# Serial communication not working when powered with external 5V **URL:** \ **Category:** Nano R4\ **Created:** [September 29, 2025, 11:42pm UTC](https://forum.arduino.cc/t/serial-communication-not-working-when-powered-with-external-5v/1407955 "2025-09-29T23:42:50Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![ezira](https://avatars.discourse-cdn.com/v4/letter/e/3be4f8/32.png) [@ezira](https://forum.arduino.cc/u/ezira)\ **Post date:** [September 29, 2025, 11:42pm UTC](https://forum.arduino.cc/t/serial-communication-not-working-when-powered-with-external-5v/1407955/1 "2025-09-29T23:42:50Z")
purchased a couple of Arduino nano r4 boards and I’m having trouble setting them in my current project. When powered by an external 5V supply via the 5V pin, the Arduino USB serial port is not discoverable. However, when I solely rely on the USB port for power, the Arduino is discoverable and I can properly interface with it. When powered with the external 5V supply on the 5V pin and running the Blink demo, the Arduino works as expected indicating that it’s running sketches but just not opening up its serial port. I’ve used many Arduino’s in the past for different projects and I’ve never had this problem before so just checking to see if there’s something unique to the Nano R4. I’ve tried rebooting the board after connecting the USB cable and using different cables. I’m using a M1 MacBook Pro and tried using all USB-C ports. ---
**Author:** ![anon94801460](https://avatars.discourse-cdn.com/v4/letter/a/278dde/32.png) [@anon94801460](https://forum.arduino.cc/u/anon94801460)\ **Post date:** [September 30, 2025, 12:34am UTC](https://forum.arduino.cc/t/serial-communication-not-working-when-powered-with-external-5v/1407955/2 "2025-09-30T00:34:10Z")
Why is this a "problem"? ---
**Author:** ![van\_der\_decken](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/van_der_decken/32/1006976_2.png) [@van\_der\_decken](https://forum.arduino.cc/u/van_der_decken)\ **Post date:** [September 30, 2025, 12:38am UTC](https://forum.arduino.cc/t/serial-communication-not-working-when-powered-with-external-5v/1407955/3 "2025-09-30T00:38:09Z")
Well, powering via the 5V pin looks like it won't provide 5V on VUSB (check the schematic), but it's not immediately obvious to me why that would matter. In fact, I won't expect it to provide 5V on VUSB. Maybe it's a USB-C thing, something I cheerfully admit to knowing nothing about its internals? What's more interesting right off that bat is that powering the board via the 5V pin is not listed as a valid power option in the [Nano R4 docs](https://docs.arduino.cc/hardware/nano-r4/#tech-specs): > The board supports multiple power input options: > > The VIN pin accepts an input voltage range of +6–21 VDC, which is internally regulated down to +5 VDC. > > The board can also be powered directly via the board's USB-C® port, which supplies +5 VDC to the system. If you power the board via VIN with 6-21V, is the serial port discoverable then? Edit: are you plugging the USB cable in after or before applying 5V to the board? ---
**Author:** ![anon94801460](https://avatars.discourse-cdn.com/v4/letter/a/278dde/32.png) [@anon94801460](https://forum.arduino.cc/u/anon94801460)\ **Post date:** [September 30, 2025, 12:44am UTC](https://forum.arduino.cc/t/serial-communication-not-working-when-powered-with-external-5v/1407955/4 "2025-09-30T00:44:36Z")
From the Nano R4 schematics (relevant section below) I don't see that powering from the +5V would present a problem, nor do I have any idea why the USB is not discoverable under those circumstances. As suggested, the reported "issue" might be with the host. ![Capture](https://europe1.discourse-cdn.com/arduino/original/4X/6/b/1/6b103c5e376b27cc48cb8bb3d775781dbeca4ce1.jpeg) ---
**Author:** ![ezira](https://avatars.discourse-cdn.com/v4/letter/e/3be4f8/32.png) [@ezira](https://forum.arduino.cc/u/ezira)\ **Post date:** [September 30, 2025, 12:49am UTC](https://forum.arduino.cc/t/serial-communication-not-working-when-powered-with-external-5v/1407955/5 "2025-09-30T00:49:39Z")
its not an functional issue in terms of the overall hardware working but being able to connect via usb for communication is part of how I prototype and debug ---
