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Why use pull-up resistor instead of pull down?
Pull-up resistors are used to solve the dilemma for the microcontroller by pulling the value to a logical high state, as seen in the follow figure. Without the pull-up resistor, the MCU’s input would be floating when the switch is open and pulled down to a logical low only when the switch is closed.
How does a pull-down resistor prevent a pin from floating?
Recall that when an input pin is in high-impedance mode and not driven by external sources, it is floating at a residual voltage level. Pull-up resistors prevent the pin from floating by forcing the signal to VCC when it is not being actively driven.
Do pull up resistors reduce noise?
The pull-up resistors will make a low pass filter between the power supply (a source of noise) and the input. They also limit the current that can flow during transients in more extreme cases.
What is the use of pull-down resistor?
The pull-down resistor holds the logic signal near to zero volts (0V) when no other active device is connected. It pulls the input voltage down to the ground to prevent an undefined state at the input. It should have a larger resistance than the impedance of the logic circuit.
How is a pull up resistor connected to a Vcc supply?
This condition means that their output is either grounded when LOW, or floating when HIGH, so an external pull-up resistor, (Rp) needs to be connected from the open-collector terminal of the pull-down transistor to the Vcc supply.
How are the pull up and pull down resistors connected?
By using these two pull-up resistors, one for each input, when switch “A” or “B” is open (OFF), the input is effectively connected to the +5V supply rail via the pull-up resistor.
Can a TTL gate be connected to a pull up resistor?
Two or more open-collector outputs of TTL gates can be directly connected together and tied through a single external pull-up resistor. The result is that the outputs are effectively AND’ed together as the combination behaves as if the gates were connected to an AND gate. This type of configuration is called wired AND logic.
What does pull up and pull down signal mean?
So you either drive 0V on the output or turn the output off (perhaps by configuring it as an input). When all the outputs are off, the wire is pulled to ‘1’ (Vcc) by the pullup. This is called a “wired AND” signal. (You can do the same driving ‘1’ or off, with a pulldown R, then it is called a wired OR).