Powering Arduino with 5V pin and using USB for serial comms

To wrap this up, I think the easiest thing to do is provide footprints for a USB-UART circuit so that if necessary I can bypass the USB power issue altogether!  Something like a Microchip MCP2221a.

I can't work out whether that should be powered from the USB VBUS or whether I can power it from my own 5V supply (preference); nor whether the ground connections for the USB connector, MCP2221a and the Arduino (or MCU) should be the same even if powered from different sources (I always get confused by that!).  I'd rather just connect the D+, D- and GND and leave the VBUS disconnected as not needed.

EDIT: Providing this, where +5Vdd and -Vdd is used as power connections for everything (MCP2221A, Arduino/MCU, USB connector), ignoring USB power, looks like a good fallback position that might cost a couple of quid to add to a PCB if needed.

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Hi Andrew,

If you're planning to use a USB-UART chip like this, an option could be to use the USB supply for only powering up the MCP2221A chip. It seems the easiest, since there will always be power available via USB if is connected, and the MCP2221A chip is doing nothing when the USB connector is not plugged in anyway.

In other words, you could power the MCP2221A via USB, and power the remainder by whatever is required/preferred for your project (USB or other supply or both), and the USB connector GND and MCP2221A VSS and the remainder circuit ground will still be connected all together, since that's the reference for the RX/TX signals. I wasn't sure if -Vdd is ground or a different voltage in the diagram.

Hi Shabaz,

That makes a lot of sense given the way I've drawn it, but I've spent some more time thinking through the right approach.

I'm not sure yet whether a 5V MCU will turn out to be the right solution - memory issues, pin counts etc - so I've provided an option for VDD being 3.3V.  That gives me access to a greater range of MCUs, LCDs with built in processors, and so on.

I have a jumper to link VDD and VUSB on the MCP2221A if VDD is 3.3V.  If I power VDD with VBUS and VUSB with 3.3 I think there will be an issue as the internal 3.3 LDO won't be disabled; also Rx and Tx will be 5V levelled and I would want them to be 3.3V levelled.  In a tradeoff, having the chip powered but doing nothing is not ideal but I'd rather that than have problems if I have to power it with the lower voltage.  Hope I've explained that well enough.

-VDD is ground tied to the +5V (or +3.3.V) regulator.

Here's a more complete schematic of this section.  VDD will be either +5V or +3.3V; JP1 will be populated 1 to 2 if it's +3.3V and 2 to 3 if 5V.  I suppose I could also jumper VDD pin between +Vdd and VBUS so that if 5V is used, then I could run the IC off VBUS. 

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Hi Andrew,

I see what you mean, it makes sense for the two scenarios (3.3V and 5V logic).

I tried creating a standalone USB-UART, for both logic levels, and I used a separate voltage regulator as shown in the circuit diagram below.. but I've also been dropping this circuit into microcontroller projects as-is, but hard-wired to the correct logic levels without the jumpers shown in the circuit below. It doesn't add much to the cost since it's a very cheap LDO, but admittedly it is a bit unnecessary if the 3.3V supply is available elsewhere!

Recently I've decided to try to use a different USB-UART chip (Cypress Cy7C65213) since it supports a few features I wanted for a specific project (in particular flow control pins) I have not tested it yet. For that too I've just gone with adding the 3.3V LDO.. the unnecessary "luxury" option but I don't lose much sleep over it since I'm not doing anything price-sensitive, and has the slight benefit that it's decoupled from the microcontroller supply.. sometimes that might run at 3V or something I suppose, if it's running off a battery etc. It could be an option if you can't entirely predict how you may use the project or the board may be used for other purposes one day. Using the existing supply on the board makes total sense too.

3463.contentimage_207307.png

Hi Andrew,

I see what you mean, it makes sense for the two scenarios (3.3V and 5V logic).

I tried creating a standalone USB-UART, for both logic levels, and I used a separate voltage regulator as shown in the circuit diagram below.. but I've also been dropping this circuit into microcontroller projects as-is, but hard-wired to the correct logic levels without the jumpers shown in the circuit below. It doesn't add much to the cost since it's a very cheap LDO, but admittedly it is a bit unnecessary if the 3.3V supply is available elsewhere!

Recently I've decided to try to use a different USB-UART chip (Cypress Cy7C65213) since it supports a few features I wanted for a specific project (in particular flow control pins) I have not tested it yet. For that too I've just gone with adding the 3.3V LDO.. the unnecessary "luxury" option but I don't lose much sleep over it since I'm not doing anything price-sensitive, and has the slight benefit that it's decoupled from the microcontroller supply.. sometimes that might run at 3V or something I suppose, if it's running off a battery etc. It could be an option if you can't entirely predict how you may use the project or the board may be used for other purposes one day. Using the existing supply on the board makes total sense too.

3463.contentimage_207307.png