**Author:** ![ezira](https://avatars.discourse-cdn.com/v4/letter/e/3be4f8/32.png) [@ezira](https://forum.arduino.cc/u/ezira)\ **Post date:** [September 30, 2025, 12:55am UTC](https://forum.arduino.cc/t/serial-communication-not-working-when-powered-with-external-5v/1407955/6 "2025-09-30T00:55:15Z")
I haven’t tested with Vin yet but the hardware the Arduino plugs into supplies power to the board only via 5V so it’s useful to test just to see but isn’t a viable solution long term. I checked the schematics and datasheet and agree that there shouldn’t be an issue. The data sheet indicates that you can use the 5V pin as a power input, assuming the power supply is properly regulated. I plug the USB after powering 5V and have tried rebooting the Arduino as well. ---
**Author:** ![anon94801460](https://avatars.discourse-cdn.com/v4/letter/a/278dde/32.png) [@anon94801460](https://forum.arduino.cc/u/anon94801460)\ **Post date:** [September 30, 2025, 1:06am UTC](https://forum.arduino.cc/t/serial-communication-not-working-when-powered-with-external-5v/1407955/7 "2025-09-30T01:06:52Z")
> [@ezira](#): > > but being able to connect via usb for communication is part of how I prototype and debug Of course you do, but why also supply power via the 5V pin? In any case, the trivial solution to your dilemma is to send debug info out of the hardware UART subsystem (Serial1). > I haven’t tested with Vin yet Why not? ---
**Author:** ![van\_der\_decken](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/van_der_decken/32/1006976_2.png) [@van\_der\_decken](https://forum.arduino.cc/u/van_der_decken)\ **Post date:** [September 30, 2025, 1:07am UTC](https://forum.arduino.cc/t/serial-communication-not-working-when-powered-with-external-5v/1407955/8 "2025-09-30T01:07:40Z")
> [@ezira](#): > > The data sheet indicates that you can use the 5V pin as a power input Neither the online docs nor the data sheet say that. They both say VIN or USB. And looking at section 3.2.1 Power Tree in the datasheet, the 5V pin is clearly indicated to be an output, not an input. ![image](https://europe1.discourse-cdn.com/arduino/original/4X/2/8/f/28f3fcd4a644f3c95bc5f12e0682cac2a8abdb80.png) ![image](https://europe1.discourse-cdn.com/arduino/original/4X/a/b/6/ab6372fdd0361d43239b2074be295548d498c756.png) ---
**Author:** ![ezira](https://avatars.discourse-cdn.com/v4/letter/e/3be4f8/32.png) [@ezira](https://forum.arduino.cc/u/ezira)\ **Post date:** [September 30, 2025, 1:25am UTC](https://forum.arduino.cc/t/serial-communication-not-working-when-powered-with-external-5v/1407955/9 "2025-09-30T01:25:46Z")
![image](https://europe1.discourse-cdn.com/arduino/original/4X/9/5/6/95627bb28f916adf0ea6210b4e73e2adcbc5e02c.png) sorry, meant to say the online docs [https://docs.arduino.cc/tutorials/nano-r4/user-manual/#uart-communication](https://docs.arduino.cc/tutorials/nano-r4/user-manual/#uart-communication) ---
**Author:** ![ezira](https://avatars.discourse-cdn.com/v4/letter/e/3be4f8/32.png) [@ezira](https://forum.arduino.cc/u/ezira)\ **Post date:** [September 30, 2025, 1:27am UTC](https://forum.arduino.cc/t/serial-communication-not-working-when-powered-with-external-5v/1407955/10 "2025-09-30T01:27:54Z")
I have external components that also use 5V power and the USB cable connection is only meant for debugging, not a final part of the hardware. I havent gotten around the Vin input test since I’m away from the bench but will test and come back with info ---
**Author:** ![van\_der\_decken](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/van_der_decken/32/1006976_2.png) [@van\_der\_decken](https://forum.arduino.cc/u/van_der_decken)\ **Post date:** [September 30, 2025, 1:28am UTC](https://forum.arduino.cc/t/serial-communication-not-working-when-powered-with-external-5v/1407955/11 "2025-09-30T01:28:24Z")
I'd say then that you need a clarification from someone who actually works for Arduino and has actual knowledge of what the R4 Nano was designed for. Because in two places it says VIN or VUSB. And in a third, it adds 5V. Picking the one you _want_ to be right and ignoring the others is probably not the most logical course of action. One or the other is wrong; time to find out which it is. ---
