Teensy 4.1, AudioShield and OLED noise

[david71]

Hi all,

I am trying to connect a small 128x64 OLED screen to my teensy - using I2C - I have connected as below:
GND to Teensy GND (pin before D0)
VCC to Teensy 3.3V (pin between D12 and D24)
SCL to Teensy pin 19
SDA to Teensy pin 18

The screen is working as expected, however I am experiencing a fair amount of noise from the audio out. I added a 2.2K resistor between VCC and teensy 3.3V so connection was:
VCC to 2.2K resistor to Teensy 3.3V (pin between D12 and D24)

and this removes the noise completely (also means the OLED is slightly dimmer - which is fine by me).

I just wanted to check adding a resistor is okay in this way (and not causing some unexpected damage anywhere?), and also to ask if anyone else and experienced this issue at all? The OLED screens I am using are these (again not sure if these are causing the issue?): https://www.amazon.co.uk/gp/product/B08QJ7BL3R/ref=ppx_yo_dt_b_asin_title_o04_s00?ie=UTF8&psc=1

Okay thanks very much!
David

 

[MarkT]

The typical current draw of the screen is stated as 9mA. So the 2k2 resistor can't really be powering the display, its probably
being phantom-powered via the I2C pins which is not a good idea at all. Try an RC filter on Vcc of say 33 ohms
and 100uF or more. That limits the voltage drop to about 0.3V on Vcc which will prevent the risk of phantom
powering.

 

[david71]

Thanks very much for the response.
Just to clarify (apologies am still very much a newbie), below is my circuit and then below that with the suggested change (as I understand it) does this look correct? (N.B. In the pictures I used a Teensy 3.1 - where as I have a 4.1 and the OLED is just a generic one that was available on Fritzing)


With 100uF and 33 ohm resisitor:


Many thanks again!
David

 

[david71]

Further to this, in case anyone has experiences anything similar, i believe I possibly have a ground loop issue. When powering the Teensy from a separate "wall wart" rather than my PCs USB port, the noise goes away completely. My setup was: PC connected to Teensy via USB -> Teensy audio shield out to external soundcard (miniJack to 1/4 inch jack), and my external soundcard is also connected to my PC via USB. So looks like possibly this setup is ripe for ground loop issues (I'm not an expert by any means)