The **FST32211G** (often prefixed as 74FST32211G) is a high-speed, low-power **24-bit to 48-bit Multiplexer/Demultiplexer Bus Switch**. Originally designed by Fairchild Semiconductor (now part of ON Semiconductor), this integrated circuit (IC) is primarily used in high-performance digital systems for memory interleaving, bus swapping, and signal routing. Here is a detailed breakdown of its electronic characteristics, functions, and specifications. --- ## 1. Core Functionality The FST32211G operates essentially as a bank of high-speed electronic switches. It allows a 24-bit wide digital bus (A) to be connected to one of two alternative 24-bit wide buses (B1 or B2), mapping a total of 48 destination bits. * **Bidirectional Operation:** Signals can flow from A to B, or from B to A with near-zero propagation delay. * **Zero Delay:** Because it acts as a closed switch (FET-based) rather than a buffer/amplifier, it does not add significant propagation delay to the circuit. --- ## 2. Key Technical Specifications | Parameter | Specification | Description | | :--- | :--- | :--- | | **Supply Voltage ($V_{CC}$)** | 4.0V to 5.5V | Standard 5V logic supply range | | **On-State Resistance ($R_{ON}$)** | $\approx$ 4$\Omega$ (Typical) | Ensures minimal voltage drop and signal distortion | | **Propagation Delay** | $\le$ 0.25 ns | Essentially negligible | | **Input/Output Capacitance** | Low ($C_{IN} \approx$ 5pF) | Minimizes bus loading | | **Package Type** | FBGA / TSSOP (The 'G' suffix often indicates RoHS compliant/Green package) | High-density packaging for space-saving | --- ## 3. Pin Function and Control Logic The device is controlled using Select (**SEL**) and Output Enable (**$\overline{OE}$**) pins. The internal NMOS pass transistors are turned on or off based on these control inputs. ### Truth Table | Control Inputs
($\overline{OE}$) | Control Inputs
(SEL) | Disconnected Buses | Active Channel/Function | | :---: | :---: | :---: | :--- | | **H** | X | A, B1, B2 | **Disconnect** (High-Impedance / Hi-Z) | | **L** | L | B2 | **A Bus Connected to B1 Bus** | | **L** | H | B1 | **A Bus Connected to B2 Bus** | *Note: **H** = High Voltage Level, **L** = Low Voltage Level, **X** = Don't Care.* --- ## 4. Internal Architecture Features ### Low ON-Resistance ($R_{ON}$) The internal switch consists of an N-channel FET. When the switch is active, the resistance between Port A and Port B is extremely low (typically 4 ohms). This prevents signal degradation and allows the IC to support high-frequency data toggling. ### Undershoot Hardened The FST32211 family features undershoot protection circuitry. If severe signal reflections cause the voltage to drop below ground (-2.0V), the switch remains turned off, preventing data corruption on the active bus. --- ## 5. Common Applications The FST32211G is commonly found in complex digital architectures, such as: * **Memory Bank Switching:** Alternating data pathways between dual DDR memory banks. * **Bus Isolation:** Isolating sections of a motherboard or backplane during hot-swapping or power-saving modes. * **Network Switches & Routers:** High-speed routing of parallel data packets.