DSO nano bug? >85V coming out of Seeeduino MEGA!

I have a weird problem. I am using a seeeduino MEGA, which is functioning as it should as far as I know.
When I toggle pin 52 up and down using direct writes to PORTB, I get a nice and clean signal out of my DSO nano (square wave at some frequency).

When I run the program below, the waveform on the nano has a frequency of 977Hz (as expected), but the signal has a very sharp rising edge, and a very slow exponential delay. So far so good, but the real problem is that the peak of the waveform is over 85Vpp!! (I can’t zoom out more than 10V/Div).

I am using firmware 2.21 (the original one gave the same signature), teh gnd connection of the nano is connected to the Seeeduino gnd, and the signal is connected to pin 52. There is no difference whether the nano runs on USB power or on batteries.

Here’s the program. It uses the library from arduino.cc/playground/Learning/TLC5940

#include “Tlc5940.h”

void setup() {

void loop() {


Sounds like maybe the nano is in x10 mode, maybe 8,5V is more correct? It shouldn’t do that by default. Do you have something else to measure with like another osciloscope?

I don’t think that it’s a 10X problem. When I bit-bang the output with the same setting on the nano, everything looks fine (in 1V/div, 2V/div and 10V/div).
I verified that the issue exists on both the Seeeduino as well as the Arduino MEGA.
When I look at it on a different scope, I get the attached waveform (1V/div, 200us/div, same as DSO nano)