I am using an Arduino Uno R3.
It is not clear on the schematic whether there are pullups on the A4 and A5 I2C pins.
Are there hardwired pullups or do I have to add them myself?
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There are very weak internal pullups, but they are not strong enough to be considered compliant with I2C specs. Supply your own external 2.2k-10kohm pullup resistors.