Using the FT245BM in a 3.3V only design...

[Jaye Gallagher]

Hi there,

I'm designing a microcontroller based system in which I'd like to use the
FTDI FT245BM as a simple USB interface. My design is battery powered and
runs on a regulated 3.3V. The FT245BM, however, is a 5V part, but can be
configured to use a 3.3V logic interface.

Here's what worries me: the data sheet for the FT245BM
(http://www.ftdichip.com/Documents/ds245b14.pdf) provides a schematic for a
USB powered design (Figure 9, Page 19) that provides 3.3V I/O to the rest of
the system. This looks good, however, what I'd like to do is only have the
FT245BM run from the 5V power from the USB, and have the rest of my design
run on the regulated 3.3V from my power supply.

I'm wondering:

Whether I should connect the ground used from the FT245BM and the USB to the
ground of the rest of my circuit? This seems like a good since everyone
needs the same reference ground level.

Whether I should worry about the fact that the 3.3V being used to power the
I/O lines of the FT245BM is generated by the FT245BM's internal regulator
from the USB +5V, whereas the voltage being used everywhere else is from
another source? Will something, like say, the FT245BM, the microcontroller,
or other bits sharing the same data bus get fried somehow by differentials
in voltage, transients or my general failure to observe something spooky and
analogue?

Whether I should somehow decouple the connection point between the
USB/FT245BM ground and the ground used everywhere else? After all, scary
stuff might happen when a USB plug is initially inserted.

Hope someone can help,

Thanks,
Jaye.

 

[Quack]

Hiya Jaye,

I think i know you actually - i believe we are actually working in the
same company ? lol

Whether I should connect the ground used from the FT245BM and the USB to
the
ground of the rest of my circuit? This seems like a good since everyone
needs the same reference ground level.

Yep, should have no problems doing that.
Just ensure you dont forget the add the decoupling capacitors between ground
and power supply pin to each IC.

Alex.

 

[Paul Burke]

Jaye Gallagher wrote:

Hmm -- but the FT245 requires a 5V supply, and the data sheet specifies that
3.3V isn't going to be enough for it. Are you saying that it's okay to
underpower the FT245 at 3.3V?

The FT245 will get its 5V (ish) power from the USB when it's plugged in.
If you are really worried about doing harm to the FT245 when unplugged
(I don't think it's likely), or if it might upset your local bus or
whatever, put in a 3 state buffer, powered from your 3.3V but with the
enable controlled by the FT245s VCC (or 3.3V output) so that the FT245
is isolated when unpowered.

Paul Burke