Pull-up resistor and pull-down resistor

The so-called high level is referred to as the low level. Pull up, is through a resistor to connect the signal to the power supply, usually used for clock signal data signal. A drop-down is a signal grounded by a resistor and is generally used to protect the signal. This is designed according to the needs of the circuit, the main purpose is to prevent interference, increase the stability of the circuit. If there is no pull-up, clock and data signal error prone, after all, the CPU power is limited, with a lot of bus lines, the provision of high-level signal a little hard. Once these signals are pulled down to a certain voltage by load or interference, the CPU is unable to receive the information and send instructions correctly, and can only reset the reboot continuously. If there is no pull down, the protection circuit is very susceptible to external interference, so that the CPU is mistaken for the protection of the object to take protective action, resulting in false protection. Pull down, to be set according to the circuit requirements.

Adding a pull-up resistor or pull-down resistor is to add a resistor from the power v+ or V-end to the output of the integrated circuit device, which is simply to solder a resistor directly in the output pin of the device to the power supply v+ or V-end.
1, the pull-up resistor to the device injection current, the common purpose of the installation is two:
(1) Increase the output level. This is necessary if the TTL output drives COM's level match.
(2) Increase the output drive capacity, but for non-OC or OD output type circuit its role is limited, if used to drive a similar led does not add pull or drop resistance is also possible, should be considered from the load current-limiting resistance and other aspects of the solution, if the load is heavier, should be installed output buffer or power drive circuit.
For OC or OD circuits, the output current channel must be provided by the pull-up resistor, otherwise it will not work, so it is not necessary to install it when it is designed and manufactured.
2, the pull-down resistor increases the device output current, mainly used to set the low level or impedance matching.
3, the addition of the resistance value of the size of the installation purpose, load conditions and the device limit parameters and other conditions, the size of the resistance determines the weak strength of the installation action.
The schematic diagram is then drawn and uploaded.

Upload schematic diagram. By the way, in order not to cause misunderstanding, the pull-up resistor and the pull-down resistor are drawn in the output and input, respectively, at the input of this stage can be seen as the output of the front-end installation of the pull-down resistor. In fact, the choice of the installation of the pull-down resistor is basically located in the rear stage of the input, one is the front stage without output pull to ensure that the input level of the lower level, and the second is the impedance matching with the previous level.

Pull-up resistor and pull-down resistor