Ethernet intermittent connection drop on customer board using SwitchBlox Nano

– Copied from customer email –

We started seeing a weird problem that points to one of our products on which we use the ( SwitchBlox Nano – 1 Inch Ethernet Switch.

The issue seems to be that the Ethernet link drops after a while of heavy constant ethernet load. The time it takes to reproduce the problem varies from 15min to a couple of hours but it’s repeatable.

Our setup is as follows:

We have a custom board that communicates to a USB host device (tablet) using LAN9514 USB Hub through USB2.0 on the IC’s USB upstream port. The LAN9514 has a 10/100 Ethernet MAC/PHY controller that we are using to communicate with the SwitchBlox Nano. We are using PCB traces (length-matched between the differential pairs) from the LAN9514 Ethernet PHY to one of the SwitchBlox ports (please note that we don’t have magnetics on the LAN9514 side).

The 2nd port of the SwitchBlox connects to a KSZ8795 Ethernet switch (100Mbps) using a 5m Ethernet cable. Then a 2nd port of the KSZ8795 switch connects to a robot through another 5m Ethernet cable (please see the picture attached).

The Ethernet network is established and starts fine. We stream positions from the USB host device (tablet) to the robot at 500Hz. After a while (from 15min to 2 hours), the robot stops moving. After some investigation, we see that the Ethernet link drops for 2 seconds.

We replaced our custom board with an off-the-shelve USB-to-Ethernet adapter (kept everything else), and we are not able to reproduce the issue. The problem seems to be caused by the combination/implementation of the LAN9514 IC and the SwitchBlox Ethernet switch.

  1. Did you see a similar issue to this happen before with the SwitchBlox ?

  2. Do you think that not using magnetics on the Ethernet in-board connection between the LAN9514 and the SwitchBlox side could be problematic ?

Any help on this would be greatly appreciated.

There are probably two options as to what this could be.

Option 1: A low level PHY connection issue

If it is a low level PHY issue, then (in theory) it should happen regardless of data rate. Do you observe that, or do you only see a link drop with network load. Also, which specific port do you see drop on SwitchBlox Nano.

I note you are using LAN9514 in a transformerless configuration, relying on the transformers on SwitchBlox Nano. From looking at the evaluation schematic, I note that LAN9514 is a current mode PHY, not a voltage mode PHY (read about the difference here). This means that it outputs open drain and needs a pullup to 3.3V on the center tap of a transformer. I am not sure it will work well without transformers. Can you share your schematic for the LAM9514 section.

Option 2: A bandwidth/bottleneck on SwitchBlox Nano

This could be a case of too much data overloading the SwitchBlox Nano, if you reduce update rate, do you see the issue still occur?

Also check temperature of the LAN. The USB Ethernet ICs tend to overheat quite quickly on heavy load.

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That’s a really good point, I’ll feed this back to our customer. Thank you!