- | | **Low (L)** | A = B | Connected (Low impedance state) | | **High (H)** | Disconnected | High-Impedance (Hi-Z) | #### Pin Designations: * **A1 - A8**: Data Input/Output (Port A). * **B1 - B8**: Data Input/Output (Port B). * **$\overline{OE}$**: Active-low output enable. * **Vcc / GND**: Power supply and ground. --- ### 3. Core Features and Functionality * **Near-Zero Propagation Delay:** Because it uses an N-channel FET as a switch, there is no "drive" logic in the path. The delay is purely based on the RC time constant of the switch's on-resistance and the load capacitance. * **Bidirectional Operation:** Since it is a physical switch, data can flow from A to B or B to A without a "Direction" control pin. * **Low On-Resistance:** The low $R_{ON}$ ensures minimal voltage drop across the switch, maintaining signal integrity. * **5V to 3.3V Translation:** It is often used for voltage translation. When powered at 5V, the N-channel transistor naturally clips the output voltage, allowing it to interface 5V signals with 3.3V logic levels. --- ### 4. Common Applications 1. **Bus Isolation:** Isolating a CPU from a series of peripherals to reduce capacitive loading on the main bus. 2. **Hot-Plugging:** Protecting sensitive components when cards or modules are inserted into a live system. 3. **Voltage Level Shifting:** Bridging 5V TTL systems with 3.3V CMOS systems. 4. **Multiplexing:** Using multiple 74FST3245 units to route data from different sources to a single destination. --- ### 5. Example Schematic Logic ```text Vcc (5V) | [74FST3245] +---------+ ---| A1 B1 |--- (To Peripheral) ---| A2 B2 |--- ---| A3 B3 |--- ---| OE |--- (GND to Enable) +---------+ ```