Seeeduino MEGA 4.02v on 3.3v Pin

I just received my Seeeduino MEGA. I am powering it through the USB cable. I connected a multimeter to the 3.3v pin and am reading 4.02v. Is it supposed to be so much higher than 3.3v? I also checked the 5v line and am seeing about 5 volts so that is normal.

I just measured my voltages using external battery power input:

Vcc = 4.964
3.3 = 3.305

On USB power

Vcc= 4.996
3.3 = 3.305

That was using a Fluke model 45 bench meter that should be in pretty good calibration.


Hi guys,

The 3.3v regulator works fine when extern drive 10mA~500mA.
Please test it work with a >10mA devce.



I have the same problem. When the switch “3V3_VCC_5V” is on 3V3 the 3V3 pin is @ 4.10V.

If you switch it back to 5V the 3V3 pin is @ 3.3V !!



Mine reads the near the same, 4.129 when the switch is in the 3.3 volt position, and 3.3 when in the 5 volt position. That seems to be strange behaviour?


As suggested above about putting a load on the 3.3vdc pin, I wired a pot from ground to the 3.3v pin and slowly decreased the ohms until the voltage lowered to 3.3volts DC, while the 3.3/5 switch was in the 3.3v position. I had to go down to 14 ohms before the voltage stayed regulated to 3.3vdc, that is 235ma !. That seems like a pretty poor regulator that requires that much of a load to stay at it’s rated regulation voltage? Almost as strange as why with no load the 3.3vdc pin measures 3.3v if the 3.3/5v switch is in the 5 volt position, but raises some when placed in the 3.3v position, to 4.13 in my case.

It appears to me that this 3.3/5 volt switch option is pretty useless if the 3.3v regulator is going to act like this? I’m certainly reluctant to try and power any external ICs that require 3.3vdc with this regulator.

Tech support, any ideas on what we are seeing? It almost seems like there is some kind of backfeed voltage or current coming from the AVCC pin on the processor that is interacting with the 3.3vdc regulator’s output voltage?

Until this is better understood, I recommend anyone that needs to use the 3.3vdc pin to power external components to leave the 3.3/5 switch in the 5 position.


Hi Guys,

Thanks for your good post.
We have found the 3.3v issue.
Here is the reason from atmega datasheet:
The ADC has a separate analog supply voltage pin, AVCC. AVCC must not differ more than ±
0.3V from VCC
Very sorry for our design mistake.

Our suggestion is do not toggle the 3v3_vcc_5v swich to 3v3 side.
We will fix the bug next version.
Advice welcome!


Does this design error and advice also apply to your V328 and V2.12 boards ? They both seem to have the same switch for 5/3.3 operation.



Seeeduino v2.12 and v328 can use the 3v3_vcc_5v switch as expect.
Toggle to 3v3 side, the atmega168/328 is working as a 3.3v device.


Also, I do not think you can use the AVcc to modify the digital voltage level on banks F and K. From the datasheet:

Port K also shares pins with the A/D Converter, but I don’t think you can say that AVCC will control the logic level of that channel, even if we ignore the ± 0.3V problem.

I am about to order one of these, because I am working on a project with two other people who already ordered these and like them; however, we need 3.3v I/Os, so I am going to come up with a circuit mod to run the entire ATMega1280 at 3.3v.

I can also try to help you fix the PCB layout for the next version, but can you tell me where to find the Eagle files for the latest rev. of the PCB? (My friends have v1.1, the Eagle files say v1.0.)

Please find the attached one: (269 KB)

Okay, I tried to fix this in the Eagle files, but I personally find working with Eagle to be extremely frustrating, so I gave up after an hour and a half. The schematic is fine, but there are a couple of airwires still left on the PCB – it should be easy to fix for someone who is comfortable with Eagle.

Here’s what changes need to be made:
[]AVcc cannot be used as a VccIO line – it was never intended for this purpose, so instead we need to change Vcc of the entire ATMEGA1280 to run off VCC_SW instead of VCC_LOG.[/]
[]If we change VccIO, we need to connect VCC_SW to the VccIO pin of the FT232RL.[/]
[]AVcc should be connected to VCC_SW – the datasheet specifies that a 10uH inductor should be placed in series between these two lines, but a 0 ohm resistor could probably be substituted.[/]
[]After the above changes are made, the only things that should be connected to VCC_LOG are the voltage switch, the 3.3v regulator, the fuse and maybe the pin marked 5v on J11. Everything else that was previously connected to VCC_LOG should now be connected to VCC_SW.[/]

The only tradeoff to be made here is that, according to the datasheet, the ATMEGA1280 must not be run at full speed if you run it at 3.3v (see page 373 of the datasheet. However, my experience with other ATMEGA chips has been that this is a binning issue, and many chips will work at 16MHz at 3.3v anyway. If not, the fuses on the chip can be set to either use the internal 8MHz oscillator, or use the clock divider. This seems to be a small price to pay for the ability to interface to 3.3v logic without blowing it up. (72.3 KB) (202 KB)

Yes, thank you very much.
We will improve Seeeduino Serial and fix the known issues.
Here I can tell you guys are:

  1. fix the power switch on Mega.
  2. move off the power select switch(USB/EXT switch) improve to auto detect…


Great to hear!

Unfortunately, I just noticed that you have no more stock of the Seeeduino Mega – is this because you plan on revising the PCB? Any idea when you will have more stock?

(If so, I’d still buy one of the v1.1 boards, and make the rework myself.)