【 Weak current College 】 TriState doors and high-impedance state 】

3-door, a logic gate output with high and low level two States, there is also a third State – high-impedance state gate circuits. High-impedance state equivalent to the partition status (resistance is very large, open circuit). 3-door has a EN control enable to control the door circuit on/off. You can have three States of the device is called the Tri-State (door, bus, ...).
Computer inside 1 and 0 indicate that non-two logical, however, sometimes this is not enough, for instance, he is not rich, but he would not necessarily poor Ah; she is not pretty, but not necessarily the ugly Ah, in the middle of the two extremes, that neither + nor-intermediate state, called the high-impedance state. High level and low level can be an internal circuit, high and low. While high-impedance state when PIN to ground resistance infinite, read the PIN level can read real level values. The important role of the high-impedance state is I/O (input/output) port in input is read into the external level.
High-impedance state equivalent to the door and it connected the circuit is in a disconnected state. (Because the actual circuit you don't might go to disconnect it, so setting a State so that it is in a disconnected state). 3-door is an expansion of output-level logic functions, it is also a control switch. Mainly used for bus connections, because the bus only allows only one user at the same time. Usually data bus with multiple devices, each device by OE/CE, signal gating. If the device does not have a gated, it is in the high-impedance state, which is equivalent to not received on the bus, without affecting other devices work.
If your device port to hang in a bus, 3-State buffer must be passed. Because in a bus on only one port for output, then other port must be in the high-impedance state, at the same time you can enter this data output port. Therefore you also need to have a bus control management, access to which port, the port of 3-State buffer can turn into an output state, this is a typical 3-door applications. If online does not have two or more of the output device, of course, less than 3-door, and the other line or logic.

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This is a high-impedance state digital circuit in common predicate, refers to a kind of output circuit, neither an high level or low level, if the high-impedance state and then enter the next level circuit, to subordinate circuit has no effect, and could answer, if you use multimeter so there may be a high level may also be low level, as it followed by something.

High-impedance state at the time of the essence: circuit analysis can do open-high-impedance state. You can think of it as output (input) resistance is very large. His limit can be considered vacant. That is in theory not vacated high-impedance state, it is on the ground or on the power resistor great State. But the practical application and PIN vacant few are the same.
(When the gate circuit output pull-up SCR and drop-down tube cut-off, the output is a high level; on the contrary it is a low level; as La pipe and tube are closed down, the output is equivalent to floating null (no current flows), its high and low level with the external level, i.e. door circuit giving up on the output side of the circuit of control)


Typical applications:
1. the structure of the bus connection. Bus on more than one device, the device to the bus to connect high-resistance. This device does not take the bus to automatically release the bus (the bus), to facilitate other device for the right to use the bus.
2, when used in most single-chip I/O can be set to a high-impedance input, e.g. Ling Yang, AVR, etc. High-impedance input (similar to the CMOS input impedance) can be considered input resistance is infinity, that I/O on the fore minimal impact, but does not have a current (no attenuation) and to a certain extent also increases the impact of anti-voltage chip.