Ftdi programming 3.3v

Dear all,
I'm trying to improve my electronic knowledge for several months ...
I just built my first my self dev board in order to interface a pellet stove including a atmega328p, a Lora transceiver and a ftdi usb interface. All based on 3.3v voltage.
All seems to works well except usb programming!

Actually, I already bought a ft232 board and I never achieved a usb programming.
So I investigate more with the ft232, atmega328p and a breadboard.

I finally performed a programming in this cases:
A 5v supply with an external oscillator ( 8 mhz or 16 mhz) and a RC circuit between DTR from ftdi and atmega's reset (10K and 0.1uF)

But it didn't work when I switch to 3.3v or when I used internal oscillator (with proper bootloader burning).

Do you have some ideas why I can't program with 3.3v?

I looked on reset time (with the very good OLS logicsniffer project) which is different linked to voltage: around 0.6ms for 5v and 1.2ms for 3.3v so I put 2x0.1uF in series to be similar but no way, it still doesn’t work ...
I have no more idea now ...
I hope someone can help me.

Thank's in advance, and sorry for my bad english...


The ATmega328P is not rated to run at 16 MHz at 3.3 V but it does usually work and you say you also tried at 8 MHz so that doesn't seem to be the cause.

The FT232 board will only be able to provide 50 mA when you switch it to 3.3 V because then you're using the FT232 to regulate the 5 V from the USB down to 3.3 V. It can provide 500 mA when you have it set to 5 V.

Thank's for your answer,
Yes you’re right, I’m trying to program at 3.3V with 8Mhz external oscillator.
Your remark is good so this morning I tried to supply the proc and breadboard with an external 3.3V supplier. But it’s still the same : it’s doesn’t work.

To summarize, I burned bootloader on atmega as Arduino pro mini 3.3V 8Mhz in arduino IDE with a Raspberry so with 3.3V voltage via SPI. Now when I tried to upload a sketch with my FT232RL on 3.3V jumper it doesn’t work, and when I switch to 5V jumper it works ….

I never had an Arduino pro mini 3.3V in my hands, but I assume that it can be program directly by FTDI. My breadboard circuit is a clone of this board with the same Reset circuit so I really don’t understand why I can’t do it

What are your fuses set at? Could be the Brown Out Detection voltage is set for a higher voltage, so with 5V it works, and at 3.3V it is in BOD reset.
Bits 2-1-0 of the Extended Fuse byte (upper 5 bits are not used)
fuse byte 0x07 or 0xFF, 111 - disabled
fuse byte 0x06 or 0xFE, 110 - 1.7 to 2.0, 1.8 typical
fuse byte 0x05 or 0xFD, 101 - 2.5 to 2.9, 2.7 typical
fuse byte 0x04 or 0xFC, 100 - 4.1 to 4.5, 4.3 typical << Problem for 3.3V power device

Have you confirmed that your FTDI adapter actually works at 3.3v setting, outside of this context? Is it a real FTDI-based one? I would suggest doing some analysis on what's different at 3.3v - stuff like what voltages are on the relevant pins, is the oscillator oscillating when it's plugged in, etc). Does the LED blink to indicate the bootloader is running?

I've seen some adapters that have a jumper (physical or cut+jump pads) for 3.3v mode that actually don't work in that mode. I use only CH340G serial adapters in my projects, so I can't comment on whether this is a problem with real FTDI ones - but it certainly is with some cheapo CH340G ones. I haven't been able to figure out what they did wrong - it's sort of pathetic, since my own original CH340G based board design worked beautifully first try, and it was like the fourth PCB I designed; it's not rocket surgery.

I now use these for cheapo commercial ones (no specific endorsement of the vendor - but you want the design with the little switch; the ones with the jumper for voltage don't break out DTR, and the red ones that do break out DTR don't work at 3.3v, or at least the last batch I bought 20 of didn't work): http://www.ebay.com/itm191146797242

I also sell my own (for the time being - nobody's buying them - probably because of the price (I can't compete with china assembly costs), so I may drop them all together to present a more focused storefront) CH340G serial adapters that break out the modem pins: CH340G Serial Adapter - fused w/all pins available from Azduino by Spence Konde on Tindie