How do you size a pull-down resistor?

To calculate the pull-down resistor value, it’s slightly different from the pull-up resistor value. Knowing that current is 100uA, we’ll take 0.5v as our pull-down voltage since the input is 0.8v. Thus, applying our R = V/I once again, but this time we don’t have to minus, so our formula remains constant.

How many ohms should a pull-down resistor have?

The use of 100Ω pull-down resistors are the most common but they can range in resistive value from 50 up to 1k ohms.

What is the reason for each pin of port 0 must be connected externally to a 10k ohm pull-up resistors?

All i know is that Port 0 has open drain output drivers so the pull-ups must be provided externally. Ports 1, 2, and 3 have built in internal pull-ups.

Why do we require external pull up resistors in port 0?

An external pull-up resistor is needed when Port 0 is defined as an output port. Port 0 of the 8051 has two main functions: To be used as a simple input-output port and to access external memory in conjunction with Port 2.

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.

What is the maximum pull up resistor for a logic gate?

By knowing the information above, we can calculate the maximum pull-up resistor value required for a single TTL 74LS series logic gate as: Then using Ohms Law, the maximum pull-up resistance required to drop 3 volts for a single TTL 74LS series logic gate would be 150kΩ.

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.

When do you need pull up resistors for microcontrollers?

In addition to thwarting problems associated with floating microcontroller I/O pins, pull-up resistors are a necessity when using an open-drain topology. We discussed open drains at length in this Bench Talk post.

How do you size a pull down resistor?

To calculate the pull-down resistor value, it’s slightly different from the pull-up resistor value. Knowing that current is 100uA, we’ll take 0.5v as our pull-down voltage since the input is 0.8v. Thus, applying our R = V/I once again, but this time we don’t have to minus, so our formula remains constant.

What size pull-up resistor would you need if you want to limit the current to 10ma when a button is pressed in a 5 volt circuit?

The lower the resistance, the more power will be used when the button is hit. You generally want a large resistor value (10kΩ), but you don’t want it too large as to conflict with condition 2.

What is the maximum pull up resistor for a logic gate?

By knowing the information above, we can calculate the maximum pull-up resistor value required for a single TTL 74LS series logic gate as: Then using Ohms Law, the maximum pull-up resistance required to drop 3 volts for a single TTL 74LS series logic gate would be 150kΩ.

When do you use a pull up resistor?

You need to use a pull-up resistor when you have a low default impedance state and wish to pull the signal to ‘high’. In the figure above, a pull-up resistor with a fixed value was used to connect the voltage supply and a particular pin in the digital logic circuit.

How does a pull down resistor work on a microcontroller?

A pushbutton switch is connected between the supply voltage and a microcontroller pin. In such a circuit, when the switch is closed, the micro-controller input is at a logical high value, but when the switch is open, the pull-down resistor pulls the input voltage down to ground (logical zero value), preventing an undefined state at the input.

How does the pull up resistor work on a VCC?

When the button is pressed, the input pin is pulled low. The value of resistor R1 controls how much current you want to flow from VCC, through the button, and then to ground. When the button is not pressed, the input pin is pulled high. The value of the pull-up resistor controls the voltage on the input pin.