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Why are pull-up resistors necessary in microcontroller systems? [Copy link]

In the microcontroller system, pull-up resistors have gradually become the most stable and reliable main component.
Most people know that pull-up resistors play a big role in circuits, but at the same time, microcontroller systems are also composed of circuits, so the role of pull-up resistors in microcontroller systems is also very important.
Let's take a look at why pull-up resistors are so important in microcontroller systems? Why do most microcontroller pins need to be connected to pull-up resistors? As we know, a pull-up resistor is a resistor that clamps an uncertain signal at a high level. When inputting current, the resistor also acts as a current limiter. The strength of the resistance is just the difference in the resistance of the pull-up resistor. In fact, there is no strict distinction. For non-open collector output circuits or open drain output circuits, the pull-up resistor has limited ability to increase current and voltage in this type of circuit. Its main function is to provide a current channel for open collector output circuits. Generally speaking, there are two reasons for the design of pull-up and pull-down pins that directly determine the connection of pull-up resistors. First, in normal operation or single fault state, the pin should not be in an uncertain state, such as the pin floating after the connector falls off. Rather, from the perspective of power consumption of the circuit, the pin is in a waiting state for a long time, and the resistance of the pin port cannot consume too much current. This is particularly important for the service life and safety of battery-powered devices. From the perspective of anti-interference, the signal port should also give priority to the pull-up resistor. When the pull-up resistor is connected, the source input is often in a high-impedance state in the standby state. At this time, if there is no pull-up resistor connected, the input wire will show an antenna effect. Once the pin is interfered by electromagnetic radiation, the pin input state is very easy to be sensed and changed.
In addition, the most important point after the pin is connected to the pull-up resistor is to provide a discharge channel to prevent high-level interference. If strong radiation interference occurs at this time, and the intensity even exceeds the VCC level, the high-level interference on the wire will be discharged to VCC through the discharge channel provided by the pull-up resistor. Therefore, no matter what kind of radiation interference, there will be no false triggering, which provides great protection for the safety performance of the system.
When dealing with the working problems of the IIC microcontroller interface, many people will be confused about the practice of connecting an external pull-up resistor. Since the pull-up resistor has been set inside the microcontroller, there have been different opinions and views on whether the external pull-up resistor is still needed.
Since some microcontrollers are equipped with pull-up resistors, some of them do not need to add pull-up resistors if the pull-up capability is sufficient, but some of them are not enough, so pull-up resistors must be added externally. In this case, it mainly depends on whether the microcontroller used by the engineer has a standard IIC interface. If the microcontroller uses a standard IIC interface, then when the interface is enabled by software, the pin will enter the open-drain mode, and the external pull-up resistor can be omitted. However, if the ordinary I/O pins of the microcontroller are used to simulate the IIC protocol, it is necessary to combine the microcontroller pins to support the open-drain mode or the pull-up mode to make a judgment. In this case, a pull-up resistor is generally required.
In addition, connecting a pull-up resistor to the IIC interface can also play a protective role. Since the IIC interface is mainly responsible for high and low level detection when working, once the pull-up resistor is not protected and the power supply is directly connected, the device will be pulled low, and the entire system will be very dangerous. According to the requirements of the IIC bus specification, both lines must be high when the bus is idle. However, since the IIC interface adopts the Open Drain mechanism, it can only output a low level but cannot actively output a high level. Therefore, the signal line can only be pulled to a high level through an external pull-up resistor. Therefore, the pull-up resistor on the IIC bus must be connected.

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learning!!  Details Published on 2018-12-21 08:50
 

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