Using 5V ftdi breakout on custom 3V board

Using Arduino Microcontrollers
1

Hi. I have a FTDI Basic Breakout - 5V from Sparkfun. I am now making a 3.3V 8MHz custom setup, with an atmega328 and some peripherals that are not tolerant of 5V. I want to still use the 5V breakout so I don't have to buy a new 3.3V version.

Right now I have the VCC line from the FTDI connector connected to the MCU VCC and the VCC of my peripherals. So when I program the MCU currently, I disconnect the 3.3V peripherals, connect the FTDI breakout, program, disconnect the FTDI and reconnect the peripherals. That works now because it's on a breadboard, but it won't work on a PCB. Plus it's annoying.

There is a 3.3V regulator onboard, which is used to even out the voltage from the battery.

So here's what I wonder...

  • Can I safely route the 5V from the FTDI connector into the VIN on the voltage regulator?
  • Can I safely disconnect the 5V from the FTDI entirely? The battery will provide power during flashing.
2

Another option might be a USB-to-3.3V LVTTL Serial cable. I hear that some cellphones use such a cable and can be had for very little money.

I don't know if the 5V TTL Serial signals will be safe for your 3.3v peripherals.

3

Yes fortunately the signals are ok. Peripherals tolerate 5v inputs. It's just supply voltage that is 3.3 or less.

4

The FTDI chip as an 3.3v power supply, but it might not be enought to supply an entire micro-controller, you can also connect VccIO to 3.3 and the Rx/tX/DTR and all the other signals in the FTDI will be 3.3 ones instead of 5v, just read the datasheet.

5

FTDI chip only supplies 50mA.

6

I’ve standardized on using a 5V FTDI board for all custom designs to keep it simple and avoid mistakes. When I need to reprogram a controller, the same FTDI board will work for any of my boards irrespective of being a 3V3 or 5V design.

On bootloaded designs I incorporate a FTDI connector. On 3V3 boards there is a voltage divider (3k3/1k8 to ground) for the mcu serial rx line. The mcu serial tx line is directly connected as is the reset signal from the FTDI breakout. Reset is ok as it is high voltage tolerant and the 3V3 from the mcu tx line back to FTDI is seen as logic high with no level shift required.

In terms of power, I have 5V on the FTDI power output as this can supply up to 500mA. This 5V is either used by the custom board or just ignored. On 3V3 designs it will be connected to the onboard regulator input and on 5V designs it is used directly as regulated output. On some boards I add a jumper that will allow me to connect 5V from FTDI or just leave it powered with whatever is normally used (you don't want to connect two independent supplies to the same rail).

This way I can program all bootloaded boards with a single FDTI without worrying about level conversion and having the flexibility to supply power if needed/desired.

7

BenF:
I’ve standardized on using a 5V FTDI board for all custom designs to keep it simple and avoid mistakes. When I need to reprogram a controller, the same FTDI board will work for any of my boards irrespective of being a 3V3 or 5V design.

Awesome, Ben, thanks for all the great info. Looks like I will do the same.

On 3V3 designs it will be connected to the onboard regulator input

Ok, cool, this is what I did, and it's working great. This seemed like the right way, but I'm still finding my way on the electronics part of things.

you don't want to connect two independent supplies to the same rail)

Good to know, I was not aware of this.

So how is this working on a stock Uno? Let's say you have it plugged into the wall, so the 5V output of the regulator is on the same line as the 5V coming from the USB cable, right?