I am working with a project where the I2C pull-up resistors need to altered. Unfortunately i cannot find any documentation that can tell me: (a) if there is any pull-up resistors and (b) what size they may be.

I hope someone can shine some light on this matter.

2 Answers 2


The resistors are 1k8 pull-ups to 3V3.

At a guess they are the tiny devices (smd) marked R24/R23 next to J8 pins 3/5.

I'm not sure if detailed schematics have been published yet.

See the B+ schematics at Link

The above applies to the I2C bus connected to pins 3/5. There is another I2C bus connected to gpios 0/1 (J8 pins 27/28) which is not intended for general use. This bus does not have hard wired pull-ups.

You can check for the presence of hard wired pull-ups by setting the gpio as an input and then setting the internal (50k) pull-down. If a hard wired pull up is present the gpio will read 1 even if the (far weaker) internal pull down is enabled.

  • When i get back to the lab i will take a look at those two and get back to you with an answer.
    – JavaCake
    Mar 26, 2015 at 13:10

I don't know why you think the "I2C pull-up resistors need to altered". It would not be advisable to use values much higher than the on-board 1.8k (they will draw ~1.5mA). You could use lower values, if necessary, but don't even worry where they are on the board (they are quite visible near the pins). Just use external resistors in parallel. Unless you have sophisticated soldering equipment you won't be able to replace them.

  • 2
    wouldn't it be nicer to ask "why do you need to modify the I2C pull-up resistors?"
    – benathon
    Mar 27, 2015 at 2:47

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