fungus:
majenko:
fungus:
Don't use digital pins as grounds. Sending current through a transistor is NOT the same as connecting it to ground (it's more like sending it through a resistor).You should qualify that with "Sending current through a MOSFET transistor...", as a BJT is more like a diode than a resistor.
I wrote that because when you're using it as a switch a fully open BJT is more like a resistor then a fully open MOSFET is (ie. the BJT will have a much bigger voltage drop across it than the MOSFET).
PS: I'm assuming they're BJTs inside the chip...
"Never assume anything as it makes an Ass out if U and Me" 
They're CMOS chips, so they use MOSFETs. It's rare for BJTs to be in logic / MCU chips these days.
A MOSFET is like a variable resistor, with the resistance being set by the gate. A BJT is a fixed voltage drop with little or no resistance - just like a diode.