Converting a USB BUB II to use a 3.3V regulator

Anyone who works with Arduino for any length of time will become familiar with the FTDI USB to Serial bridges. These convert the USB interface found on all modern computers into a simple serial interface, required by many Arduino boards for their bootloader.

There are many FTDI cables and boards available. A lot of Arduino boards have a built-in FTDI chip (the older ones with a USB port). Many vendors sell FTDI cables which have USB on one end and a pin header on the other (the chip is embedded in the USB connector). There are also a lot of breakout boards available.

There are two important voltage specifications on each cable:

  • Signalling voltage – they are generally available in 5V or 3.3V signalling. It is possible to damage some 3.3V boards using 5V signalling. With the ATmega microcontrollers, you should never exceed 3.8V on any pin with Vcc at 3.3V, but it doesn’t seem to cause damage. Conversely, a ATmega running with Vcc at 5V will pick up 3.3V as logic high with no issues. If in doubt, go with the 3.3V cable.
  • Power supply voltage (Vcc) – the normal FTDI pin out provides a power supply on one of the pins. Generally you get either 5V or 3.3V. The 5V versions often supply VUSB direct – so you can draw 500mA. Most of the 3.3V ones however use the pin 3V3OUT on the FTDI chip, which cannot supply more than 50mA (and the FTDI chip gets very hot when doing so!). This often isn’t enough to power a project. Again, a lot of boards don’t really care, but some ancillaries like radio chips will die if given 5V instead of 3.3V. The ATmega is a very forgiving chip.

There’s one big thing to catch you out here. A lot of the 3.3V cables still provide 5V on the Vcc pin. I didn’t realise this until I frazzled a CC1110 board with 5V.

So there are a lot of combinations of voltages available. Rather than have a four or more FTDI cables handy, I’ve found one particular board is versatile enough to use in all situations, with a bit of modification.

The USB BUB II is available from Jeelabs and Modern Device. It’s a small board, and by default it has 3.3V signalling with a 5V power supply. With a few small changes, it can be a lot more versatile.

It has a some good points that many other solutions don’t have. A small polyfuse protects your computer from over-current (although most machines seem fairly tolerant of this nowadays). It also has RX/TX lights, which are absent on many DIY and smaller boards.

Onto the modifications.

1. There are two pairs of small solder bridges on the front of the board, one labelled VOUT and the other LGLV. Use solder braid to remove solder from whichever combination is currently soldered.

2. Solder a pair of 3-pin headers into the space next to VOUT and LGLV. This allows you to chose between 5V from USB or 3.3V from 3v3OUT using jumpers.

3. Turn the board over and cut the 3v3_OUT trace using a scalpel. Bridge the VR_OUT pads using solder.

photo 2

4. Finally, solder on a 3.3V regulator on the back of the board. Modern Devices suggest a MCP1703 which can provide 250mA. I used a MCP1825 which can provide 500mA as it was what I had lying around.

Now you have a FTDI board which will work for all of your 5V and 3.3V boards, as well as being able to provide power for most small projects.

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