AVR leaching power from serial line

eddy

Hello Everyone,

I searched through the site but didn't find anything on this topic. The AVR chips require very little power to operate and I have a project where the AVR remains running after the power switch is turned off. The serial port is connected to a USB to serial adapter and even though I don't have the USB 5V line attached, the AVR gets enough current from the serial line to stay on.

Is there an elegant way to keep the serial line from power the AVR? I can take a brute force approach and add a relay between the serial adapter and the AVR but it seems there should be another way.

Thanks!

Eddy Wright

http://www.wrighthobbies.com

nil_orally · Joined: 07 Jun 2010 Posts: 18

Hi Eddy,

a resistor is series with tx and rx lines will reduce the available current. Selecting the Brown Out Detection voltage to near rail voltage will mean when the voltage drops due to the removal of 5V, the uP will shut down.

Just a thought.

eddy · Posted: Wed Dec 18, 2013 1:35 pm Post subject:

I looked at the brownout detection settings but since that looks at voltage, I wasn't sure it would accomplish what I wanted. But using your suggestion to limit the current along with BD, it may work if limiting the current causes the 5v to sag.

I'll give it a try and post back the results.

Eddy

eddy · Posted: Thu Dec 19, 2013 4:40 am Post subject:

Turning on brownout detection did the trick. The voltage at the chip via the serial line was about 3.2v so setting BOD to 4.3 keeps the chip from staying awake when the power switch is turned off.

Thanks,
Eddy

eddy · Posted: Fri Jan 17, 2014 1:37 am Post subject:

Just a follow up on this issue with power. I've been running the circuit non-stop for a month now and the problems are gone. The combination of putting current limiters on the serial lines and enable brownout detection has kept the serial line from keeping the chip running with the power off.

Since the USB serial adapter I'm using is powered by USB, the problem may not exist if the serial adapter is powered by the circuit. All of my adapters are USB powered so I don't have a way of testing this.

Eddy