1

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
3

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.

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  • 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

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