Would you please share your flash problem link?

It's here in the same forum:
viewtopic.php?f=12&t=28486

GPIO_12 = MTDI

When pulled high it sets the VDD_SDIO voltage to 1.8V, pulled low sets it to 3.3V. This matches your observed change in flash voltage.

Level is sampled on release of reset only, so level after that does not matter.

See ESP32 datasheet: 2.4 Strapping Pins

The strapping pins can be ...

Thanks, I was not aware of that option.

I had already wired the 3.3V to VDD_SDIO and that solved the flash problem.
Now I removed the wire and burnt these fuses to 1

XPD_SDIO_FORCE Ignore MTDI pin (GPIO12) for VDD_SDIO on reset = 1 R/W (0x1)
XPD_SDIO_REG If XPD_SDIO_FORCE, enable VDD_SDIO reg on ...

Indeed, define more exactly what your requirements are.

1kHz sampling is not very fast for a 200MHz processor, as long as you have no blocking code in, for example, your ethernet (TCP/IP) stack. On a dual core processor (which most ESP32 are) you can run your sampling on one core and ethernet on ...

In the mean time I have found one possible cause:

Pin MTDI (strapping pin) is pulled to 3.3V on this board, this means VDD_SDIO is connected to the 1.8V LDO.

In the datasheet, in the recommended operating conditions, it says the minimum operation voltage for VDD_SDIO (3.3V mode) is 2.3V for parts ...

On a board with a ESP32-U4WDH mcu, I cannot program the the embedded flash. The board used to be populated with a ESP32-D2WD and there programming worked as it should.

As recommended in the datasheet, GPIO16, GPIO17, SD_CMD, SD_CL, SD_DATA_0 and SD_DATA_1 have no external connection.

I already ...