越边检护照被写脏话:three-state三态门_高阻态

When used as an input, the general-purpose I/O portpins can be configured to have a pullup, pulldown or three-state mode ofoperation. By default, after a reset, inputs are configured as inputs withpullup. To deselect the pullup or pulldown function on an input, theappropriate bit within the PxINP must be set to 1.

Indigital electronics three-state, tri-state, or 3-state logic allowsan output port to assume a high impedance state in addition to the 0 and 1 logic levels, effectively removing the outputfrom the circuit. This allows multiple circuits to share the same output lineor lines (such as a bus).

Three-stateoutputs are implemented in many registers, bus drivers, and flip-flops in the 7400 and 4000 seriesas well as in other types, but also internally in many integrated circuits.Other typical uses are internal and external buses in microprocessors, memories, and peripherals. Many devices are controlled by an active-low inputcalled OE(Output Enable) whichdictates whether the outputs should be held in a high-impedance state or drivetheir respective loads (to either 0- or 1-level).

Theterm tri-state^[1] shouldnot be confused with ternary logic (3-value logic).

Thewhole concept of the third state (Hi-Z) is to effectively remove the device'sinfluence from the rest of the circuit. If more than one device is electricallyconnected, putting an output into the Hi-Z state is often used to prevent shortcircuits, or one device driving high (logical 1) against another device drivinglow (logical 0).

Three-statebuffers can also be used to implement efficient multiplexers,especially those with large numbers of inputs.^[2] Inparticular, they are essential to the operation of a shared electronic bus.

Three-statelogic can reduce the number of wires needed to drive a set of LEDs (tristatemultiplexing or Charlieplexing).

Use of pull-ups and pull-downs

Whenoutputs are tri-stated (in the Hi-Z state) their influence on the rest of thecircuit is removed, and the circuit node will be "floating" if noother circuit element determines its state. Circuit designers will often use pull-up or pull-down resistors (usually with in the range of1–100 kΩ) to influence the circuit when the output is tri-stated. Forexample, the I2C bus protocol (a bi-directionalcommunication bus protocol often used between devices) specifies the use ofpull-up resistors on the two communication lines. When devices are inactive,they "release" the communication lines and tri-state their outputs,thus removing their influence on the circuit. When all the devices on the bushave "released" the communication lines, the only influence on thecircuit is the pull-up resistors, which pull the lines high. When a devicewants to communicate, it comes out of the Hi-Z state and drives the line low.Devices communicating using this protocol either let the line float high, ordrive it low – thus preventing any bus contention situation where onedevice drives a line high and another low.

The PCI local bus provides pull-up resistors, but theywould require several clock cycles to pull a signal high given the bus's largedistributed capacitance. To enable high-speed operation,the protocol requires that every device connecting to the bus drive theimportant control signals high for at least one clock cycle before going to theHi-Z state. This way, the pull-up resistors are only responsible formaintaining the bus signals in the face of leakage current.

三态门，是指逻辑门的输出除有高、低电平两种状态外，还有第三种状态——高阻状态的门电路高阻态相当于隔断状态。

三态门都有一个EN控制使能端，来控制门电路的通断。 可以具备这三种状态的器件就叫做三态(门,总线,......).  

举例来说：

内存里面的一个存储单元，读写控制线处于低电位时，存储单元被打开，可以向里面写入；当处于高电位时，可以读出，但是不读不写，就要用高电阻态，既不是＋5v，也不是0v 计算机里面用 1和0表示是，非两种逻辑，但是，有时候，这是不够的，比如说，他不够富有 但是他也不一定穷啊，她不漂亮，但也不一定丑啊，   处于这两个极端的中间，就用那个既不是＋ 也不是。

三态门电路的一种状态,其他两个状态是低电平,高电平,这两个是工作状态.而处在高阻态的三态门是与总线隔离开的,这样总线可以同时被其他电路占用.

高阻态就是电阻非常高的状态，通常当作电阻无穷大的情况处理。 

　　高电平就是高电压，是电工程上的一种说法。 

　　电阻高和电压高肯定不是一回事的。 

　　有高阻态就有低阻态，它接近于零电阻，一般的导线我们都可以等同于低阻态，但实际他还是有电阻的，只是非常小而已。 

　　同理，低阻态和低电平也不是一回事，只不过在一些条件下他们经常同时存在。也比如在一根导线的两端，即是低阻态，也是低电平，要知道导体两端的相对电压是相等的。一般用这些状态来判断一些故障和问题。

    三态-高电平，低电平，高阻（断开）

参考：高阻态和高电平是一回事吗？

三态门,三态逻辑与非门的电路结构及工作原理