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

Hello,

Here's what worries me: the data sheet for the FT245BM

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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.

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.

Yes, the grounds obviously have to be connected together.

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?

You may want to add a small-value resistor (watch out for that time constant) in the signal lines to protect against cases where one of your supplies is on but the other one not. This setup works fine for me (I am using FT232BM).

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.

I also place some LC filtering and some overvoltage protection on the

5V USB supply to suppress spikes etc.

greetings, Tom

Reply to
Tom Torfs
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Use the design for a self- powered device, but replace references to 5V with 3.3V, and select the 3.3V interface on the FT245.

Yes.

That would happen (if it's possible) with a bus- powered device anyway. Use the usual techniques- optoisolate the logic inputs and outputs, have a "dirty" ground at the input, and a "clean" analog ground inboard, connected by an inductor to limit the surge when you plug in, and use differential inputs for your analog. Or isolate them too. If there's a DC component between the grounds you'll probably fry the wiring anyway.

Paul Burke

Reply to
Paul Burke

Hiya Jaye,

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

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.

Reply to
Quack

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

Reply to
Paul Burke

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

formatting link
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.

Reply to
Jaye Gallagher

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?

Thanks, Jaye.

Reply to
Jaye Gallagher

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