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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 time it attempts to end the transmission.

It acts as if the SD21 is not powered by the external power, I did verify that power does exist at the appropriate pins on the SD21.

Am I missing something obvious ?  Have noticed this problem with other sketches as well that use the servo library to fire up an R/C ESC.

Any troubleshooting suggestions
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When you do this
"If I try to use 5 volts to the SD21 and simply use the SDA/SCL from the Ardunio the sketch hangs, the 1st time it attempts to end the transmission. "
are you connecting the 2 grounds together?
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Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

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Hi,

Thanks for the very quick reply

No actually I am not, to connect the grounds, I am assuming the ground on the SD21 will go to ground on the Arduino, does it matter which one ?  I am assuming you would go to the ground beside the 5 volt pin ?

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Yes. All grounds must connect together. The Gnd on the header is a convenient location.
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Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

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Thanks,

That resolved the problem.  I appreciate the assistance.
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Glad to help.
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Designing & building electrical circuits for over 25 years. Check out the ATMega1284P based Bobuino and other '328P & '1284P creations & offerings at  www.crossroadsfencing.com/BobuinoRev17.
Arduino for Teens available at Amazon.com.

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