What do strong pull-up, strong pull-down, weak pull-up, and weak pull-down mean?

tiankai001

What do strong pull-up, strong pull-down, weak pull-up, and weak pull-down mean? [Copy link]

 

When reading integrated circuit manuals, for I/O ports, some device manuals will say that the port can have built-in pull-up/pull-down resistors. Generally, these built-in pull-up/pull-down resistors are called "weak pull-ups" or "weak pull-downs". In actual circuits, the pull-up/pull-down resistors connected to the chip ports are often called "strong pull-ups" or "strong pull-downs". (Of course, this is the general case. It does not mean that the pull-up/pull-down resistors inside the chip are all weak pull-ups, and the external pull-up/pull-down resistors are all strong pull-downs. Specific). So, what exactly does the "strong" and "weak" mentioned here refer to? What is "strong"? What is "weak"? 1. "Strong" output "Strong 1" means that the high level of the output is equal to or close to VCC, and "strong 0" means that the low level of the output is equal to or close to GND. As shown in the figure below. You can intuitively see what is "strong" and "weak". 2. "Weak 0" and "strong 1" output are obvious. This is the connection method of the pull-down resistor. When the output is low, due to the existence of the pull-down resistor, the low level is higher than GND, and when the output is high, the output level can reach VCC. 3. The effect of "weak 0" on low-level recognition is shown in the figure below. For the input of device 2, its input voltage is the voltage divided by R1 and R2. If the resistance value of R1 is not appropriate, the low-level voltage value output by device 1 may exceed the maximum low level of device 2, which will cause a logic level judgment error. 4. Due to the existence of the pull-up resistor, the output high level of "weak 1" and "strong 0" output cannot reach "VCC", so when it is output 1, it is "weak", and the output low level is equal to GND, which is "strong 0" 5. The effect of "weak 1" on low-level recognition is shown in the figure below. For the input of device 2, its input voltage is the voltage divided by R1 and R2. If the resistance value of R1 is not appropriate, the high-level voltage value output by device 1 may be lower than the minimum high level of device 2, which will cause a logic level judgment error. 6. Why are the internal pull-up/pull-down resistors of the device generally "weak pull-up" or "weak pull-down"? This is to facilitate users to turn it into a strong pull-up/strong pull-down through an external pull-up/pull-down resistor. For example, if the internal pull-up resistor is 50K, then an external 10K pull-up resistor is connected, then the total pull-up resistance of this port will be less than 10K, becoming "strong". Regarding this point, forum netizen drjloveyou has said it very well. You can go and have a look. Weak pull-down and strong pull-down, weak pull-up and strong pull-up

懒猫爱飞

The most intuitive way to understand and remember is the size of the resistor

tiankai001

Lazy Cat Loves Flying published on 2019-2-26 08:32 The most intuitive way to understand and remember is the size of the resistor

yanbao

Nice sharing!

chunyang

The pull-up/pull-down resistors connected to the chip ports are often called "strong pull-up" or "strong pull-down" - there is no such term, it will be misleading. The concept of strong and weak refers to the internal resistance of the signal source. The so-called "strong" refers to the electronic switch composed of transistors/MOS tubes, which connects the load to the power supply or ground. The saturation on-resistance of the tube is low, so it is called "strong". The so-called "weak" refers to the load connected to the power supply or ground through a resistor. The resistance of the IO bias resistor is much larger than the saturation on-resistance of the transistor, so it is called "weak". The two are relative, and the comparison benchmark is the saturation on-resistance of the transistor and the resistance of the resistor. The description of the concept must be as rigorous as possible.

chunyang

For the case of adding external bias resistance when necessary, the rigorous description should be "enhanced pull-up or pull-down" rather than "strong pull-up or strong pull-down", which will lead to confusion with the saturated on-resistance of the transistor.

tiankai001

chunyang posted on 2019-2-26 16:33 The pull-up/pull-down resistors connected to the chip ports are often called "strong pull-up" or "strong pull-down" - there is no such term, it will be misleading. The concept of strong and weak...

Such a professional explanation really makes people feel enlightened. I will be grateful for your advice

tiankai001

chunyang posted on 2019-2-26 16:36 For the case of adding external bias resistors when necessary, the rigorous description should be "enhanced pull-up or pull-down" instead of "strong pull-up or strong pull-down" ...

This explanation is easy to understand the principle

yangxf1217

Learned