Position: Index > Unclassified >

I2C bi-directional level shifter

2017-12-22 13:41  
Declaration:We aim to transmit more information by carrying articles . We will delete it soon, if we are involved in the problems of article content ,copyright or other problems.

 

I2C bi-directional level shifter


Introduction

This page is about a bi-directional level shifter for the I2C-bus: what it is; why you need it; and when you can do without.

A level shifter shifts the voltages of the bus signals so that devices of different voltages can communicate over the bus.

This page is to collect all information (for example from the forums), and you are invited to add your own solution.


Connecting 3.3V I2C components directly to the Arduino.

A 3.3V I2C component can be connected directly to the Arduino, by using the 3.3V of the Arduino board, and using two 4k7 pull-up resistors to the 3.3V.

Almost every AVR chip (the microprocessor) on the Arduino Board runs at 5 Volts, and almost all Arduino's have an onboard voltage regulator for 3.3 Volt. This 3.3V can be use for components that require 3.3V.

BSS138

More 3.3V I2C components can be added to this bus. Many I2C devices operate with a range of voltages. It is very well possible that all your I2C components can be used with 3.3V.

The 3.3V is recognized by the Arduino as high. Since the I2C-bus is rather slow, the 4k7 pull-up resistors will pull the signal fast enough to 3.3V.

Even 5V I2C components can be connected to the SDA and SCL lines of the 3.3V I2C-bus. A high level will be 3.3V and not 5V, but in most cases it's enough to make it work.

Internal pull-up resistors activated by Wire libary.

The AVR microcontroller in the Arduino uses a TWI interface, which is I2C compatible. The SDA and SCL lines can be programmed with internal pull-up resistors. The Wire library of Arduino 1.0 (in 2012) enables those internal pull-up resistors with the function Wire.begin(). Those internal pull-up resistors are connected to +5V which could violate the specifications of the I2C component.

If two pull-up resistors of 4k7 to 3.3V are used, the voltage is only raised a little by the internal pull-up resistors. This is a controversial subject with many opinions, but no one has ever noticed that a chip has been damaged this way (but you do need those two 4k7 resistors).

Outside specs, (usually) works anyway

Connecting the 5V Arduino directly to a single 3.3V-powered I2C chip usually works, even though it violates official specifications in multiple ways. In practice, Arduino's internal pullups are so weak that ESD protection diodes inside the 3.3V chip limit the voltage.

(if any exceptions are discovered, where a chip is damaged or just doesn't work properly, please edit this page)

Of course, following the official I2C specification (stronger pullup resistors) and avoiding higher voltage signals at any 3.3V chip is best. Any "serious" project should follow all technical specifications.


Incompatible voltages

I2C components that use 1.8V or 2.8V can not connect directly to the Arduino. In that case a level shifter is used. Also for 3.3V I2C components is sometimes a level shifter used.


Bi-directional level shifter.

A level shifter can be used to do a voltage shift of the bi-directional signals of the I2C-bus (SDA and SCL).

This creates two groups on the I2C-bus, one group with the 5V and one group with a lower voltage. Once the level shifter is implemented, any combination of 3.3V and 5V components can be made and all within the I2C specifications.

Below is a list of possibilities:

Use two mosfets.

This is the most used one.

The Philips Applications Note AN97055 explaines it.

BSS138

The mosfets must be types that activate with a very low gate voltage. For example mosfet BSS138 or TN2501.

BSS138

The resistors are not a part of the level shifter. They are the pull-up resistors required by the I2C bus. In this case they have to be on both sides of the level shifter.

The gates of the mosfets becomes active if either one of the signals is pulled down. The internal diode is used to make the gate active if the drain of the mosfet is pulled down. The gate is always connected to the lower voltage.

A device that pulls a line down, has to handle the current through both resistors (on the 3.3V side and on the 5V side). That's why a higher value for the pull-up resistors is choosen, mostly 10k.

Using a module.

Many common sensors are available as a module with an onboard level shifter. Sometimes those modules are specifically for the Arduino, others are more common modules for any microprocessor.

Using a level shifter IC.

There are common IC's for level shifting: 74HC4050

There are also special IC's for I2C level shifting: PCA9512

Using 4 transistors

BSS138

The outer most resistors are pull-up resistors for the I2C bus.

A device that pulls a line down, has to pull down the current through both resistors (on the 3.3V side, and on the 5V side) and also the current through the base of the transistor. Typical values for the resistors 10k for all of them.

Warning: There are some disadvantages with these 4 transistors. If a line is pulled low, a current from the base to the emittor will turn that transistor on. But the other transistor will leak current from the base to the collector. That current will reduce the current in the first transistor. I have build this circuit and it is doing well, but it doesn't meet the fullI2C specifications when the pull-up resistors are less then 4k7.