Pull-Up Resistors I2C [DS1307 RTC , HTU21D Humidity Sensor + Slave UNO]

Projects Programming
1

I'm using a MEGA as Master and I want to use these 3 devices mentioned in the subject! The sensors have boot pull-up resistors assembled.. Is it necessary to add another external resistor to them, or do I have to do it only for the Uno Slave? When reading the description of the Humidity Sensor HTU21D Humidity Sensor Hookup Guide - SparkFun Learn

I saw that "If you’ve got other devices on the I2C bus you may need to disable the on-board resistors." Why?

2

Because all the resistors are in parallel. Too many, and the resistance might be too low.

3

Thanks! And how can I dimension the pull up resistances?

4

Read the written documentation or schematics. If you don't have those, use a multimeter to measure it. Connect everything (but with no power) and measure between VCC and SDA, VCC and SCL. The combined resistance should not be less than about 4.7k.