Gigablox nano switch compatible with PHY DP83848TSQ/NOPB Chip?

BotBlox Hardware
1

Hello,

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.

Thank you.

Ethernet TI PHY Pixhawk schematic
Ethernet TI PHY Pixhawk schematic1936×960 308 KB

2

Hi Sid,

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.

image
image1502×906 37.1 KB

1 Like
3

Thank you for the info on current-mode vs voltage-mode Ethernet drivers. It was very helpful. I’m still facing an issue and would really appreciate a second set of eyes on my schematic.

Changes made:

  • We removed common-mode chokes F19 and F20 from the Ethernet interface.
  • These were replaced with an Ethernet transformer (IC3: S558-5999-T7-F).
  • The transformer is air-wired exactly as shown in the attached schematic.
  • On the line side, I have AC coupling capacitors placed after the transformer, as shown in the previously shared image.

What we know so far:

  • The Ethernet switch shows a solid red LED blinking at a fixed interval.
  • PHY status registers indicate that the link is up.
  • However, no data can be transmitted or received.

Could you please review the attached schematic and let me know:

  • If there are any obvious issues with the transformer wiring.
  • If the placement or value of AC coupling capacitors (0603YC104KAT2A) could be problematic.
  • If there are any missing biasing, termination, or common-mode requirements.
  • Or anything else that could explain link-up without data transfer.

Any insights or suggestions would be greatly appreciated. Thank you.

PHY Transformer schematic
PHY Transformer schematic1000×837 98.2 KB

4

Your circuit has some issues but I’m not sure if they’re show stoppers.

  1. You use the same ground on the line side of the transformer as you do on the chip side. It should be an isolated ground.
  2. You use 22nF on the center tap, it should be closer to 0.1uF.
  3. I don’t see your AC capacitors, where did you put them? You don’t need them because GigaBlox Nano already has AC capacitors.

What does your routing on your PCB look like? And how do you connect it to GigaBlox Nano? What connects to what?

5

Thank you for your quick response.

  1. Regarding AC caps - they are connected in series after transformer to gigablox nano switch.
    PHY → Termination resistors → Transformer → AC capacitors → Gigablox Switch Port 1

From the previous attached schematic:
100BaseT_OUT_TX_P → AC cap → Pin 36 of connector LSHM-120-02.5-L-DV-A-S-K-TR
100BaseT_OUT_TX_N → AC cap → Pin 34 of connector LSHM-120-02.5-L-DV-A-S-K-TR
100BaseT_OUT_RX_P → AC cap → Pin 32 of connector LSHM-120-02.5-L-DV-A-S-K-TR
100BaseT_OUT_RX_N → AC cap → Pin 30 of connector LSHM-120-02.5-L-DV-A-S-K-TR

So I was thinking to use a voltage mode PHY chip that is compatible with Gigablox nano switch. Can you please suggest the most used and tested 10/100 Base-T PHY chip (supports RMII) that is compatible with Gigablox Nano?

I really appreciate the help. Thank you.

6

First let’s check your pinout, assuming you’re using port 1 on GigaBlox Nano).

100BASE-T_TX_P → A_P = Pin 36
100BASE-T_TX_N → A_N = Pin 34
100BASE-T_RX_P → B_P = Pin 32
100BASE-T_RX_N → B_N = Pin 30

Your pin mapping looks good.

The AC capacitors are not ideal but probably not a big problem.

I think the issue may be related to your layout; in that case changing to a voltage mode chip won’t help. Can you share some screenshots of your layout and routing so I can check?

I can’t suggest a specific 10/100BASE-T PHY, as we don’t have an example schematic for that, but happy to verify one that you select based on your requirements.

7

Wait, did you try connecting this to to another ethernet device? Can we verify your RMII PHY<->MAC connection is actually working?