Quote:
Originally Posted by SiriusHardware
Someone did once tell me you can power an Arduino by feeding power INTO the +5V pin on the socket rows although I would worry about that because it would be backfeeding power to the output of whatever normally provides the +5V supply.
Re: Your approach, let's say the Arduino is powered and providing strong output signals, some high, some low, to the logic inputs of the programmer hardware while the chips etc of the programmer itself are not yet powered. I could see that potentially causing problems.
You might need a kind of 'interlock' which enables your programmer board supply only when the Arduino is powered, and disables the programmer board supply whenever power is removed from the Arduino. That way you can never have a powered Programmer shield trying to back-power the unpowered Arduino, or a powered Arduino trying to drive the unpowered programmer hardware.
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I already have a diode to feed power to the Arduino when the sheild is powered, as the phantom powering through the pullup resistors on the sheild was causing high current flows and heating of the microcontroller. All power to the PROM is switched by requiring pins to be pulled low that are by default pulled high, and the high voltage can only be switched on if the 5v and 10.5v pins are pulled low simultaneously
Back powering the Arduino isn't a problem, because the Arduino has an op-amp that disconnects the USB power when 5v comes in through the power connector, and the regulators used (MC33269ST) shut off if back-powered. Some cheap clones use clones of AM1117-5 regulators that can object to this treatment, so in the case of those boards I'd recommend just keeping the USB plugged in before applying the 12v (I'll confess to having to replace 2 regulators with "reputaable" components before working this out). Or buy a better Uno!