This may be me but here is the situation I am communicating via the I2C bus to a Devantech SD21 using an Ardunio Mega 2560 If I use the Arduino on board 5 volt and ground everything works ok. If I try to use 5 volts to the SD21 and simply use the SDA/SCL from the Ardunio the sketch hangs, the 1st…

odd issue with Arduino Mega

CrossRoads October 31, 2012, 8:07pm 4

Yes. All grounds must connect together. The Gnd on the header is a convenient location.

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