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Topic: Power bleeding over serial link from one Mega to second Mega (Read 476 times) previous topic - next topic

seguinc

Morning folks.

Here's an interesting situation: I have 2 Arduino Megas connected to each other over the Serial3 port and attempting to communicate data (yes, they have their individual grounds connected over the same comms cable).  Mega 2 receives signals to activate relays, Mega 1 (a Sainsmart TFT Screen package programmed to read an SD file containing activation sequences) sending signals over Serial link to Mega 2.  When I connect power to one of the Megas (whether through external power or the USB port), it feeds the other Mega whose power has not yet been connected.  It doesn't matter which one is connected to power, the one not connected will have it's power LED glowing brightly. 

In fact, when the relay-Mega is connected to power, the Sainsmart TFT-Mega's screen begins to glow (but not boot up), but only after a few seconds; as if the power filtering caps were slowly charging.

What's up with that???

tack

So, how many wires are connected between the two Megas?

seguinc


CrossRoads

Power is leaking thru diodes that provide IO protection on the 2nd device.
The data sheet clearly says IO signals are not to exceed Vcc by more than 0.5V.
You have 5V signals with Vcc at 0 - now you see what can happen.  Make sure to apply power to both devices!
Designing & building electrical circuits for over 25 years.  Screw Shield for Mega/Due/Uno,  Bobuino with ATMega1284P, & other '328P & '1284P creations & offerings at  my website.

seguinc

That makes sense!

Electronics Engineering 101.  This behavior is to be expected, not a defect, but a protective design.

Thanks for the info.

MarkT

Much simpler solution is to place a 4k7 resistor in line with the serial signal line at each Mega - this will protect against this
and other scenarios which could damage the Megas - it will limit bandwidth a bit but few people use serial at more than 115k baud
which should be fine.

Incidentally if you power a CMOS chip this way through one of its inputs you may damage/destroy the chip - it pays to be really
careful when designing circuitry to add protection to the signals coming in and going out to external boards...
[ I won't respond to messages, use the forum please ]

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