**Author:** ![ezira](https://avatars.discourse-cdn.com/v4/letter/e/3be4f8/32.png) [@ezira](https://forum.arduino.cc/u/ezira)\ **Post date:** [September 30, 2025, 1:38am UTC](https://forum.arduino.cc/t/serial-communication-not-working-when-powered-with-external-5v/1407955/12 "2025-09-30T01:38:41Z")
I'll reach out to Arduino for more information, hopefully they respond in a timely manner. Assuming the 5V pin is designed as an output, any idea why it can it still be supplied with power and run programs? The schematic suggests to me that it should still work. ---
**Author:** ![MaximoEsfuerzo](https://dub1.discourse-cdn.com/arduino/user_avatar/forum.arduino.cc/maximoesfuerzo/32/923798_2.png) [@MaximoEsfuerzo](https://forum.arduino.cc/u/MaximoEsfuerzo)\ **Post date:** [September 30, 2025, 2:23am UTC](https://forum.arduino.cc/t/serial-communication-not-working-when-powered-with-external-5v/1407955/13 "2025-09-30T02:23:34Z")
I would test feeding through the 5V pin but through a Schottky diode to avoid any possible voltage conflict with the computer's USB port (Although looking at the R4 scheme in detail I don't know if anything is avoided...). ---
**Author:** ![anon85221860](https://avatars.discourse-cdn.com/v4/letter/a/59ef9b/32.png) [@anon85221860](https://forum.arduino.cc/u/anon85221860)\ **Post date:** [September 30, 2025, 2:40am UTC](https://forum.arduino.cc/t/serial-communication-not-working-when-powered-with-external-5v/1407955/14 "2025-09-30T02:40:09Z")
As Remington shows by the looks of it - there's no VBUS ("vusb") from either VIN or a 5V applied to "+5V". They don't use D1 on NanoEvery where this external 5V is an issue (and could whack the SMPS IC). ---
**Author:** ![ezira](https://avatars.discourse-cdn.com/v4/letter/e/3be4f8/32.png) [@ezira](https://forum.arduino.cc/u/ezira)\ **Post date:** [September 30, 2025, 1:57pm UTC](https://forum.arduino.cc/t/serial-communication-not-working-when-powered-with-external-5v/1407955/15 "2025-09-30T13:57:33Z")
haven’t heard back from Arduino yet but the Arduino is recognizable when powering via Vin. ---
**Author:** ![anon85221860](https://avatars.discourse-cdn.com/v4/letter/a/59ef9b/32.png) [@anon85221860](https://forum.arduino.cc/u/anon85221860)\ **Post date:** [September 30, 2025, 2:00pm UTC](https://forum.arduino.cc/t/serial-communication-not-working-when-powered-with-external-5v/1407955/16 "2025-09-30T14:00:44Z")
Is it "connected to" USB at the same time? ---
**Author:** ![ezira](https://avatars.discourse-cdn.com/v4/letter/e/3be4f8/32.png) [@ezira](https://forum.arduino.cc/u/ezira)\ **Post date:** [September 30, 2025, 2:15pm UTC](https://forum.arduino.cc/t/serial-communication-not-working-when-powered-with-external-5v/1407955/17 "2025-09-30T14:15:14Z")
yes. I have the usb-c cable connected to my computer and it’s being powered via Vin with 12V. I powered the 12V first then connected the USB-C cable to my computer. ---
**Author:** ![anon85221860](https://avatars.discourse-cdn.com/v4/letter/a/59ef9b/32.png) [@anon85221860](https://forum.arduino.cc/u/anon85221860)\ **Post date:** [September 30, 2025, 2:43pm UTC](https://forum.arduino.cc/t/serial-communication-not-working-when-powered-with-external-5v/1407955/18 "2025-09-30T14:43:00Z")
When "the Arduino" is plugged into USB it will, of course, be 'recognizable'. ---
**Author:** ![anon94801460](https://avatars.discourse-cdn.com/v4/letter/a/278dde/32.png) [@anon94801460](https://forum.arduino.cc/u/anon94801460)\ **Post date:** [September 30, 2025, 2:50pm UTC](https://forum.arduino.cc/t/serial-communication-not-working-when-powered-with-external-5v/1407955/19 "2025-09-30T14:50:54Z")
If USB connection works when the Arduino is powered by Vin, then it is reasonable to assume that there is a problem with the wiring, or the "5V" power supply you forgot to describe. Good luck with your project. ---
**Author:** ![ezira](https://avatars.discourse-cdn.com/v4/letter/e/3be4f8/32.png) [@ezira](https://forum.arduino.cc/u/ezira)\ **Post date:** [September 30, 2025, 3:00pm UTC](https://forum.arduino.cc/t/serial-communication-not-working-when-powered-with-external-5v/1407955/20 "2025-09-30T15:00:05Z")
The problem here is that you cannot provide 5V using the 5V pin and use the USB serial communication simultaneously, which isn’t obvious based on current documentation and experience with using prior arduinos. [Next page](https://forum.arduino.cc/t/serial-communication-not-working-when-powered-with-external-5v/1407955.md?page=2)