Colibri iMX6ULL: On module PHY not detected

Colibri iMX6ULL 256MB Rev 1.1A
Linux 5.6.0+build.18
Custom carrier board


We have a product that uses a Colibri VF50 module on a custom carrier board. There is a fairly large number of products in the field over a number of years. We have experienced no Ethernet issues with the product whatsoever.

We have recently investigated replacing the VF50 module with an IMX6ULL module using the same carrier board. However, we cannot get the IMX6ULL Ethernet to work with our carrier board. The carrier board Ethernet circuit appears exactly the same as described in the “Colibri Carrier Board Design Guide”.

For test, I have installed both IMX6ULL Linux LTS 3.0.4 and 5.6.0+build.18, both exhibit the same behavior.

On boot, the log shows:

[ 1.730287] libphy: fec_enet_mii_bus: probed
[ 1.748658] fec 20b4000.ethernet eth0: registered PHC device 0
[ 18.370966] fec 20b4000.ethernet eth0: Unable to connect to phy

The carrier board Ethernet connector Link and Link Activity LED’s flash on and off repeatedly.

A schematic of the Ethernet circuit is provided.

The issue seems very similar to this one:

The IMX6ULL module Ethernet port appears to work fine in the Iris Rev 1.1 carrier board.

Any help will be most appreciated.

Hello ashinton,

I see that you do not have center tap caps to GND in you design. See attached schematic.

Also I do not know how good you followed our layout design guide. →
Can you check your PCB layout and sent you PCB layout to as pdf?

Best Regards,
Matthias Gohlke

Hello ashinton,

did you fix it ?

Best Regards,

Matthias Gohlke

Hello Matthias,
Thanks for your response.
No we don’t have it working yet.
Unless I am mistaken, the design does have center tap caps to GND.
CT_TXD and CT_RXD both have caps to GND.
The design looks the same to me.

hello ashinton,

both center taps need to have their one cap to GND as close as possible.

Best Regards,

Matthias Gohlke