Troubleshooting Noise on Teensy 4.1 & PT8211

R

rmills

Active member
I have two different noise sources, one I assume is from the power supply but it is very low. I have a 2nd issue with a lot more noise (hum/buzz, video with noise below) coming from a TFT display connected via SPI. This is not my area and wanted to see if anyone has run into this before. I thought it was the backlight but its the chipset 3.3V that seems to be the cause it. I'm using the Teensy 4.0x hat but I had to solder jumpers from it to a breadboard to test. Removing the 5V and GND for the backlight does not change anything, as soon as the 3.3V is removed the noise stops, backlight on with no 3.3V chipset connected, no noise. Not sure how to go about filtering this? I added some extra caps on the 3.3V output from the teensy, no change. Tried driving with a separate 3.3V supply, no change, separate 3.3 volt with caps no change. Powered with a bench top regulated power supply , no change. A 2nd switch mode 5V power I had lying around did not help either. I have seen some PT8211 using output drivers, but I assume that will just boost the noise. It's possible its noise from the breadboard, I gave everything the wiggle test to but made no difference. I tried a 2nd 4.1 and a second brand new display, no change. Note, I have the trace cut so I can use the USB with the bench supply, disconnecting USB does not change anything. Open to any advice.

Example Video: https://vimeo.com/694191704
Display: https://newhavendisplay.com/5-0-inch-premium-eve2-resistive-tft/
Display Datasheet: https://store-ybeckn7x79.mybigcommerce.com/content/specs/NHD-5.0-800480FT-CTXL-T.pdf
 
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Video link is broken...

BTW what do you mean by "the chipset 3.3V" - which chipset are you refering to?
 
MarkT said:
Video link is broken...

BTW what do you mean by "the chipset 3.3V" - which chipset are you refering to?
Click to expand...

Video link fixed. The Display has a FT812 on an EVE2 display board to drive the TFT. The 3.3V for the EVE causes the noise, the separate 5V for the backlight is fine. I let the screen init, then disconnected the SPI bus completely and still had the noise. In fact I can disconnect everything but until i remove the 3.3V I have noise.
 
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Narrowed it down a little more, if the screen is connected but I don't init it with SPI in the software, no noise.
 
MarkT said:
Doesn't work. "Video is not yet rated, login to watch".
Click to expand...

Odd, never had to rate my videos before. It should be fine now. And to make this more interesting, the PT8211 works perfectly with the 3.3V disconnected and that drastically reduces the noise. So I really have no idea what to blame at this point. Noise is coming from the screen, going to solder another teensy hat just to check but I'm not expecting any change.
 
Hmm, suspiciously like mains hum frequency, but that might just be the rate some code is hitting the external bus of course - its sounds to be square wave rather than sine.

One thing that's really useful for something like this is run the audio through spectrum analysis - for instance Audacity can do this. That will identify the precise frequencies involved.

ADCs and DACs like very clean power - ie its normal for completely separate regulators to be used for the analog side. If that's not feasible try adding a reasonably large amount of capacitance to the 3.3V rail to ground, perhaps 220µF to 1000µF, see if that makes a difference.

Avoid misrouting power and ground - wire straight to each load from the supply/regulator, don't daisy-chain.
 
MarkT said:
Hmm, suspiciously like mains hum frequency, but that might just be the rate some code is hitting the external bus of course - its sounds to be square wave rather than sine.

One thing that's really useful for something like this is run the audio through spectrum analysis - for instance Audacity can do this. That will identify the precise frequencies involved.

ADCs and DACs like very clean power - ie its normal for completely separate regulators to be used for the analog side. If that's not feasible try adding a reasonably large amount of capacitance to the 3.3V rail to ground, perhaps 220µF to 1000µF, see if that makes a difference.

Avoid misrouting power and ground - wire straight to each load from the supply/regulator, don't daisy-chain.
Click to expand...

After a bit of tinkering I think its a combo of issues. Cheap benchtop power supply, single rail, sloppy breadboard wiring. Without a PCB I might be spinning my wheels. Depending on if I use the 3.3V out from the Teensy or the main 5V rail both have different noise because I assume the teensy regulator and caps are filtering it a little. To be safe I'm adding two low noise LDO's to create separate 3.3V rails as well as extra bypass caps for the PT8211. The TFT is connecting via a box header and around 24" of cable so it's got some bulk capacitance. The 1000uf on the input and 5V is probably overkill but this board is going to be driving a bunch of relays, some other random stuff. I was curious, both Paul's board and the application notes have 47uf and 100nf bypass caps. I added an additional 1uf and .1uf ceramics. Is that 47uf electrolytic just bulk or is it a specific value to target low frequency noise? Outside of proper routing and isolation in the PCB layout. Is there anything else I can do to limit the noise from the TFT?

Schematic:
 
Mains hum is _very_ low frequency from a decoupling perspective - at 50Hz a 47µF capacitor has 68 ohms of reactance, a 1mF cap
has 3 ohms which is much more effective, but at some point the series resistance will become dominant and you get diminishing
returns.
 
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