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DP83869HM: PHY functional mode and address strapping

Olivier Frenette
Prodigy 50 points
Part Number: DP83869HM
Other Parts Discussed in Thread: DP83869

Good day,

I am using the DP83869 in a design, and am currently struggling with strapping for the address and functional mode. The problem I have is that this PHY is connected to a microcontroller that have pull-ups enabled at boot. The pins can provide between 68uA to 250uA of current. Only after booting can I disable these pull-ups, causing the PHY to boot with the wrong address and functional mode, as I did not ass any external resistors : 

 

For the functional mode, I would simply add 2K pull-down resistors on RX_D2 and RX_D3. Note in my case that JTAG_TDO/GPIO_1 is not connected anywhere. The TI spec recommends a value of 2.49kohm, though:

Now, with the strapping of the address, it is not so simple, as this time these pins have 4 possibles modes. Even without adding any resistors, I can theoretically go off-spec if my pull-up current is 250uA in the 9Kohm internal resistor of the PHY : 250u*9K = 2.25V. My VDDIO is 2.5V, so this is a 0.9VDDIO, exceeding the recommended max of 0.888VDDIO. Initially, I wanted to use mode 0 for both pins, but this would force me to use a small resistor (arround 1K) as pull down. I don't know if that is acceptable from the PHY standpoint. I could also use mode 3 and use an external pull-up, but again, I would be exceeding the maximum 0.888VDDIO requirement.

So my questions are the following : 

1. Is it OK to use a 2K resistor instead of the 2.49K as pull-down for the functional mode straps?

2. How low can a strapping pull-down resistor be?

3. In 4-level strap, what requirement prevents exceeding 0.888VDDIO for mode 3?

Thank you!

Olivier

  • 0
    TI__Expert 7040 points

    Hi Oliver, 

    To answer your questions:

    1. Using a 2k pull-down resistor is acceptable. The TI recommended resistors are a way to quickly select common resistor values but not the only choices. 

    2. A 1k pull-down resistor should also work, as you get lower the current draw will increase but the bootstrap looks for the voltage on the pin at the time of power-up or reset. 

    3. The 0.888*VDDIO Vmax threshold is a recommended limit but because there is no mode you can strap into above mode 3, you can exceed mode 3's Vmax threshold. 

    Regards,

    Justin