I2C pull-up resistors on modules and breakout boards

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The maintained copy of this page is here:
http://www.prolifictec.com/blog/2015/10/i2c-pull-up-resistors-on-modules-and-breakout-boards.html .

- Nick

 

Some of the modules and breakout boards for I²C slave devices contain 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-up that's 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 means the same as strong, when describing pull-up resistors.  This terminology is 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 +5V.  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 are connected in parallel.

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 the possible parallel pull-up scenario.  Many of the newer breakout boards have been designed with that in mind.

• Perhaps to partially address this, some of the breakout boards have I²C pull-ups which are half strength.  They are weaker than the correct 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 combined parallel pull-up becomes too strong.

• On some of the 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 SparkFun breakout board.)

• Some breakout boards have unpopulated pads for pull-ups.  (For example, see R1 and R2 on this board.)

Related

I²C bus specification and user manual.  Anyone working with I²C should at least skim through it.
How to deal with multiple pull-up resistors on modules?  That forum question was a point of departure for writing this post.
A case when ATmega's internal pull-ups were used for I²C.  The oscilloscope screenshots show I²C waveforms with internal pull-ups, and with proper external pull-up resistors.
Forum thread Is there a correct resistance value for I2C pull-up resistors? and application note I²C Bus Pullup Resistor Calculation.
EDN article Design calculations for robust I²C communications.  Concise article with diagrams showing I²C physical layer principles of operation.
Effects of varying I²C pull-up resistors.  The article shows oscilloscope screenshots of good and bad I²C signals.
Pull-up resistor and pull-down resistor explained