DC Boarduino gets low supply voltage from FTDI cable

For other supported Arduino products from Adafruit: Shields, accessories, etc.

Moderators: adafruit_support_bill, adafruit

Please be positive and constructive with your questions and comments.
Locked
User avatar
ubiyubix
 
Posts: 9
Joined: Thu Oct 27, 2011 2:42 pm

DC Boarduino gets low supply voltage from FTDI cable

Post by ubiyubix »

Hi,

I have a DC Boarduino which I usually power from an external 9V wall wart and flash using an AVR ISP mkII. The power selector is always set to "ext". When I disconnect the wall wart and connect an FTDI cable, the green power LED lights up very dimly and I measure 1.85V at the 5V pin. There is nothing apparently wrong with the Boarduino, i.e. the serial port and everything else works, but I wonder about the source of the low supply voltage and whether I should worry about it.

In order to find the source, I disconnected RTS#, RXD, and TXD of the FTDI connector pins one after another. When I disconnect TXD, the LED is off. Any explanation why this is would be very much appreciated.

-ubiyubix

User avatar
adafruit_support_bill
 
Posts: 88087
Joined: Sat Feb 07, 2009 10:11 am

Re: DC Boarduino gets low supply voltage from FTDI cable

Post by adafruit_support_bill »

The Boarduino doesn't have automatic power source switching like an UNO. You need to set the jumper when switching power sources: https://learn.adafruit.com/boarduino-ki ... ftdi-cable

User avatar
ubiyubix
 
Posts: 9
Joined: Thu Oct 27, 2011 2:42 pm

Re: DC Boarduino gets low supply voltage from FTDI cable

Post by ubiyubix »

Thank you for your reply. I understand the role of the power select jumper, but I do not actually want to use the USB power. I am simply curious to find out where the low voltage comes from when nothing but the FTDI cable is plugged in. Even after reading the schematic it was not clear to me why the LED lights up. It just did not make any sense to me, but I think I figured it out by now.

The LED lights up, because there is some (low) voltage on the reset pin (#1) of the microcontroller and the reset pin is connected to the +5V net via R1. There are no power sources, except the RTS#, RXD, and TXD lines from the FTDI cable. When I unplug the TXD, the LED is off. TXD is connected to the RX pin (#2) of the microcontroller. A voltage on pin #2 also appears on pin #1 with some voltage drop and this is what makes LED light up.

-ubiyubix

Locked
Please be positive and constructive with your questions and comments.

Return to “Other Arduino products from Adafruit”