Circuit common sense concepts - the concept and application of tri-state gate and high impedance state

We all know that a three-state gate is a gate circuit whose output has a third state in addition to the two states of high and low level - high impedance state. The high impedance state is equivalent to the isolation state (the resistance is very large, equivalent to an open circuit). The three-state gate has an EN control enable terminal to control the on and off of the gate circuit. The device that can have these three states is called a three-state gate .

In computers, 1 and 0 are used to represent the two logics of yes and no, but sometimes this is not enough. For example, he is not rich but he is not necessarily poor; she is not beautiful but she is not necessarily ugly. In between these two extremes, it is represented by the intermediate state that is neither + nor -, which is called high impedance state. High level and low level can be pulled up and down by internal circuits. In high impedance state, the resistance of the pin to ground is infinite, and the real level value can be read when reading the pin level. One of the important functions of high impedance state is to read the external level of I/O (input/output) port during input.

The high impedance state is equivalent to the gate being disconnected from the circuit it is connected to. (Because you can't disconnect it in the actual circuit, so set such a state to make it disconnected). The tri-state gate is an output stage that extends the logic function and is also a control switch. It is mainly used for bus connection because the bus only allows one user at the same time. Usually, multiple devices are connected to the data bus, and each device is selected by a signal such as OE/CE. If the device is not selected, it is in a high impedance state, which is equivalent to not being connected to the bus and does not affect the operation of other devices.

If your device port is to be connected to a bus, it must pass through a tri-state buffer. Because only one port can be used for output at the same time on a bus, the other ports must be in high impedance state, and the data of this output port can be input at the same time. Therefore, you also need bus control management. When accessing a port, the tri-state buffer of that port can be transferred to the output state. This is a typical application of tri-state gates. If there are no more than two output devices on the line, of course, tri-state gates are not needed, and wired OR logic is another matter.

High impedance state is a common term in digital circuits, which refers to an output state of the circuit, which is neither a high level nor a low level. If the high impedance state is input into the next level circuit, it will have no effect on the next level circuit, which is the same as not being connected. If measured with a multimeter, it may be a high level or a low level, depending on what is connected to it.

The essence of high impedance state : When analyzing circuits, high impedance state can be understood as an open circuit. You can think of it as a very large output (input) resistance. Its limit can be considered as floating. In other words, in theory, the high impedance state is not floating, it is a state with extremely large resistance to the ground or the power supply. In practical applications, it is almost the same as the floating of the pin. (When the output pull-up tube of the gate circuit is turned on and the pull-down tube is turned off, the output is a high level; otherwise, it is a low level; if both the pull-up tube and the pull-down tube are turned off, the output end is equivalent to floating (no current flows), and its level depends on the external level, that is, the gate circuit gives up control of the output circuit)

Typical applications:

1. In the structure of bus connection. There are multiple devices on the bus, and the devices are connected to the bus in the form of high impedance. In this way, when the device does not occupy the bus, it automatically releases the bus (gives up the use of the bus) to facilitate other devices to obtain the right to use the bus.

2. Most MCU I/O can be set to high-impedance input when used, such as Lingyang, AVR, etc. High-impedance input (similar to CMOS input impedance) can be considered as infinite input resistance, and it is considered that I/O has little effect on the previous stage, and does not generate current (no attenuation), and to a certain extent, it also increases the chip's ability to resist voltage shocks.