I am trying to interface a GigaBlox Nano Ethernet switch with the TI DP83848TSQ/NOPB Ethernet PHY chip. With the current circuit, the red LED turns on, indicating that the PHY is operating in 100BASE-T mode, and it blinks in certain intervals. However, I am unable to transmit or receive any data over the link. I have attached the Ethernet PHY schematic from the Pixhawk Standard which is in question.
I would like to understand whether additional components are required on the PHY side for proper operation. Specifically:
Are common-mode chokes (F19, F20) required on the differential pairs?
Are AC coupling capacitors (C71, C70, C77, C76) mandatory between the PHY and the magnetics?
Are 100-ohm differential termination resistors (R93, R94, R95, R96) required at the PHY side?
Please let me know if my current implementation is missing any critical components or if there are specific requirements when interfacing the DP83848 PHY with the GigaBlox Nano switch.
The issue here is that the DP83848TSQ is a current mode PHY. It’s designed to sink and source current through a transformer to generate the correct voltages for the ethernet signals.
This is in contrast to a voltage mode PHY, which directly generates the voltage levels for ethernet.
GigaBlox Nano is a voltage mode PHY, and can generally be interfaced with other voltage mode PHYs without needing transformers (transformerless ethernet).
But a current mode PHY will always need a transformer to work, which you don’t have in your design. So I wouldn’t expect this to work. There’s actually a related forum post on this which I’ve linked below.
To solve this, either add transformers to your design or switch to a voltage mode PHY (There are many options).
Side note - How do I know this is a current mode PHY? Looking at the example schematic in the datasheet for the DP83848TSQ, we can see that the center taps of the transformers are connected to 3.3V. That’s a key giveaway.
