“In circuit analysis, the 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 regarded as floating, that is to say, in theory, the high-resistance state is not floating, it is a state of extreme resistance to ground or to the power supply. The actual application is almost the same as the floating of the pin.
When we first came into contact with the 51 single chip microcomputer, a pull-up resistor must be added to port P0, otherwise P0 will be in a high-impedance state.
You may be wondering about this question, why is it high impedance? Add a pull resistor? Today we will briefly explain this concept.
High resistance state
High-impedance state is a common term in digital circuits. It refers to an output state of the circuit, which is neither high nor low.
If the high-impedance state is input to the next-level circuit, it will have no effect on the next-level circuit. It is the same as if it is not connected. If it is measured with a multimeter, it may be high or low, depending on what is connected behind it.
The essence of high resistance state
In circuit analysis, the 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 regarded as floating, that is to say, in theory, the high-resistance state is not floating, it is a state of extreme resistance to ground or to the power supply. The actual application is almost the same as the floating of the pin.
The meaning of high resistance state
When the output pull-up tube of the gate circuit is turned on and the pull-down tube is turned off, the output is high, otherwise it is low. If both the pull-up tube and the pull-down tube are 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 end circuit.
In the structure of the bus connection. There are multiple devices on the bus, and the devices are connected to the bus in a form of high impedance. In this way, the bus is automatically released when the device does not occupy the bus, so that other devices can obtain the right to use the bus.
Most of the single-chip I/O can be set as high-impedance input when in use. High-impedance input can be considered as infinite input resistance, and that I/O has minimal impact on the previous stage, and does not produce current (no attenuation), and to a certain extent also increases the chip’s ability to withstand voltage shocks.
High-impedance state is usually expressed by the letter Z.
In a system or in a whole, we often define some reference points, just like the sea level we often say, and the same is true in a single chip. Whether we say high level or low level, we are relatively speaking. of. Knowing this point may be easy to understand this issue.
High impedance state in single chip microcomputer
In 51 single-chip microcomputer, the P0 port without a pull-up resistor is compared with the P1 port with a pull-up resistor. The connection between the I/O port pins and the power supply is realized by a pair of push-pull FETs. The specific structure of 51 is as follows picture.
Composed of a push-pull structure, theoretically, it is possible to easily realize large current output by adjusting the parameters of the tubes and increase the load capacity. The two tubes have four different combinations according to the on-off state. The upper and lower tubes are equivalent to short-circuiting the power supply. In this case, it must not appear in the actual circuit.
In terms of logic circuit, IO is directly connected to VCC when the upper tube is on-the lower tube is off and on, and the IO outputs a low level of 0. If there is no external pull-up resistor in this structure, the output of 0 is an open-drain state (low-impedance state) , Because the I/O pin is grounded through a tube, it is not directly connected with a wire, and the general MOS will also have an on-resistance of mΩ in the on-state.
At this point, it is very clear that whether it is in the low-impedance state or the high-impedance state, it is relatively speaking. Putting the lower tube in the cut-off state can isolate the GND and the I/O port to an open circuit state. At this time, push-pull one The tube is in the off state. If the read logic is ignored, the I/O port pin is equivalent to an open circuit with the internal circuit of the microcontroller. Considering that there will be a little leakage current when the actual MOS is off, it is called the “high impedance state”.
Due to the influence of the junction capacitance caused by the PN section of the tube, some materials are also called “floating”. It takes a certain time to charge the capacitor through the I/O port, so the true voltage and water surface at the IO pin are The floating of the buoy with the wave is similar. The magnitude of the voltage is not only related to the external input but also to the time. This phenomenon cannot be ignored under high frequency conditions.
In short, the high impedance state is a relative concept. When using it, we only need to do it according to the requirements, let us add, and we add, all of which make sense.