The difference between open-drain output and push-pull output of microcontroller I/O port

Push-pull output: can output high and low levels and connect digital devices;

Open drain output: The output terminal is equivalent to the collector of the transistor. A pull-up resistor is required to obtain a high level state. It is suitable for current-type driving and has a relatively strong ability to absorb current (generally within 20ma).

The push-pull structure generally refers to two transistors being controlled by two complementary signals, and one transistor is always turned on while the other is turned off.

To realize line-and, an OC (open collector) gate circuit is required. It is two transistors or MOSFETs with the same parameters, which exist in the circuit in a push-pull manner, each responsible for the waveform amplification task of the positive and negative half cycles. When the circuit is working, only one of the two symmetrical power switch tubes is turned on at a time, so the conduction loss is small and the efficiency is high. The output can both inject current into the load and extract current from the load.

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Open-drain circuit characteristics and applications

When designing circuits, we often encounter the concepts of open drain and open collector.

The "drain" mentioned in the concept of the so-called open-drain circuit refers to the drain of the MOSFET. Similarly, the "collector" in the open-collector circuit refers to the collector of the transistor. The open-drain circuit refers to a circuit with the drain of the MOSFET as the output. The general usage is to add a pull-up resistor to the circuit outside the drain. A complete open-drain circuit should consist of an open-drain device and an open-drain pull-up resistor.

   
The circuit that makes up the open-drain form has the following characteristics:

1. Use the driving capability of the external circuit to reduce the driving inside the IC. When the MOSFET inside the IC is turned on, the driving current flows from the external VCC through R pull-up, MOSFET to GND. Only a very low gate driving current is required inside the IC. As shown in Figure 1.
2. Multiple open-drain output pins can be connected to one line. Form an "AND logic" relationship. As shown in Figure 1, when any one of PIN_A, PIN_B, and PIN_C becomes low, the logic on the open-drain line is 0. This is also the principle of I2C, SMBus and other buses to determine the bus occupancy status.

3. The transmission level can be changed by changing the voltage of the pull-up power supply. As shown in Figure 2, the logic level of the IC is determined by the power supply Vcc1, and the output high level is determined by Vcc2. In this way, we can use low-level logic to control the output high-level logic.
4. If the open-drain pin is not connected to an external pull-up resistor, it can only output a low level (therefore, for the P0 port of the classic 51 microcontroller, an external pull-up resistor must be added if it wants to have input and output functions, otherwise it cannot output high-level logic).
5. The standard open-drain pin generally only has output capabilities. Adding other judgment circuits can have the ability of bidirectional input and output.

Notes in application:
1. The principles of open drain and open collector are similar. In many applications, we use open collector circuits instead of open drain circuits. For example, an input pin is required to be driven by an open drain circuit. Then our common driving method is to use a triode to form an open collector circuit to drive it, which is convenient and cost-saving. See Figure 3.

2. The resistance value of the pull-up resistor R pull-up determines the speed of the edge of the logic level conversion. The larger the resistance value, the lower the speed and the lower the power consumption. Vice versa.

Push-Pull output is generally called push-pull output. It should be more suitable than CMOS output in CMOS circuits because the push-pull output capacity in CMOS cannot be as large as that of bipolar. The output capacity depends on the area of ​​the N-tube and P-tube of the internal output electrode of the IC. Compared with open-drain output, the high and low levels of push-pull are determined by the power supply of the IC, and simple logic operations cannot be performed. Push-pull is the most commonly used output stage design method in CMOS circuits.

The I/O port of the 51 single-chip microcomputer is an open-drain output with weak driving capability, so a pull-up resistor is generally required to drive the next level circuit.

The AVR and STM8S series are true bidirectional I/O ports with push-pull output and current up to 20mA