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I made a custom board around STM32F407 MCU that is connected with a PHY Board with a LAN8720 IC via RMII. I also have connected an 2.42' OLED Display based on SSD1309.

I notice when I turn on the OLED display, the status/activity LED starts blinking very fast, seems there is some error. When I turn off the display, goes back to normal, the LED only blinks when there is network activity. In both cases the network connection works.

When this behaviour occurs, the board is operation at 100Mbps Full-duplex. I don't observe the same when is operating at 10Mbps.

This board is powered by a single AMS1117-3.3 @ 800mA, that converts 5V to 3.3V. When everything is powered up it uses about 300mA, so theoretically it should be enough for this current usage.

I tried to supply 3.4v directly to 3.3v rails and the same does not occur...

In the data sheet of LAN8720 I don't see any description for fast blinking activity LED.

Is this problem related with voltage supply to the components? The SPI for Display and RMII for PHY don't share any pin or should not have any interference.

Should I have 2nd AMS1117-3.3 dedicated just for display?

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I think that the problem was related with fact that LDO was not able to handle the load and voltage dropped, not enough to keep all devices.

I re-design the power supply of the board to use a switching regulator instead and now the problem seems to be gone.

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