I2C pull-up resistors on modules and breakout boards

Some modules and breakout boards for I²C slave devices come with I²C pull-up resistors. When multiple such modules are connected to the same I²C bus, their pull-up resistors appear in parallel. That results in a stronger effective pull-up resistance, which may prevent communication on the I²C bus.

You have to have pull-ups on the I²C bus.

Somewhere on the I²C bus you have to have pull-ups in order to be able to communicate on the I²C bus. The pull-ups may reside on the microcontroller board, or on the I²C slave device's breakout board, or anywhere else on the I²C bus.

Some microcontrollers have internal pull-ups. It's not recommended to use the microcontroller's internal pull-ups for I²C. Still, internal pull-ups is much better than no pull-ups at all.

Watch out though, you may end up with pull-ups that are too strong.

If pull-up resistors are too strong, that can prevent communication on the I²C bus.

Strong pull-up means that the current through the resistor is relatively large. Stronger pull-up translates into smaller resistor value. Weak pull-up translates into a larger resistor value. Stiff pull-up is same as strong pull-up. These terms are not limited to I²C. It applies to pull-up and pull-down resistors in other types of circuits too.

Suppose all three boards in this diagram came with pull-up resistors installed. The 2.2 kΩ value is common for an I²C bus running at +5 V. Notice however that R2, R4, R6 all are connected between SCL and Vdd. They are in parallel, and they act as one 733 Ω resistor. This combined pull-up is too strong I²C. That can prevent communication on the I²C bus. (Similar for the pull-up resistors on the SDA line.)

If these resistors can spoil the I²C bus, then why are they designed into the breakout boards in the first place? I'm guessing that it must have been a compromise to create an easy out-of-the-box experience. A lot of I²C first-timers don't know that pull-ups are required. Some Arduino boards don't have I²C pull-up resistors nor pads for them (Arduino Uno, Arduino Pro, for example). Some Arduino boards have unpopulated pads for I²C pull-up resistors (Arduino Mini Pro, for example). A breakout board provides the pull-ups, and "it just works". That is, until too many breakout boards — each with pull-up resistors — are connected to the same I2C bus.

Remedies

Of course, as soon as you realize that there are extra pull-up resistors you can simply remove them.

Some breakout board suppliers are aware of a possible parallel pull-up scenario. Some breakout boards were designed with provisions to address this.

• Some breakout boards have I²C pull-ups which are half strength. They are weaker than the optimal values for the I²C, but stronger than the internal pull-ups of a μC. A typical such value is 4.7 kΩ . This is a compromise. The number of modules that can be connected in parallel without removing resistors is still limited. But a greater number of modules may be connected before the combined parallel pull-up becomes too strong.
• On some breakout boards, pull-up resistors are connected to Vcc through solder blobs. A solder blob is easy to remove, then pull-up resistors will not affect the I²C bus. (For example, see SJ1 in this breakout board.)
• Some breakout boards have unpopulated pads for pull-up resistors. (For example, R1 and R2 on this board.)

Related

I²C bus specification and user manual. Anyone working with I²C should at least skim through it.

Forum thread How to deal with multiple pull-up resistors on modules? was a point of departure for writing this post.

Design calculations for robust I²C communications. Concise EDN article with diagrams.

Forum thread Is there a correct resistance value for I2C pull-up resistors?,
and application note I²C Bus Pullup Resistor Calculation by Texas Instruments (slva689).

Forum thread What happens if I omit the pullup resistors on I²C lines? The oscilloscope screenshots in the post show I²C waveforms with internal pull-ups, and with correcly sized external pull-up resistors.