Signal Switches, Multiplexers, Decoders
Logic signal switches, multiplexers, and decoders cover key logic components used for digital signal selection, routing, and conversion, and are widely used in data management, control signal processing, and programmable logic systems (such as FPGAs).
1. What are Decoders?
A decoder is a simple combinational logic block that converts a small digital input representation (such as an n-bit binary code) into a larger output representation (such as 2^n independent logic signals). This conversion is usually used to activate specific output lines, such as in a 3-8 decoder, where a three-bit input signal is decoded into one of eight output lines being selected.
Decoders implement address decoding or control signal expansion functions in digital systems, increasing the flexibility of signal processing.
2. What are Multiplexers?
A multiplexer (MUX) selects one of multiple input signals as an output through select lines. For example, an n-bit select line can multiplex 2^n input signals for efficient signal routing. It is similar to a decoder, but focuses on input selection rather than output expansion.
Multiplexers are commonly used in data selectors and signal distributors to support dynamic control of signal paths.
3. What are Logic Signal Switches?
Signal switches are used to switch or isolate signals between multiple signal paths, such as FET bus switches (such as IDT74FST163245PAG8). These components support bidirectional signal transmission and can operate in high-voltage or high-speed environments, suitable for bus management and interface control.
Switching circuits usually provide tri-state outputs (high impedance state) to avoid signal conflicts and optimize power consumption.
4. Typical Components and Applications
Common Components: For example, CD74HCT157 (quad 2-input multiplexer), CD74HCT257 (quad 2-input multiplexer with tri-state output), and IDT74FST163245 (FET bus switch). These components are standardized in integrated circuits (ICs) for easy integration into digital circuit designs.
Application Scenarios: These components play a core role in FPGA, microcontroller systems, and instrumentation (such as oscilloscopes and signal generators) to achieve efficient data flow control, signal selection, and logic conversion.
Logic Signal Switches, Multiplexers, and Decoders FAQs
1) How to choose the right signal switch or multiplexer?
You need to screen based on parameters such as number of channels, voltage range (such as VCC and I/O level matching), switching speed, and on-resistance (Ron); manufacturers such as TI provide online tools to quickly select by configuration (SPST/SPDT), number of channels, and electrical characteristics.
For example, the number of channels of a multiplexer is determined by the bit width of the selector: an n-bit selector supports 2n inputs.
2) Can the I/O voltage of a logic device exceed the supply voltage (VCC)?
No. You need to strictly follow the “Absolute Maximum Ratings” in the datasheet to avoid device damage caused by I/O pin voltages higher than VCC or lower than 0V.
3) What is the difference between a decoder and a multiplexer?
Decoder: Converts binary code into physical output (e.g., 1 n-bit input activates 1 of 2n output lines), used for memory address addressing, seven-segment digital tube driving, etc.
Multiplexer: Implements multiple-choice signal routing, while demultiplexer (Demux) is its reverse process (one input divided into multiple outputs).
4) What is the function of the enable pin?
When the enable pin input is low, the device is forced to output high impedance (floating) to avoid bus conflicts and is often used in scenarios where multiple devices share communication lines.
5) How do devices with different logic levels interconnect?
The input/output level specifications need to be matched:
VIH (minimum input high level)> VOH (minimum output high level);
VIL (maximum input low level)< VOL (maximum output low level).
When there is a mismatch, a level conversion chip (such as LVTTL to LVCMOS) or a bus switch must be used.
6) What should be noted in the design of the open collector (OC) gate?
The output of the OC gate must be connected to an external pull-up resistor, and the resistance value must be calculated based on the load current and voltage to ensure that the high and low levels are effectively switched.
7) How to deal with unused input pins?
It must be pulled up or down to a fixed level (no floating) to prevent electrostatic interference from causing abnormal logic states.
8) Can multiplexers replace combinational logic circuits?
Yes. By configuring the selection end and the input signal, Boolean logic functions can be implemented and the circuit structure can be simplified (such as using 8-to-1 MUX to replace multiple gate circuits).
9) What is the role of the decoder in embedded systems?
Convert the processor address bus signal into a memory chip select (Chip Select) or a peripheral enable signal to expand the system access capability.