DP83848K Link/Speed LED status question

Hi Eric,

The Link/Speed LED should be OFF when you are not linked and have not completed Auto-Negotiation. 

When you have no connection you are seeing the LED ON? This should not happen. 

Can you read register 0x0001 multiple times when you do not have a cable connected to the RJ45?

What is the schematic portion of the 848 in your system?

Kind regards,

Ross

Hi Ross,

The below is register 0X0001 value for your reference.

U-Boot# mii read 1 1
7849

U-Boot# mii read 1 1
7849

U-Boot# mii read 1 1
7849

The attach file is DP83848 schematic for your check again.

Thanks.DP83848_10_100 sch.pdf

Hi Eric,

Is R81 populated in your system? It seems like the driver may be latching in an incorrect value at start up for the device. Is your PHY strapping in the correct values for auto-negotiation? Read register address 0x4 to see that the auto negotiation advertisement register is set as expected.

There also may be an issue of a mismatch in voltage since the LED's + connection is to a different 3.3V bus. Can you change this from VDD_3V3B to LAN1_3V3?

Best Regards,

Hi Rob,

R81/R83 already remove from our system.

The below is 0X4 value for your reference again and VDD_3V3B & LAN1_3V3 is the same voltage as below.

U-Boot# mii read 1 4
01E1
U-Boot# mii read 1 4
01E1

So may we know what's happen for LED_SPEED status that ON before auto-negotiation?

Thanks.

Regards,

Eric

Hi Eric,

R81 should be populated in your system. Please try placing a resistor on R81.

Thank you for pointing out R85, I had missed that.

Your auto-negotiation registers are set to the correct value.

Are you putting Ethernet data into the PHY when it is not connected? Data could be coming from your MAC, trying to transmit data while the link is down. Place the device in MII isolate mode using bit 10 of the BMCR to see if this causes the LED to extinguish.

Best Regards,

Hi Rob,

Yes, we also testing put R81 at my system then get the same result -- LED_SPEED LED always on without RJ45 connection.

We also try MII isolate mode using bit 10 of BMCR but still got the same status -- LED_SPEED LED always on.

Would you kindly help to double confirm again?

Thanks.

Regards,

Eric

Hi Rob,
Would you kindly check again?
Thanks.

Regards,
Eric

Hello Eric,

The SPEED LED will be always on until the PHY enters 10M link, even if no link is present.

When no link is present, the SPEED_LED will be on.

The only way to change this behavior is to:
1. Disable auto-negotiation and force the PHY into 10M mode.
or
2. Force the LED_SPEED value to 0 using register address 0x18 written to a value of 0x24.

Both of these methods will not automatically turn the LED back on when 100M link is established.

Regards,