I encountered a very strange i2c behaviour in my project.
I use an Adafruit Feather M0 (3.3V) as i2c master and 3 slaves (MPU6050 Gyro Sensor, DS3231 RTC, M24C01WP EEPROM) schematic below
The EEPROM works flawless, the Gyro returns 0 on all existing registers and the RTC only works in minutes and seconds. Date and Hours are always returned wrong.
Due to the Adafruit Feather M0 and the standard Arduino library, the I2C bus runs on 3.3V with 100kHz. I used 1kOhm Pullups.
All the Datasheets tell that the devices are compatible with 3.3v and 100kHz. Also the 3 devices have different i2c addresses.
I built the device twice, so i dont think that the gyro or rtc are broken.
Again the devices respond to requests. Only the values are wrong.
Edit: C1 is between 3V3 and Ground, NOT scl and data