I can't find a full Pi4 schematic but I gather some previous Pis have had pullups on these two lines whilst others haven't, along with much discussion on whether hats should have a pullup or not.

Does anyone have a full Pi4 schematic (not the simplified one on the website) or a definitive answer ?

I have tried with a multimeter but as the Pi has so many switch mode power supplies I'm not sure the lack of a reading definitely proves the resistors aren't there.

  • Well, a couple of years ago, when I was a Rpi1 newbie, I used a mult-imeter to measure the pull up resistance between 3V3 and I2C SDA/SCL, and found them about 1k7. Now I am a Rpi4 newbie, I did the same thing, and found the pull up resistances 1k7 and 1k8. These two days I am programming the I2C PCA9685 PWM controller with Rpi4B and found everything OK as Rpi3B+. So I concluded that Rpi4B has the same I2C pullpups as Rpi3B+. – tlfong01 Aug 13 '19 at 14:06

I²C requires pullup to work. The unreferenced discussion seems to be misinformed.

All Pi models have on-board 1.8kΩ pullups on I²C pins 3,5.

I have not used I²C on a Pi4, but assume it is unchanged.

You can verify for yourself that pullups are present - just connect a 1.8kΩ resistor from pin 3 to Gnd and the voltage should be 3.3/2V i.e. 1.65V

NOTE measuring anything on an electronic circuit with a multimeter on a resistance range is potentially dangerous and invariably useless.

| improve this answer | |
  • Correct. My Pi4B has pull-ups on GPIO 2/3. They are stronger than the internal pulls. If I set the internal pull-downs on 2/3/4 then GPIO 2/3 still reads high but GPIO 4 reads low. – joan Aug 13 '19 at 12:53

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.