STM32H7 + LAN8742: both ETH leds blink every 2 seconds

[ValerioF]

Hello, 

I am working with a STM32H743 with RTOS and LWIP, using LAN8742 as PHY. I have developed a custom board with this setup and it has been working very well.

Now, after almost an year, the new production lot does not work anymore: when the firmware starts, the 2 leds connected to the RJ45 plug blink simultaneously. The producer assured me that he used the same components, only the PCB has been produced in a different lot, same master.

I tried to handle the nRST pin by software, delaying its assertion as descripted in the LAN8742 datasheet but it's no use. With the "old" boards that pin was left high for the whole time, no need of hardware reset via that pin.

Is there a logic table that describes what the actual LED meaning is? I think they are showing some kind of error state but can't figure out which.

Big thanks in advance!

 

[Pavel A.]

The datasheet does not mention any LEDs blinking pattern that indicates any error condition (besides of usual link activity). So it looks like repeating reset.

[ValerioF]

Hello Pavel, 

thanks for the tip.

What could cause the reset?

[Pavel A.]

It's hard to tell... perhaps something overloads or shorts? What is the frequency of these blinks? Does the MCU keep running?

[ValerioF]

Yes, the MCU keeps running.

An update: even if the MCU is never been flashed (new board) the LEDs blinks on/off (50% duty cycle) with 4 second period, 2 secs on, 2 secs off.

AND

a very low noise can be heard form the board.

I will check the 3v3

[ValerioF · Accepted solution]

Hello,

We discovered that R101 connected to RBIAS pin of the driver WAS NOT 12K ohm. As soon as we placed the correct resistor, it started working as inteded.

In the datasheet it does not say anything particular in case that resistor is not the correct value.

Thanks anyway for the help!