Custom CM4 breakout board ethernet/PoE problem

[hacor]

Hello

This way I wanted to aks a question regarding my custom designed CM4 breakout board. It is based on the CM4 IO board design specs and it works perfectly, but I have some problems regarding network or power which I can not debug.

Some facts:
- I use the TI TPS23750 PoE+ reference design HPA107 (Link) Which gives 5V @2A
- Because of the component shortages, I choose to use the Abracon ARJM11D7-104-AB-EW2 ethernet port (10/100Mbps with PoE+ support) (Digikey link)
- The differential pairs for ethernet use the same 100R values as used in the CM4 IO board Kicad reference design
- The board copper thickness, layer count etc is exactly the same as the CM4 IO board

Now I have a lot of problems using the network port on my board.
- 100Mbps-FD does not work and causes the network port to flicker very fast. First I thought this was caused by inference.
Possible mistake is that the Ethernet data line differential pairs are to close together, but this was solved by using the 10Mbps-HD software solution

Screenshot 2023-09-12 at 14.55.19.png (179.57 KiB) Viewed 486 times

- Lowering the NIC speed to 10Mbps-HD in Debian helped 1 board out of 3 to start working on my Unifi PoE switch. The other boards boot perfectly but the network does not come up and the port start flickering very fast. The changed file:

Code: Select all

cat /etc/network/interfaces.d/eth0.conf
auto eth0
iface eth0 inet dhcp
# Always use 10Mbps half duplex! Full duplex was not fixed in IEEE, so unreliable
post-up /sbin/ethtool -s eth0 autoneg on advertise 0x01

- Adding a Unifi PoE+ injector before every board and using the software adaptation to use 10Mbps-HD works for every board.

The most strange thing is that by adding a PoE+ injector before every board makes it function perfectly @10Mbps-HD. This hack makes me think the cause is a power supply error/malfunction.

I really do hope that someone here can help me debug this, because I find this so strange.
Thanks already

Kind regards

Hacor

[dp11]

looks as though you are missing your "bob smith" termination

[hacor]

Thanks for the quick reply! The "bob smith" termination principle was completely new for me . It seems indeed that the MagJack contains a 1000pF capacitor and a 75R resistor for termination inside the housing and the Abracon I used does not. I'll experiment with some solder components to see if it works.

I'll let you know!

[hacor]

Hi dp11

I experimented with the Bob Smith termination and came up with following working solution

Screenshot from 2023-09-17 19-02-07.png (53.3 KiB) Viewed 287 times

Now the board boots on every PoE switch Many thanks!

About the ethernet connection problems:
This seems to be related to an interference problem related to the lines being too close to a field of USB capacitors.

The solution here was to scale down the speed to 10Mbps-HD as stated in the original post.

May be this can help some people in the future.
Kind regards

Hacor

[titu85]

@hacor, I think you are going through a similar situation that I have gone through while I was designing a CM4 breakout board. If you look into the Tx and Rx line pairs of the ethernet patch cable (figure below), it is the first, second, third, and sixth line. First, second makes one pair, and the third and sixth second pair. As per the schematic you posted, you are connecting TRD0_P, TRD0_N, and TRD1_P, TRD1_N. My suggestion is to change TRD1_P to TRD2_P, for me, this connection is giving me 100Mbps link speed.

Ethernetpatch.png (132.24 KiB) Viewed 244 times

[hacor]

Hi @titu85

Thanks for your reply!
I looked up the schematics once more and I think the connections on the port are ok. The pins used in my schematic are not the port numbers, but the physical pin numbers on the connector itself. They seem to reference to wire numbers 1-2,3-6 (right side of the schematic) on the connector end.

Screenshot from 2023-09-20 17-57-54.png (105.29 KiB) Viewed 175 times

I can be mistaken, but I think my connections are correct. It was just about the left and right numbering

Kind regards,

Hacor