I2C Level Shifting With The TCA9517 With Reference Schematic

David Ray

TCA9517 Schematic Example

The Voltage Mismatch Problem

     I²C looks deceptively simple: two wires (SDA and SCL), lots of devices, and everything “just works.”
But the trouble starts when devices don’t agree on voltage.

  • Most modern microcontrollers run at 3.3 V
  • Many sensors, displays, and older peripherals still run at 5 V

If you directly connect them:

  • A 5 V pull-up can drive a microcontroller pin above its absolute maximum rating
  • Best case: unreliable communication
  • Worst case: permanent damage

I²C’s electrical design helps, but it doesn’t magically solve voltage differences.

What an I²C Level Translator Does

An I²C level shifter (or level translator) sits between the two voltage domains and does one simple but critical job:

It allows both sides to communicate without ever exposing either side to the other’s voltage.

Each side:

  • Sees valid LOW signals
  • Generates its own HIGH voltage locally
  • Remains electrically protected

This works because of how I²C signaling behaves.

TCA9517 Datasheet - Pinout

Why I²C Is Special: Open-Drain Signaling

Unlike push-pull interfaces (SPI, UART, GPIO), I²C uses open-drain outputs.

That means:

  • Devices never drive the line HIGH
  • Devices only pull the line LOW
  • HIGH is created by pull-up resistors

This single design choice is what makes safe, bidirectional level shifting possible.

Pull-Up Resistors: One Set per Side

These rails define what “HIGH” means on each side of the bus. The translator never forwards a voltage directly. It only mirrors LOW states.
Because no device drives I²C HIGH, pull-ups are mandatory.

A typical value for a pull-up resistor is 4.7 kΩ. Place one pair of pull-ups on the 3.3V side and another pair on the 5V side.

TCA9517 Schematic Example

How the Translation Actually Happens

Here is what is happening:

  • If either side pulls the line LOW
  • The level shifter detects that LOW
  • The LOW is mirrored onto the other side

What never happens:

  • A 5 V signal crossing into the 3.3 V domain
  • A driven HIGH being forwarded
TCA9517 - I2C Translation

Enable Pins and Decoupling

     Many level shifters include an enable pin. The TCA 9517 has an internal pull-up, so it can be left floating to be always active (often recommended).

     Each supply rail should also have a 0.1 µF ceramic capacitor placed close to the pin. These capacitors suppress noise, improve edge stability, and prevent weird,  intermittent I²C failures.

They matter more than people think.

TCA9517 Function Diagram

Companion YouTube video.