Details of decision quality, this sentence everyone knows, but, in the practice of the development of electronic products, truly fully deal with subtleties, is not easy, detail processing is not sufficient, in normal working condition, the product can run normally; But when the deterioration of working conditions (for example, the power fluctuation, noise interference), there may be unexpected negative results, and the pros and cons of electronic product quality, more is in bad working conditions can be fully reflected,

We pull up resistors of as shown in the following figure, for example, talk about the importance of the detail processing of electronic product quality,


Pull-up resistors, the main function of I/O port is to ensure that the input in the absence of external input signal, maintain certain high level, this for electronics engineers even beginner, usually also is able to understand, to master, so pull up resistors for resistance, how should choose? Usually, we will according to the example on the typical circuit or reference books, choose 4.7 K, 5.1 K, 10 K, etc., so the choice of the resistance, what are the limits? The answer is limited,

If we read the electrical characteristics of MCU manual unit, will find that the I/O part has a parameter IOL, used for characterization of the I/O port as an output port and low output electricity at ordinary times, ports of the input current value, and this parameter is the limitation of the maximum current value, pull-up resistors value restrictions, it is and the related parameters, to meet the pull-up resistance R & gt; Vcc/IOL (Max),

Pull-up resistors, type is used to enter the I/O port, why is the limit and the parameters of the output port IOL associated? This involves the pull-up resistors another role - safety protection function of an exception occurs,

Pull-up resistors are used to input, as we all know, the direction of the I/O port, and the direction of the port choice, usually by direction register set, in normal working conditions, direction, once set, will not change, unless the application through the instruction set changes its direction, however, if the working environment there is strong noise, the direction of value may be modified by the accident in the register, make originally set for the input type of I/O port change to output port, if the port of the buffer data is 0, has a current from VCC N tube through the pull-up resistors and port, if the resistance is very small, will appear too much electricity, and even cause the I/O port hardware damage,

Therefore, pull-up resistors value choice, consider not only the port as input direction, the resistance make port is used to determine the initial value, also need to consider an exception occurs, not because of the change of port direction causes the damage of port ,

Let's look at the following figure of pull-up resistors:


From logic functions, such treatment can ensure high level of each port, and save the number pull-up resistors, but, this kind of processing method is a potential risk, as stated earlier, when work in strong noise environment, the direction of register may be accidental tampering, pull up resistors if sharing the two ports are accidental tampering as output port, and just a port data buffer=1, another port of the data buffer=0, then there will be from a port of P pipe flows through another port N of large current, may cause the damage of the port,

Multiple ports is not recommended, therefore, share the same pull-up resistors processing methods, processing method is safe, each port through a separate pull-up resistor connected to the power supply, as shown in the figure below,


So, pull up resistance of simple such details, for the product quality is also very important,

CodePudding user response:

Pull up resistors common and can implement line

CodePudding user response:

Thank you for your comments @ worldy,