3v3 output from FTDI chip on Arduino Nano 3

Using Arduino General Electronics
1

Hi,
I have to use an Arduino Nano 3 for a project and wish to use the 3v3 output of the FTDI chip for reference to the ADC. However, it is said in the Nano 3 datasheet that the 3v3 output of the FTDI chip is not avialable when the board is powered via external power.

"The FTDI FT232RL chip on the Nano is only powered if the board is being powered over USB.
As a result, when running on external (non-USB) power, the 3.3V output (which is supplied by
the FTDI chip) is not available and the RX and TX LEDs will flicker if digital pins 0 or 1 are
high."

But when I look into the schematic (http://arduino.cc/en/uploads/Main/ArduinoNano30Schematic.pdf) of the board it can be seen that the FTDI is powered via the 5V supply only. I want to know why is the 3v3 output not available while on external power, because as per the schematic, it should be available, shouldn't it?

Thank you,
Makarand

2

Ten seconds with a multimeter will tell you the truth.

3

I am sorry, but our schematic needs to be designed before procurement of parts. If the output is not available, we need to make arrangements for an external ADC reference.

Thank you for your quick reply.

4

makarandsd:
I am sorry, but our schematic needs to be designed before procurement of parts.

Usually there's a 'prototype' stage...

makarandsd:
If the output is not available, we need to make arrangements for an external ADC reference.

All AVR chips have an internal ADC reference.

5

I agree, the Nano 3.0 schematic shows the FTDI chip being powered from 5V.
5V may come from USB power, from Vin regulated to 5V, or from 5V directly supplied via header pin.
Note that the FTDI chip is only spec'ed to supply 50mA of current at 3.3V.

6

But if the FTDI was being powered from USB, the net should be shown as VUSB, yes?

Because according to the current schematic, 3v3 should be available even if the board is not being powered via USB.

I do not have the board currently. I have put the query only because we will have to add a 3v3 zener for the reference, if the 3v3 output is truly not available. We finalise schematic before procurement of parts.

Thanks for your reply.

7

If the schematic is accurate, then 3.3V will be available even when not powered via USB.

8

VUSB goes thru a diode, MBR0520, to connect to 5V. The diode prevents Vin or externally supplied 5V from being driven back into the PC.
http://www.digikey.com/product-detail/en/MBR0520LT3G/MBR0520LT3GOSCT-ND/2705022
This will have up to 0.385V drop across it.
The FT232R needs 4V to use the internal oscillator, so 5V - 0.385 +/-10% USB variation = 4.15/5.07 should be sufficient for the chip to put out 3.3V.
http://www.digikey.com/product-detail/en/FT232RL-REEL/768-1007-1-ND/1836402

9

That helped. Thanks a lot all for your prompt replies.

10

Glad to help. Make sure you put a 100nF cap on the 3.3V line to help keep it clean.

11

Surely!