🔼 Pull-up / Pull-down Resistor

Calculate the right resistor value for buttons, open-drain signals, and GPIO inputs.

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Quick presets:
V
Typical: 1–100 kΩ
PCB trace + pin: ~10–50 pF typical

Typical resistor values by application

ApplicationRecommended RVccReason
Push button (5V AVR)10 kΩ5 VLow current draw, fast enough for buttons
Push button (3.3V MCU)10 kΩ3.3 VStandard — works for ESP32, STM32, RPi
I²C SDA/SCL (100 kHz)4.7 kΩ3.3 / 5 VRequired pull-up for open-drain I²C bus
I²C SDA/SCL (400 kHz)2.2 kΩ3.3 VFaster speed needs lower R for rise time
I²C SDA/SCL (1 MHz)1 kΩ3.3 VFast mode+ — low R needed
Open-drain output4.7–10 kΩanyBalance speed vs power consumption
Reset pin (MCU)10 kΩ5 / 3.3 VWeak pull-up to keep MCU running
UART RX idle high10 kΩ3.3 / 5 VPull to idle state if line floats
Long cable / noisy env.1–4.7 kΩanyLower R reduces noise sensitivity
Battery-powered device100–470 kΩ3.3 VMinimise leakage current

Pull-up vs Pull-down

Pull-up resistor

  • Connects GPIO pin to Vcc through R
  • Idle state: GPIO reads HIGH
  • Button/switch connects pin to GND
  • When pressed: GPIO reads LOW
  • Required for open-drain/open-collector outputs (I²C, 1-Wire)
  • Most common choice — less noise pickup

Pull-down resistor

  • Connects GPIO pin to GND through R
  • Idle state: GPIO reads LOW
  • Button/switch connects pin to Vcc
  • When pressed: GPIO reads HIGH
  • Used when active-HIGH input logic is needed
  • More susceptible to picking up 50/60 Hz hum

Choosing resistor value

  • Too high (>100 kΩ) — slow edges, noise sensitive, may not drive input reliably
  • Too low (<1 kΩ) — high current when button pressed, wasted power
  • 10 kΩ — good default for buttons at 3.3V and 5V
  • For I²C: R × C_bus must allow fast enough rise time

Internal pull-ups

  • Arduino/ESP32/STM32 all have built-in pull-ups (~20–50 kΩ)
  • Enable with: pinMode(pin, INPUT_PULLUP)
  • Internal pull-ups are usually sufficient for buttons
  • For I²C or long cables, use external pull-up resistors
  • Internal values vary — check your MCU datasheet for exact value