I read somewhere that the total current an Atmega328 can provide is 50mA across all its pins.
Is this true? If yes I will have to reconsider my project which uses a ton of pins (all of them) without any consideration for total current draw!
No.
Hi,
Check max specs in data sheet.
Pg 299
Tom.... 
Absolute max for supply and ground pins is 0.2A, I would suggest keeping to below 150mA
for a stress-free life, so 10mA on 15 pins, or 20mA if half the pins source and half sink.
You'd want to keep the max per pin safely below the 40mA abs-max limit anyway, which
I take to be about 25mA as a reasonable maximum source or sink per pin.
What sort of loads are we talking, and can you arrange for some to be sourced and
some sunk?
The total stress rating for the Vcc pin is 200mA so the total sourced current must be below this. In addition there are limits on what each individual 8 bit port should be made to source, see the data sheet for the breakdown.
Thanks all for the info. Many pins are input pins and only one has high power requirements (an ebay buzzer at over 20mA).
Over 20mA isn't helpful, what's the maximum current?
40 is Absolute Max. Voltage degrades above 20mA, from 4.2V downward.
If you want more, add a transistor to sink current thru the part.
Low Rds Logic Level N-channel MOSFET would be the best, can act like a .01ohm resistance switch.
If I remember correctly the source current is 50ma but with the pullup resistors enabled.
stx38834:
If I remember correctly the source current is 50ma but with the pullup resistors enabled.
Not according to the data sheet.