51 MCU IO port input and output mode

    From the characteristics of the I/O port, the P0 port of the standard 51 is an open-drain structure when used as an I/O port, and a pull-up resistor is usually added in actual applications; P1, P2, and P3 are all quasi-bidirectional I/O ports with internal pull-up resistors, which can be used as both input and output. The I/O port characteristics of the LPC900 series microcontrollers are somewhat different, and they can be configured into 4 different working modes: quasi-bidirectional I/O, push-pull output, high-impedance input, and open-drain.

    Compared with the standard 51, the quasi-bidirectional I/O mode is different in internal structure, but similar in usage. For example, when it is used as an input, you must first write "1" to set it to a high level, and then you can read the level status of the pin. ! ! ! ! ! Why is it like this? See the following graphic analysis.

   The characteristic of push-pull output is that it can drive a large current regardless of whether the output is high or low. For example, when the output is high, the LED can be directly lit (with a current-limiting resistor of several hundred ohms connected in series), which is difficult to do in quasi-bidirectional I/O mode.

    The characteristic of high-impedance input mode is that it can only be used as an input, but a relatively high input impedance can be obtained, which is necessary in analog comparator and ADC applications.

    The open-drain mode is similar to the quasi-bidirectional mode, but there is no internal pull-up resistor. The advantage of the open-drain mode is good electrical compatibility. If the external pull-up resistor is connected to a 3V power supply, it can interface with a 3V logic device. If the pull-up resistor is connected to a 5V power supply, it can interface with a 5V logic device. In addition, the open-drain mode can also easily implement the "wired AND" logic function.

    To explain the above question, there is such a document:

      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, just like not connected. If measured with a multimeter, it may be a high level or a low level, depending on what is connected to it.

When analyzing a circuit, the high impedance state can be understood as an open circuit. You can think of it as the output (input) resistance being very large. Its limit can be considered as floating.

      Typical applications of high impedance state:

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, 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.　
2. Most microcontroller I/Os can be set to high-impedance input when used, such as Lingyang, AVR, etc. High-impedance input can be considered as an infinite input resistance, and it is considered that the 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.