-[Rhett]

Resistance ACTS as a limitation of current in a circuit. Pull-up resistance and pull-down resistance are frequently mentioned and frequently used resistors. A large number of pull-up and pull-down resistors are used in the design of each system.

In the circuit of pull-up resistance and pull-down resistance, there is often a question: why pull-up resistance can pull up? Why does a pull-down resistor pull down? Why is there often a resistor next to a pull-down resistor?

It can be briefly summarized as follows: the resistance from the power source to the pin of the device is called the pull up resistance, which makes the pin high at ordinary times, and the resistance from the ground to the pin of the device is called the pull down resistance, which makes the pin low at ordinary times. The low level is connected to GND in the IC. The high level is connected to a very large resistor within the IC.

Pull-up means to pass an uncertain signal through a resistance clamp at a high level. The resistance also ACTS as a current limiting function. Similarly, pull-down. For non-collector (or drain-open) output circuits, such as gate circuits, whose ability to raise current and voltage is limited, the main function of pull up and down resistors is to provide an output current channel for collector open output circuits. Pull up is the input current to the device, pull down is the output current; Strong and weak only pull up or pull down the resistance value is different, there is no strict distinction.

When the I/O port of IC is at high voltage, the impedance between the node and GND is very large, which can be interpreted as infinite. At this time, the pull-up resistance (such as 4.7K ohm, 10K ohm) is connected to the VCC, and the partial voltage of the pull-up resistance is almost negligible. When I/O port node needs low power, GND can be directly connected. At this time, VCC and GND are connected through the pull up resistance just now (such as 4.7K ohm, 10K ohm resistance), and the current passing through is very small and can be ignored.

The level value of the size, high and low is relative to the ground level, so in the size of the level value to refer to the level value. It doesn't matter whether the pins are connected to the ground or not, because high or low is relative to the ground plane.

The indirect pull up resistance of 10K ohm or 4.7k ohm between the node and +5V can pull up the potential of this node, which usually requires the application of SCM or other controllers to control it (and the connection of this node with I/O) to high or low level. If you simply want to make this node become a high level, and the output impedance is very large, then it is ok to directly connect the power supply, but if the MCU wants to make this node low, that is, the MCU internal ground node, so the 5V power supply and ground short circuit.

In addition, when the node is required to be of high voltage, the impedance between the node and the ground is generally very large, such as 100K ohm. When a resistance of 10K ohm is pulled up, the voltage divided by this point is 100K ohm/(100K+10K) *5V=4.5V, which can also be pulled to a high level. When this node is required to be at low power, just connect it to the ground. There is a 10K resistance between the power supply and the ground, so there will be no short circuit. When low electricity at ordinary times, have formed a load between power supply and ground loop, and sometimes this node will be through a resistor, because current flow to the low impedance, so current through the resistor LiuXiangDe connected to the power supply, rather than to the resistor connected to the nodes, because this node is connected the resistance of the impedance is high, so low electricity at ordinary times the point of electric potential is low level.

It can be considered that for the I/O port of IC, controlling the high and low level in the IC is equivalent to controlling the O/O port and connecting it to the GND or very large resistance inside the IC, such as 100K ohm. When the I/O port is low level 0V, in the IC, the pin controlling the IC chip O/O port is connected to GND inside the chip. When high I/O ports for the electricity at ordinary times, such as 5 v, this time I/O port pin is in the chip with a very large resistor, such as 100 k connected, sometimes will be concatenated in the I/O node of a small resistance resistance, such as 68, because the current flow to the low impedance, so when the inside of the chip I/O port connected to GND for low electricity at ordinary times, the power supply to the GND within the pull-up resistors and chip form a circulation loop, the I/O mouth node of current will flow to the chip inside the GND, because node combination of a small resistance of the resistance, is a high impedance, relative to the GND Even a little bit more is high resistance, so the current doesn't flow through this resistor in series.

When pull-down resistance is used (the so-called pull-up and pull-down are used for high-resistance states), when the I/O port is in the high-resistance state, the pull-up resistance can be used to keep it in the high level state. Specific as described above: when the I/O port is high impedance state, using pulldown resistor to the mouth connected to GND, and high impedance state resistance is very big, can be understood as disconnected, is a large and chip internal resistances are connected, the drop-down to the ground, there is no current, level value is 0, unless it's for the pin with a high level value it can play a role.

When TTL circuit drives CMOS circuit, if the TTL circuit output high level is lower than the lowest high level of CMOS circuit, then the TTL output end needs to be connected with a pull resistor to improve the output high level. OC gate circuits must be fitted with pull resistors to increase the high level of output.

On A CMOS chip, in order to prevent damage caused by static electricity, the unused pins cannot be suspended. Generally, they are connected with a pull resistor to reduce the input impedance and provide a discharge path. At the same time, the dangling pin is easier to receive external electromagnetic interference.

The reflected wave interference is suppressed. The resistance mismatch in long line transmission is easy to cause the reflected wave interference, and the pull-down resistance makes the resistance match, which can effectively suppress the reflected wave interference.

In some CMOS inputs a pull - down resistor or pull - down resistor is applied to set the default potential. These inputs pull down to the low level or pull up to the high level when these pins are not used. The idle state on a bus such as I2C is obtained by pulling up and down resistors.

If the input end is in the state of high resistance, or the input end of high impedance is in the suspended state, then a pull or pull resistance should be added to avoid being affected by the random level, thus affecting the work of the circuit. Similarly, if the output end is in a passive state, a pull or pull resistance should be added. For example, the output end is only a collector of an audion, so as to improve the noise tolerance of the input signal of the chip and enhance the anti-interference ability.

At the base end of the BJT transistor, the pull-up resistor and pull-down resistor also play an important role. In the circuit application of triode, the resistance connected in series to the base plays the role of limiting the base level current, as shown in R2 in the figure below.

As shown in R5 in the figure below, the pull up resistor makes the input level of the audion base be high by default. When the CPU has low level signal output, the peripheral circuit responds. The pull down resistor makes the base input of the transistor be low by default, as shown in R6 in the figure below.