field guide

I2C Pull-up Resistors

Why I2C needs pull-up resistors on SDA and SCL, how to size them, and what breaks when they are missing or wrong.

updated 2026-06-14 sensing i2c

What a pull-up is

A resistor connected between the signal wire (SDA or SCL) and VCC (3.3V). Its job: pull the line to 3.3V (logic 1) when nothing is actively driving it low.

Analogy: the I2C lines are ropes that devices can only pull down (to ground). They cannot push them back up. The pull-up resistor is a spring that gently returns the rope to 3.3V whenever no one is pulling it.

Why I2C needs them (open-drain)

“SDA | Serial Data — bidirectional, open-drain, pulled high by pull-up resistors” — i2c-protocol.md

I2C uses open-drain outputs:

To send a 0: device connects the line to ground (pulls it low).
To send a 1: device does nothing — releases the line. The pull-up resistor pulls it high.

This is a “wired-AND” design. Multiple devices can share the same two wires safely — if two devices try to talk simultaneously, nothing gets damaged. If any device drives low, the line goes low. They all agree on the low state.

Without pull-up resistors the line floats at an indeterminate voltage when no device is pulling it low → the bus doesn’t work. Transitions are slow or never complete.

External vs internal pull-ups

The ESP32 has weak internal pull-up resistors built into the silicon, enableable per-GPIO. This is what your config disables:

.sda_pullup_en    = GPIO_PULLUP_DISABLE,
.scl_pullup_en    = GPIO_PULLUP_DISABLE,

These internal pull-ups are typically ~45 kΩ — far too weak for I2C. At that resistance the RC time constant (with trace capacitance plus sensor input capacitance) is too large for clean edges at 100 kHz or 400 kHz. The bus becomes flaky: intermittent ACKs, wrong data, timeouts.

Best practice: disable internal pull-ups and use dedicated external resistors on the board.

Wiring

3.3V  ───[4.7kΩ]───┬─── SDA (to ESP32 GPIO21 and MPU SDA)
                   │
3.3V  ───[4.7kΩ]───┴─── SCL (to ESP32 GPIO22 and MPU SCL)

Each line gets its own pull-up to 3.3V. Two resistors total.

Resistor value selection

Situation	Value	Why
Standard 100 kHz bus, short wires	4.7 kΩ	Sweet spot for most setups
400 kHz fast mode	2.2–3.3 kΩ	Lower resistance charges line capacitance faster for higher speed
Long wires ( >30 cm)	2.2 kΩ	Overcome higher bus capacitance
Many devices on bus (>4)	2.2–3.3 kΩ	Each device adds ~10 pF input capacitance
Breakout board (MPU-6050 module)	Usually has 4.7 kΩ already	Check with a multimeter before adding your own

Too high → slow rise times → failed communication at speed. Too low → excessive current draw, devices may struggle to pull the line low (weak drive strength).

4.7 kΩ at 3.3V draws $I = 3.3/4700 = 0.7\,\text{mA}$ when the line is pulled low. That’s within any I2C device’s drive capability by a wide margin.

When using a breakout board

Most MPU-6050 breakout boards (GY-521, etc.) already have 4.7 kΩ pull-ups soldered on. If you’re using one, you don’t need to add your own — but verify with a multimeter: probe between SDA and 3.3V (with board unpowered). If you see ~4.7 kΩ, they’re there.

If you’re using a bare MPU-6050 chip (QFN package), you must add external pull-ups. The chip has no internal ones.

Symptoms of missing or weak pull-ups

Bus works sometimes but fails intermittently
Frequent NACKs on address or data frames
i2c_master_cmd_begin returns timeout or error
WHO_AM_I reads 0x00 or random garbage
Oscilloscope shows slow rise times on SDA/SCL (trapezoidal edges instead of square)
Works at low speed (10 kHz) but fails at 100 kHz

This is the most common hardware cause of I2C bringup failures on ESP32.