Arduino NG - EXTernal Power vs USB Power

I have two Arduino Boards, one a Sparkfun bought Arduino NG (USB) and the other one of Paul Badger’s BB_Arduino boards. On the BB_A I have a demo sketch up and running using the new libraries I adapted from SoftwareSerial, transmit only with added routines to specifically drive a Serial LCD run by an Anderson chip.

On the BB_A I set it up “both ways”, that is the whole shebang (Arduino and serial LCD) powered from the FTDI USB cable and also the whole thing powered from extenally supplied +5 volts … it all works just fine.

Then I got an email (see bottom of this message) from one of my K107 (the serial controller LCD board I designed and sell) That he can get my demo program running just fine on his Sparkfun Arduino NG under USB power but not EXTernal power.

I also have an Arduino NG from Sparkfun … here is what I did and what I observed

(1) Power jumper set to USB, the serial LCD display board connected to #8 and GND on the top row and +5 on the lower row. USB cable plugged in, no external power plugged in - loaded program for the first time, works fine

(2) Changed power jumper to EXT, plugged in 9 volts external power, left USB plugged in, hit reset, after bootloader timeout, program came back up and it works.

(3) Then removed USB while it is running, it continued to work

(4) Hit reset, program does NOT come back up …

(5) Remove power connector and replace (cold restart) … program does not come back up.

I then loaded the BlinkingLED program and repeated the process with identical results …

What am I missing here? Is there a bug or what are we doing wrong? Thanks

cheers … 73 de brian riley, n1bq , underhill center, vermont Tech Blog
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-----Original Message-----
From: Brian Riley []
Sent: Monday, July 16, 2007 2:27 PM
To: Vu, Joe
Subject: Re: Bug in Arduino Software Serial Library

What Arduino board do you have?

On Jul 16, 2007, at 2:17 PM, Vu, Joe wrote:

I have a Sparkfun Arduino NG. Do you know if there is a bug in the hardware ? Your prog runs ok on USB power, but not with ext
power !

Tried on two units.

Did you ever see this ?




this has been answered many many many times

please search the forum


this has been answered many many many times

please search the forum

OK, this seems to be the same problem they have with PICAXE if Sin is left floating the bootloader thinks something's coming. I threw a 22K resistor from RX to ground and the problem went away. I have used as high as 47K on a PICAXE to achieve the same thing.

I have seen one answer that suggests putting a 10K resistor in the line from RX to the USB ... I don't see how that helps. I see 10K bridging TX-RX and pull up to +5 or pulldown to ground, but not the series 10K. I noticed Paul Badger put the 10K in series in his Rev C of his BB_Arduino ... how does that work?

cheers ... BBR

Putting a 10k to 100k between rx and ground fixes the problem placing it between rx and tx is an hack to avoid long wires going from rx to ground

in the new arduino boards, the bootloader has an internal software enabled pullup resistor. Moreover, a very clever user, realised that having only 1k between tx/rx and the usb chip is somehow enough to feed power back to the USB chip keeping it active thus generating trouble.

The new boards have a 10k between the processor and the usb chip further eliminating the problem.

mellis has also been working on a more clever timeout algorithm for the bootloader... We should be pretty safe in the near future.

I can't comment on other boards because I don't know their schematic