3v3 input to BX?


#1

Looking at the schematic, it seems that I can power the BX with a 3v3 power supply using the 3v3 pin (since the power rails to the rest of my circuit are 3v3 also). If I leave the Vin pin disconnected, that shouldn’t blow up the 3v3 regulator, right?


#2

Should work just fine. Just be careful if you power the 3.3v rail externally and also want to connect a USB cable.


#3

What precautions should you take in that instance? What’s the best way to externally power the device and still be able to have USB comms to it?


#4

That’s a good question. In my application I’d like to have the USB connected for data, but power the circuit using an external 3v3 supply. I definitely don’t want the circuit powered by USB.

I think I’m just going to leave the Tiny’s 3v3 pins unconnected. It means that I must have USB connected for the board to work, but that’s not a problem for me.


#5

The question was more for Luke - what’s an appropriate way to power the FPGA externally while connected to a USB port? Some systems use a diode to prevent contention when USB and a Vin are both used, but I don’t see that on the BX schematic.


#6

I’ve been thinking about this. I think the simplest solution can be found here:

https://www.pjrc.com/teensy/external_power.html

I would recommend option #2: modify a USB cable by cutting the red wire. This will modify a relatively cheap USB cable vs the more expensive TinyFPGA BX.

Option #1 on the BX would be removing the 3.3v regulator. I wouldn’t recommend it unless you have a hot air tool.