Connecting i2c on Raspberry and Arduino: wrong information on websites!

I wanted to connect my Raspberry and the Spider controller board with Arduino
via i2c.
On websites like
I read that I can simply connect both as long as Raspberry is the master.

But there is some important issue, if Raspberry and Arduino stay connected:
if you do nothing in the void loop() (like the blink example) and/or if you do
not set digital pin D20 and D21 to 0, these pins are at 5V per default!
I never set these pins and they were on 5V out of the box.
Then I tried a Arduino script setting these to 0, was ok.
But as soon as you put a script onto the Arduino which doesn't do that,
the SCL and SDA are on 5V again.
Even if you set the pins to 0 in void setup()
and then use the script on
the two pins are on 5V afterwards!
Luckily I measured this before connecting the Raspberry trusting the above


You should be sure you didn’t enable the

internal pull-ups on your arduino.

Or just use a 3.3V Arduino :slight_smile:

Or just use a 3.3V Arduino :slight_smile:

If there is some 3V3 version
If there is some 3V3 version of the Spider Controller ?

Solved: solution for 3V3 Raspberry stuff and 5V Arduino stuff

I had a solution here afair but it seems to be deleted.

So I simply point to another posting which I found afterwards:

which is nearly identical to my solution.