Troubleshooting Noise on Teensy 4.1 & PT8211 - Page 1
Forum section: electronics / projects

author: ryanmills
author_rank: Unknown
timestamp: March 30, 2022, 10:37:56 pm
content: 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 audio 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 examples 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 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: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

author: DavidAlfa
author_rank: Super Contributor
timestamp: March 30, 2022, 10:53:21 pm
content: Does the noise go away if you disconnect the SPI bus from the Teensy?Unshielded high speed digital lines can cause all kind of noises due EMI.The display has charge pumps to generate some required voltages.Try adding 10uf+100nf capacitors in the display to filter the 3.3V supply.

author: ryanmills
author_rank: Unknown
timestamp: March 30, 2022, 11:02:15 pm
content: Quote from: DavidAlfa on March 30, 2022, 10:53:21 pmDisconnected the entire SPI bus from the display, after letting the screen init. I will try adding more caps on the 3.3V again.

author: ryanmills
author_rank: Unknown
timestamp: March 30, 2022, 11:59:49 pm
content: Quote from: DavidAlfa on March 30, 2022, 10:53:21 pmCaps don't seem to do it either, I don't want to hack on the TFT, I added them on the breadboard. But no discernible change

author: ryanmills
author_rank: Unknown
timestamp: March 31, 2022, 12:22:08 am
content: Found out a little more, if the screen is connected but I don't init it via SPI, no noise. As soon as the display is active, then I have noise. Going thru the libary now to see if anyone thing is the cause but I have a feeling its coming from the display its self.

author: ryanmills
author_rank: Unknown
timestamp: March 31, 2022, 12:50:30 am
content: So to make it more interesting, if I disconnect the 3.3V from the PT8211 (I2S audio driver), it still works perfectly and it drastically reduces the noise. Going to solder up another hat to double check it.

author: DavidAlfa
author_rank: Super Contributor
timestamp: March 31, 2022, 01:01:02 am
content: There're at least two switching regulators on that display, I guess one is for the backlight and the other... Perhabs the LCD bias generator.It's weird for a $90 display, expecting a decent quality at that price range

author: TopQuark
author_rank: Frequent Contributor
timestamp: March 31, 2022, 03:31:19 am
content: If the noise is in the audible range, perhaps try inserting a PI filter consisting of caps -> inductor -> caps into the 3v3 line powering the display. Your display is probably missing an input EMI filter and is pushing noise back into the rest of the circuit. Use a mix of low ESR ceramic MLCC for good ripple rejection and higher ESR electrolytic or tantalums to dampen any potential oscillations introduced by the LC network.If that fails, maybe try powering the display with a separate 5v to 3v3 regulator.