I would like to use your dCompass, a Lego ultrasonic sensor (maybe more) and a third party Lego i2c sensor on an arduino. I read in the Lego Hardware Developer kit that Lego recommends an 82k pullup resistor on the SCL and SDA lines. If each Lego sensor has an internal 82K pullup resistor, won’t that reduce the overall resistance of the pullups since they are in parallel? Is there a limit to how many lego-type sensors I can use on i2c because of these internal pullup resistors? Thanks.
Yep, the HDK recommends an 82k pullup and yes, adding more sensors to a bus can affect the total pullup value. Your best bet is to try, and see if the I2C is able to pullup or not ( you can use a logic analyzer for this).
If the resistors in series aren’t able to pullup, the good news is that with the DI board, you can alwasy disconnect the 82k resistors with a flick of the switch (on your custom sensor at least, you can’t disable the ones on the dCompass).