Shulou(Shulou.com)06/01 Report--

It seems that we have mentioned pull-up resistors and pull-down resistors many times before. what kind of resistors are up and down resistors in detail, and what are the effects of pull-up resistors?

The pull-up resistor is to pull the uncertain signal through a resistor to a high level, and the resistor also acts as a current-limiting effect, and the pull-down means to pull down to a low level.

For example, when our IO is set to open-drain input high level or high resistance state, the default level is uncertain, the internal through a resistor connected to the VCC, that is, pull-up resistance, then the response pin is high; after a resistance to GND, that is, the pull-down resistance, then the response pin is a low level.

Pull-up resistors are used a lot, what kind of influence can they play? We now need to understand the following four points that are most commonly used:

OC gate to input high level, must be added internal pull resistance to normal operation, in fact, OC gate is equivalent to single-chip microcomputer IO open-leak input, the reason can refer to figure 9-1 open-leak circuit.

Increase the driving ability of the popular IO port. The pull-up resistor of the external IO port of the standard 51 single-chip microcomputer is usually dozens of K ohms, for example, the external STC89C52 is a 20K pull-up resistor, so the maximum input current is 250uA, so the internal pull-up resistor can be combined with the external pull-up resistance to increase the input ability of high current.

In the level conversion circuit, such as the 5V to 12V circuit we mentioned earlier, the pull-up resistor actually affects the current-limiting resistor. You can recall figure 3-8.

Single-chip microcomputer unused pins, such as bus pins, when they are suspended, they are easily disturbed by electromagnetic interference and are in a messy form, although it will not affect the program, but it usually adds the power consumption of single-chip microcomputer, plus a pull-up resistance to VCC or a pull-down resistance to GND, it can effectively resist electromagnetic disturbance.

So when we stop the circuit design, how to choose the appropriate resistance value of the up-and-down resistance?

Consider that it should be large enough in terms of reducing power consumption, because the greater the resistance, the smaller the current.

From ensuring that there is enough pin drive to think that it should be small enough, the resistance is low, and the current is large.

When opening the leakage input, the excessive pull-up resistance will cause the rising edge of the signal to slow down. Let's explain: most of the changes in the practice level are time-consuming, which is small, but they can never reach zero, and the size of the pull-up resistance during the open-leak input directly affects the time required for this upward process, as shown in figure 9-2. Come to think of it, if the resistance is very high and the frequency of the signal is very fast, it will eventually cause the signal to become low again before it rises to a high level, so that the signal cannot be transmitted accurately.

Fig. 9-2 influence of pull-up resistance on waveform

Considering all kinds of situations, most of our commonly used pull-up resistance values are between 1K and 10K, and the details of how much we usually have to choose according to the needs of practice, usually within the standard scale, is a fixed value.

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