2024-01-03 280 View

Pull-up Resistors

Imagine your digital circuit is like a light switch that can be either on (1) or off (0). Sometimes, we want to change it from on to off or vice versa. To do this smoothly, we must ensure the switch is firmly in the off position before turning it on and vice versa. We can't just let it be in the middle.

That's where Pull-Up and Pull-Down Resistors come in handy. They help us make sure our switch (or digital pin) is in the clear on or off state, avoiding any confusion in between, like this:

Pull-up Resistors

When we want the default state to be high and only change it to low with an external interaction, we can employ a pull-up resistor, as illustrated in the image below:

The digital logic input pin can switch between logic 1 (High) and logic 0 (Low) using switch SW1. The R1 can be 4.7K resistor that acts as a pull-up resistor, connecting to the logic voltage from a 3.3V supply. When the switch isn't pressed, the input pin maintains a default voltage of 3.3V, staying High. Pressing the switch shorts the pin to the ground, making it logic Low.

Although we can't directly short the pin to ground or Vcc to avoid damaging the circuit, the closed switch seems to create a short in this case. However, on closer inspection, it's not a direct short. Thanks to the pull-up resistance and Ohm's law, a small current flows from the source through the resistors and the closed switch before reaching the ground.

Without the pull-up resistor, pressing the switch would directly short the output to the ground. Conversely, leaving the switch open would leave the logic level pin floating, potentially causing undesired outcomes.

Pull-down Resistors

Pull-down resistors function similarly to pull-up resistors, but instead of pulling the pin to a logical high value, they bring it to a logical low value. These resistors are linked between the ground and the relevant pin on a device. The illustration below depicts an instance of a pull-down resistor in a digital circuit.

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