3.3v logic using an FTDI cable at 5v and external power at 3.3v

[345876]

Hello,

Sorry for the newb question. I am working on a fun little project that requires a greater current than an FTDI cable and USB port can provide. Therefore I need to use an external power supply. I can't just use a 3.3v FTDI cable because in some instances I will need to connect the project to an arduino like board that only outputs 5v.

I need the TX line from the FTDI cable to be brought down to 3.3v logic. I can do this with a 2.2K resistor in the TX line to the project, in parallel with a 4.3K resistor to ground.

But which ground do I use? Would connecting the two grounds solve the issue? I don't want to destroy my dc-dc voltage regulator for the external power supply. I have a feeling there is a bit more to it then that. Any help you can give is great, I am sure I also left out a key piece of info as well so don't feel bad letting me know!

Thanks!

 

[shahbaz.ele]

dear you must attach a circuit diagram so we can understand your problem.
without circuit it is difficult to understand.
dc-dc regulator is normally isolated from external power

 

[345876]

Sorry I realize my explanation of the issue was poor. I have attached a diagram. The issue is that the hardware does not function properly when the TX hardware pin is plugged in, but does when only the TX hardware pin is unplugged (but then I can't receive data from the hardware). I believe this is somehow due to grounding of the FTDI TX to the FTDI ground rather than the system ground, but I am not sure. Any ideas or obvious mistakes?

Thanks!

 

[sky_123]

There were a couple of threads on logic level voltage conversion recently, so you may want to search then out. Also, there is this doc:
**broken link removed**
Basically, the resistor method is only useful occasionally. The reason is that you now cannot supply much current, so it will take longer to
charge the capacitance at the input of the circuit, so your waveform doesn't look square at high frequencies. The way around this is to
use a dedicated logic converter ic, or to use a switching device (e.g. MOSFET) to do the level conversion. Anyway, the doc I mentioned
describes it a lot better than I can. But you might be ok, and the resistor method may be ok.

Also, you're right, your issue is due to the different grounds that you have got, I couldn't see why you did not connect the two
grounds together. When your TX pin goes high or low, it is quite undefined what voltage level the FTDI RX pin will see, since the common ground
reference is not connected.
For example: Lets say your Hardware RX pin is at 3.3V. That means that the reference GND that the FTDI chip 'sees' is that 3.3V through the
lower resistor, i.e. 3.3V. So, if your Hardware TX pin is at 3.3V, then it looks equivalent to GND to the FTDI chip. If the Hardware TX pin went
to 0V, then the FTDI chip would 'see' -3.3V with reference to it's GND.
I think you should confirm your DC converter is generating 3.3V, and that your Hardware TX/RX pins don't seem to have anything more than 3.3V
on them, and then you can quite safely connect the two grounds together.
If you're worried, stick a low value resistor temporarily in series with the supply and your circuit, to limit the maximum current that can be drawn,
while you're testing.