WROOM-32 ground pin connections and EMI/EMC

[longtimer]

Hello,

We are encountering some issues with clock noise emissions on our WROOM-32 mounted on a PCB and were wondering if all ground pins need to be connected? Given the size of the module, having multiple grounds connected creates ground loops which we think might be causing some of the emission problems for us. We have multiple capacitors between the Vcc and GND pins near pin 9 on the module and there, the clock related EMI is quite tolerable. However, near the other 2 ground pins we get a lot of clock related noise.

From the pinout image as shown below, it looks like all grounds are on the same ground plane so would expect that not all grounds would be required. Looking for input on this.



Thanks,

Jason

[guillep2k]

I'm no expert on the subject, but are you using a two-layer board layout? With a four-layer PCB you'd get rid of most emmision problems. Otherwise, I'd recommend investigating the rise/fall time and drive strength of all of your signals, since many pins can have times of 2ns or less (the ESP32 can drive 80 MHz parts!) depending on its configured drive strength (FUN_DRV/MCU_DRV). On high(ish) loads, like long tracks, it's advisable to insert a small RC filter just at the output of the ESP32 to limit these effects; a continuous ground plane just under the track, returning directly to the ESP32, is also very advisable. Short tracks are a must if you need your signals to work at those speeds.

On the software side, make sure to lower the drive strength of all signals that don't need a lot of it:

Code: [Select all] [Expand/Collapse]

1. ESP32 Datasheet section 5.3:
2.  
3. Each digital output pin is associated with its configurable drive strength. Column “Drive
4. Strength” in Table IO_MUX lists the default values. The drive strength of the digital output
5. pins can be configured into one of the following four options:
6. • 0: ~5 mA
7. • 1: ~10 mA
8. • 2: ~20 mA
9. • 3: ~40 mA
10. The default value is 2.
11. The drive strength of the internal pull-up (wpu) and pull-down (wpd) is ~75 µA.

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