Powering Nano from usb AND 9V wall wart. Usb only 4.7V

If nano is plugged to usb, can i supply it with 9V wallwart to Vin-pin? It uses its own regulator then like Uno does?

Reason for this is that usb only gives arduino about 4.69V. And im not sure does it cause this: http://www.youtube.com/watch?v=ACMMp1S_9NU So when serial reads 1-wire network. LCD goes dark and measurements go way off. Check the video to see it clearly.

So is it okay to supply 9V to Vin same time when usb is connected? And would it fix that or whats wrong with it?

Reason for this is that usb only gives arduino about 4.69V. And im not sure does it cause this:

The USB spec allows the voltage to range from 4.75V to 5.25V. The on-board polyfuse could be dropping the remaining 60mV, especially if you are drawing near the maximum 500mA.

I think its only taking <200mA.

But could i try to fix it by connecting 9V to Vin pin? It should regulate it to 5V and use external power?

There's a Schottky diode after the USB supply for auto selecting the power source. Probably drops a little of voltage.

Voltage select is different on the Nano 9V will make 5V http://arduino.cc/en/uploads/Main/ArduinoNano30Schematic.pdf

5V will keep the Schottky diode referred to by 48x24x48x from turning, keeping USB voltage out. http://www.fairchildsemi.com/ds/MB/MBR0520L.pdf

So you should be fine running board from 9V, and the USB drop 4.6-4.7 is expected.

Okay i've tried with external 9V. Noted that you have to plug usb first then 9V. If 9V goes in first, Debian says few errors and ttyUSB0 does not come up. Is it normal?

Anyway, I tried it but still same problem:

It all works fine but when you read sensors from serial(usb) the lcd goes off for about a sec. What could it be then? When laptop reads sensors it clears LCD and one result goes -127.00 and the other 85.00.

laptop gets randomly right measurement and randomly -127.00 and 85.00. Sometimes nothing. Wheres the problem?