Logic States, pull-up & pull-down

Lecture 2 of Digital Electronics Course

Before you know how to make actual digital electronics, there is one more thing that you need to know. What are the 3 possible logic states and how do I use them in my circuits?

Logic States

There are 3 logic states:

  1. HIGH
    • A HIGH logic state can sometimes also be referred as ON, 1, Positive Current
    • It simply means that this pin is Active
  2. LOW
    • A LOW logic state can sometimes also be referred as OFF, 0, Negative Current or even Grounded
    • It simply means that this pin is Inactive
  3. FLOATING
    • Floating may also be referred as Not Connected or Undefined
    • It is a state that occurs when the pin is not connected to a source or drain. It means that there is no precise logical state and that it can be either 1 or 0

Floating State

Today the topic of our discussion will be the Floating State.

What are the effects of floating state on the circuit?

  • The FLOATING state causes errors in the circuit’s logic by randomly changing values and not triggering circuits as expected.

What causes a floating state to occur?

  • The FLOATING state is caused by shoddy wiring and poor design.
  • It can also occur due to hardware malfunction but it is best to look for faults in the wiring before deciding to change all of the hardware.

How to ensure floating state doesn’t occur in my circuit?

  • This brings us to our other topic of discussion for today, Pull-up and Pull-down.

Pull-up

Pull-up refers to attaching the pin to a source or a positive current through a resistor. This means that the pin will always be in the HIGH state unless a negative current is passed. It is important to use resistor to avoid a short circuit between the pull-up and the input.

Below is an example where a pin has been pulled up. A pushbutton attached to GND is acting as the input.

Pull-down

Pull-down refers to attaching the pin to a drain or GND through a resistor. This means that the pin will always be in the LOW state unless a positive current is passed. It is important to use resistor to avoid a short circuit between the pull-down and the input.

Below is an example where a pin has been pulled down. A pushbutton attached to VCC is acting as the input.

When to use which?

Using Pull-up and Pull-down depends entirely upon your input. If you wish to send LOW as your input to the pin then you should use Pull-up, and if you wish to send HIGH as your input then you should use Pull-down.