2 Questions about adding USB FTDI Interface to Standalone Arduino/Atmega

I'm adding a usb to a PCB I'm making for the first time with the ATmega chip. The 'standalone' arduino.

As part of my project, I have a 5V 10A power supply to run a bunch of LEDs and other components.

But I want to add USB functionality.

Question 1: The FTDI boards you can buy have the 5V power pin. My understanding is that is 5V OUT, to give power to the board/chip when connected to USB. If I already have a separate 5V power source supplying my chip/board, should I leave the 5V unconnected on the USB FTDI board? Or should I still connect it up. I feel like the chip would be getting TWO power sources at that point and that that would be bad. So with external power, do I still connect the 5V line on the chip?

Question 2: The DTR pin connects to pin 1 of the 328 chip for reset, through a capacitor (I'm using 0.1 uF). Can I also add a reset switch at the same place? So basically pin1-->0.1uF-->DTR and then pin1-->same 0.1uF-->spst switch?

Or can only one thing be connected to the reset switch? If I add the button, then when pressed it will also connect DTR to ground as well as the pin1.

I should mention I do have the 10k pullup resistor on pin1 as well.

Thank

edit: I've seen one board suggesting for question #2 that you CAN add a button as well. But that the button does NOT go through the capacitor. Does that sound right?

Question 2: The DTR pin connects to pin 1 of the 328 chip for reset, through a capacitor (I'm using 0.1 uF). Can I also add a reset switch at the same place? So basically pin1-->0.1uF-->DTR and then pin1-->same 0.1uF-->spst switch?

No do not connect the output of anything to a switch to ground. Connect the reset pin direct to the push button switch to ground. The capacitor will protect the USB adapter from a short.

Question 1: The FTDI boards you can buy have the 5V power pin.

We need to know what FTDI board you have before that can be answered. In general you do not connect two 5V supplies together.

Thanks for the reply. I think it makes sense about what to do with the button switch now.

This is the exact chip I have.

Here is a pic

http://cdn.instructables.com/FE9/H8H1/I3VHABJX/FE9H8H1I3VHABJX.LARGE.jpg

I bought it off ebay, the chip on the board is labelled as

FT232RL

Hopefully that helps.

But yeah I kind of assumed I should just leave the +5v and +3.3V pins unconnected.

But I would still connect grounds.

Yes, if you are supplying power to your ATmega chip separately, you do not need to connect the +5 V on the FTDI adapter. Of course you must connect the grounds as well as Tx, Rx and DTR.

You connect your reset switch to pin 1 of the ATmega, not on the DTR side of the capacitor. And don't forget the 10k pull-up on Reset - pin 1 - and preferably a diode also with cathode to +5 V.

I see that link contained the dreaded word Instructables, you do know they are crap.

Look at the data sheet for that board it has about four different powering options it can be configured for.

Paul__B: Yes, if you are supplying power to your ATmega chip separately, you do not need to connect the +5 V on the FTDI adapter. Of course you must connect the grounds as well as Tx, Rx and DTR.

Sounds about what I was thinking. The Tx,Rx and DTR are all connected as well.

Paul__B: You connect your reset switch to pin 1 of the ATmega, not on the DTR side of the capacitor. And don't forget the 10k pull-up on Reset - pin 1 - and preferably a diode also with cathode to +5 V.

Sounds good on the reset stuff. That makes sense now.

But the diode? If I'm not connecting the +5v, why would I need a diode on it at all? With the cathode facing the +5V, are you just trying to ensure no power leaks out? If its not even connected, why would the diode help at all?

Sorry, I'm lost on the diode part.

Grumpy_Mike: I see that link contained the dreaded word Instructables, you do know they are crap.

Yeah it was just the first picture that showed up on google search for the board.

But the diode? If I'm not connecting the +5v, why would I need a diode on it at all?

It is complex but basically the diode will prevent latch up when the reset line is taken above 5V due to the pulse from the capacitor on the DTR line.

The diode prevents the part from thinking it is gong into high voltage programming mode, and then lookng like it is hung when the next step in that process does not occur.

Grumpy_Mike:
It is complex but basically the diode will prevent latch up when the reset line is taken above 5V due to the pulse from the capacitor on the DTR line.

CrossRoads:
The diode prevents the part from thinking it is gong into high voltage programming mode, and then lookng like it is hung when the next step in that process does not occur.

Okay I think that makes sense in theory. I can see how the capacitor could mess that up.

I searched around and found this thread and this thread that kind of helped explain it even further to me.

Looks like that second thread has this image as the schematic for the diode.

I’m going to follow that and use a 1n4148

Does this all seem reasonable?

Kind of cool to read some of the history of how people found a hardware fault and found they needed the diode in later uno revisions.

The thing is that if you have a capacitor with 5V across it, like when the DTR is zero and the reset is pulled up to 5V and you take that end that has a zero and make it 5V, in that instant the end that was 5V now becomes 10V because the charge on the capacitor can not change instantly. This is the basis of of voltage doublers.

The diode shorts out that 10V to the 5V rail and so quickly discharges the capacitor.
Reset Diode.jpg