4.9V on the 3V3 Output

Products & Technology XIAO
1

Hey guys, try and figure this one out with me.

I’m integrating the ESP32-C3 Xiao board into a design I have for temperature controlled storage box - essentially I have a bunch of mains relays operating a refrigeration compressor, ceramic fan heat, dehumidifier, based on temperature and humidity sensing over I2C, with an OLED screen and capacitive touch switches.

Breadboarded the whole lot (not the mains part, obviously) and powering the Xiao at the time over USB cable attached to my Mac, the entire design working just dandy. Set about getting the PCB designed, with 5V LDO reg to supply the Xiao, relays and some other bits…

Imagine my surprise when I supply 5V into the VUSB pin that I get 5V OUT of the 3V3 line, almost killing all my 3V3 active comps (specifically the two BME280 temp/humidity sensors and OLED screen), thankfully I can easily disconnect them from my board, leaving essentially the Xiao feeding 5V into that 3V3 trace on my board.

The datasheet suggests you can use the VUSB/5V pin as input or output and to protect the USB with a schottky diode, assuming you’re using both external power to the Xiao AND USB simultaneously. Which I’m not, nor never will. The USB-C socket on the Xiao was only used to flash ESPHome into the board, then OTA updates takes over. Only external 5V to the Xiao is used in my design, so therefore that protection diode is not needed.

On several bare modules (I have a good few Xiao’s in stock here), if I power the module only over the USB-C socket (plugged into my laptop), I get 5V out of the VUSB pin and 3V3 out of the 3V3 pin as expected.

If I then disconnect my USB cable, then power the Xiao on the VUSB/GND pins, I get 5V out of the 3V3 pin.

Just to satisfy my curiosity, I cut a USB-C cable up so I could continuity check between +VE in on the USB plug goes to VUSB, yes it does. -VE in the cable buzzed out fine to the GND pin also.

Where is this 5V coming from on the 3V3 output? Did I order a duff batch of Xiao boards from Ali Express?

While I can bodge my design to take my 5V LDO reg output off the board into my chopped up USB cable, it’s less than ideal. Those of you powering the Xiao ‘externally’ (not over USB cable) AND needing the 3V3 output must’ve had similar issues, no?

2

Hi there,

So having made several (20) PCB’s for Xiao…
That is not normal behavior for a healthy XIAO ESP32C3.

On Seeed’s documentation, the 5V pin is the USB 5V rail, and they do say it can be used as an external input, but they also explicitly recommend placing a diode between the external 5V source and the 5V pin. They also describe 3V3 as the regulated output from the onboard regulator.

So if you are truly seeing 5V on the 3V3 pin when powering from the 5V/VUSB pin, I would not consider that normal or expected.

I would suspect one of these first:

  1. Faulty or non-genuine module
    If the onboard regulator/power path is bad, 5V may be leaking onto the 3V3 rail.
  2. Backfeeding from the rest of your PCB
    Disconnect everything and test the XIAO completely by itself:
  • only 5V to 5V pin
  • only GND to GND
  • measure 3V3 directly on the bare module
  1. PCB solder bridge / wiring error
    Especially if 3V3 and 5V run close together on the custom board.
  2. Clone board variation
    If these came from AliExpress, I would not assume they match Seeed’s exact hardware.

I would try this in order:

  • test a bare XIAO module only, off-board
  • power it from a clean 5V bench supply into 5V + GND
  • measure 3V3 directly on the module
  • then repeat with a known genuine Seeed board if available

If a bare module still puts 5V on 3V3, I would consider that board defective.

And even if USB is never used in the final product, I would still follow Seeed’s guidance and add the recommended Schottky diode on the external 5V feed.
My own gut says: either clone boards or something on your PCB is backfeeding 3V3. A missing ground usually causes unstable behavior, bad readings, or no boot — but 5V appearing on 3V3 points harder at a power-path fault.

HTH
GL :slight_smile: PJ :v:

and BTW, say what you want …Idunno why some folks want to go against the guidance anyway,
What I would not tell anyone is:

“No diode is fine because you never use USB.”

That is exactly the trap. :face_with_hand_over_mouth: Seeed does not phrase the diode as “only needed if USB and external power are present at the same time.” Their published guidance for the 5V pin says if you use it as an input, you should add the diode. :backhand_index_pointing_left: :grin:

3

Something’s little odd there. Check your grounds :wink:

Rather than rely on an AI response, I have just powered my XIAO ESP32 C3 with 5V on the VUSB (Pin14) and GND (Pin13), and as expected the 3V3 (Pin12) shows 3.3V and the GPIO has varying voltages never exceeding 3.3V.

I have various GPIO connected (5V WS2812’s, 5V I2C clock and SPI CAN with 5V transceiver etc) and I don’t have voltage issues.

I don’t use a series diode either… not necessary as the 5V is disconnected when USB is connected. Also, when USB is connected, the VUSB can still power the connected 5V devices from the XIAO (Pin14).

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This link also mentioned the same issue, didn’t it?

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I think the point is it is best to power the XIAO with an external 3v3

The 5v is just a remnant of the old days

It only exists because USB is 5v and XIAO is 3v3

It only exists to bring the USB 5v out on a pin.

