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DP83848I and Bi-Color-LED

Steffen Koepf
Prodigy 40 points
Other Parts Discussed in Thread: SN74LVC2G125, SN74AHC1G125

Hello,

i would like to connect a Bi-Color-LED to the DP83848,

so that yellow signals "Link and 10MBit" and

green "Link and 100MBit".

I think it is best to add a 74AHC1G125 single gate, feeded with LED_LINK as output enable and LED_SPEED as input.

The output connects to one pin of the LED.

Is it now possible to connect the LED_SPEED pin to the other LED pin (with a resistor)?

What does the LED_SPEED Pin do when there is 10MBit operation? Is it then tristate or driven active?

Thanks in advance.

  • 0
    TI__Mastermind 31600 points

    The LED pins will actively drive the signals to the desired state.  As noted in the datasheet, the pins will initialize as active high or active low drivers based on the external strap resistors connected to the pins.  The LED pins include internal pull ups so by default (without any external pulls), they will initialize as active low drivers. 

    The speed LED indicates when the device is configured for 100M mode.  Therefore, for an active low driver, the speed LED will drive the pin to 0V in 100M mode.  Similarly, for an active low driver, the speed LED will drive the pin to the supply (nominally 3.3V) in 10M mode. 

    Note that the device must be able to receive 100M scrambled idles in order to support parallel detection when it is configured for auto-negotiation.  Therefore when configured for auto-negotiation, the device will assert the speed LED by default until link has been established. 

    For an active low driver, the link LED will drive the pin to 0V when link has been established.  Similarly, for an active low driver, the link LED will drive the pin to the supply (nominally 3.3V) when there is no link. 

    Assuming you plan to use active low drivers, I would suggest a configuration similar to the drawing below:

    Using a Dual Bus Buffer Gate with 3-State Outputs (e.g. SN74LVC2G125) or a pair of Single Bus Buffer Gates with 3-State Output (e.g. SN74AHC1G125) will provide a more symmetric solution and should avoid any issues with strap initialization of the LED pins.

    A pull up resistor on the speed LED would not be required.

    A pull up resistor on the link LED would be recommended to ensure the high impedance output state during power up or power down. 

    Does this configuration look like something that would work in your system?

    Patrick