Can't Get Past setup() When Combining I2C Sensor Chain

Projects Programming
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There are a few more problems. If it is not reliable, you might want to read these notes:

  1. The Sparkfun Openlog is a ATmega328P running at 16MHz at 3.3V. It works, but not according to the datasheet.
  2. The Adafruit STEMMA QT is the same connector as the Sparkfun Qwiic. However, the Qwiic is only 3.3V and the STEMMA QT can be both (more or less). The voltage regulator of the Adafruit module will lower the voltage by about 250mV.
  3. Both the STEMMA QT and Qwiic are wrong. See page 54 of the standard I2C document UM10204.

[ADDED] I also noticed that the Sparkfun OpenLog has AREF connected to VCC. That is a fault. It causes a internal shortcut when the internal voltage reference is selected. The board is missing a diode from /RESET to VCC, that might result in a corrupted bootloader when the /RESET pins enters the High Voltage mode by a pulse at DTR.