It only exists because we cant get rid of it now

It was never intended to power external 5v loads or logic/ as XIAO is not 5v logic tollerant

It has a micro 5V to 3v3 regulator to take USB power to XIAO power.

It can do all it can do to run the XIAO with signal levels

It is not designed to be a power distribution system

With that being said, the wiki probably needs to be updated.

If you want to power with 5v try powering thru the battery terminals underside

if you want to power externally, power REGULATED at 3v3 from the 3v3 pin.

if you don not have a regulated 3V3 external source, connect near 3v3 UNREGULATED power to the 5v

pin but do not connect a USB, anything on the 5v will feed back or be fed by the USB and could damage external hardware…

In my opinion the 5V pin on the XIAO is only used for proper indexing

( on a side note: no offense to the OP but i am so sick and tired of answering questions like this, I took a 3v3 tolerent unit and tried to power it with 5v and it doesnt work… is something wrong?… my next favorite is I got a brand new unit and wiped the firmware first thing before ever doing anything with it and now its bricked… can you help me? I dont know what i did wrong..) what da?

6

LOL,

Ding-Ding…Friday night … Right on schedule… :grin:
Go look… :face_with_hand_over_mouth:
AA :tropical_drink:is just a call away BRO!
LOL

GL :slight_smile: :v:

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7
  1. What kind of external 5V power supply are you using to power the 5V pin? Is it reliable?
  2. Have you tried powering the XIAO’s 5V pin using a USB cable connected to a Mac?
  3. Older 5V power supplies can sometimes experience a voltage overshoot that exceeds 5V when turned on. Could this overshoot be damaging the LDO inside the XIAO?
8

Hey it was my birthday… i deserved a break..lol

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9

So to come back to this… someone mentioned grounds… Yup. Pin13 isn’t grounded! Just found a glaring error in the DRC rules for my copper pour! Relying on the copper pour to pick up all the gnds around the board, I have two isolated pour regions. No idea how/why that wasn’t picked up in the DRC!

Either way, I still don’t need that diode :wink:

Cheers Grobasoz for the cheeky reminder :smiley:

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10

Hi there,

So Glad you found your mistake. Yes the diode is needed and advised according to Seeed own documentation. You’re new here so I would read a few posts and get some understanding on Xiao and the little things that can stop a design quick.

GL :slight_smile: PJ :v:

11

I’ll come back with my reasoning why the official PDF schematic and documentation is flawed.
The blocking diode as mentioned will only protect your own external 5V source, not the USB socket. Given the VUSB net on the schematic ties the VUSB pin of J1 to A9/B9 of the USB-C socket (VBUS) and the left hand side of fuse F1, that net itself is responsible for the tracks between three points.
The only place you can put a blocking diode is the input to J1. As I said, this only blocks USB 5V going back to your supply. There is no such other diode in the connection from A9/B9 of the socket to the rest of the circuit, so you can quite happily chuck 5V or more straight back into your host computer connected over USB.

Being new here, doesn’t mean new to electronics and questioning official documentation is everyones business, don’t take it for granted it’s correct. Their advice is only partially correct, and partially so if you happen to use both the USB for comms and your own power source.
As I’ve said, if you do not use USB for power, you do not need any such diodes, anywhere, and, if you want to protect your host computer, you need a diode in line with VUSB on the USB socket, which means cutting tracks on the Xiao board…

Egg on my face was relying on the DRC check, which I’m still baffled as to how it missed two unconnected regions! Tagged pin 13 to GND with some verowire and I’m back in business, sans diodes…

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12

Thanks - glad you found the problem and got it fixed.

Sadly, I cannot take full credit for the “grounding” suggestion as there was a similar issue posted the day prior with a ground problem causing incorrect readings.

13

Hi, sorry for highjacking the thread but I think this is related. If the VBUS pin can be used both as input and output power, is it possible to connect it to both the VIN and SYS pins of a BQ25180 battery charging IC which is connected to a battery? This way the BQ could both recharge its battery when the user connects the USBC of the Xiao, and also power the Xiao when not connected to USBC. Something tells me there is some kind of impossible loop here but I’m not sure.

14

Post #2 at the link below includes a wiring diagram for the power supply connections of the nRF52840 and ESP32C3. I think you’ll find it helpful.

15

no I was the one who gets the credit for the grounding solution… what da?

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In actuality, you went on a mini ‘rant’ about USB & 5V. In your post, you didn’t mention grounds once.

USB is 5V and perfectly fine to be used for low loads.. It’ll be around significantly longer than you care to shake a 3V3 stick at that’s a dead cert.

As for the wiki, if it’s open for public updates, I may be inclined to correct it. But, without a schematic change, it would be fruitless.

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18

It sounds like the onboard 3.3V regulator might be getting bypassed or is potentially faulty on that specific batch. Have you had a chance to check the resistance between the VUSB and 3V3 pins on an unpowered board? If it’s a direct short, it’s definitely a hardware defect. It might be worth checking the schematic for your specific revision to see if the battery charging circuit is somehow creating a path when VUSB is powered externally.