Digital pin voltage drops under load

In poking around a led circuit I was surprised to find that the voltage of an output pin was only 4.4 volts. This was driving a led and resistor - about 15ma being drawn.

Does that seem right? I have to say I was surprised but I've never had occasion to measure this before.

What is the Vcc at the ATmega pins?
If you are powering from a USB port you could be down around 4.5V.

Also the minimum Voh is 4.2V for Vcc=5V and Ioh=-20mA so a value
of 4.4V does not seem unreasonable.

(* jcl *)


www: http://www.wiblocks.com
twitter: http://twitter.com/wiblocks
blog: http://luciani.org

What is the Vcc at the ATmega pins?
If you are powering from a USB port you could be down around 4.5V.

it is a USB port as it happens but VCC measures 5.1v.

Also the minimum Voh is 4.2V for Vcc=5V and Ioh=-20mA so a value of 4.4V does not seem unreasonable.

There you go, I just didn't know that. I would have guessed it would stay close to 5v until it got quite near the 40ma limit.

Thanks a lot.