Powered device including a detection signature resistor

1. A method comprising:

receiving a detection voltage at a powered device from a powered network;

applying the detection voltage to an external resistor to provide a detection signature to the powered network; and

applying a reference voltage to the external resistor to generate a reference current after the detection signature has been provided to the powered network;

wherein the reference current comprises a substantially fixed current.

2. The method of claim 1 , further comprising:

mirroring the reference current to provide the reference current to other components.

3. The method of claim 1 , wherein the reference voltage comprises an on-chip precision voltage.

4. The method of claim 1 , wherein the external resistor is coupled between a first pin responsive to a first voltage supply terminal and a second pin, wherein applying the detection voltage to the external resistor comprises:

activating a transistor to couple the second pin to a second voltage supply terminal; and

isolating a reference current generator from the second pin.

5. The method of claim 4 , wherein applying the reference voltage comprises:

deactivating the transistor to isolate the second pin from the second voltage supply; and

applying the reference voltage to the second pin to provide a desired voltage differential across the external resistor to generate the reference current.

6. A powered device comprising:

an interface responsive to a powered network;

a first node responsive to an external resistor;

a switch to selectively couple the first node to a voltage supply terminal;

a power over Ethernet (PoE) detection circuit responsive to a detection voltage from the interface and to selectively activate the switch to couple the first node to the voltage supply terminal; and

a reference current generator responsive to the first node to generate a reference current related to the external resistor when the switch is inactive;

wherein the external resistor comprises a resistor having a resistance value that is approximately 25 K-ohms.

7. The powered device of claim 6 , further comprising:

a diode responsive to the node to isolate the reference current generator from the first node when the switch is activated.

8. The powered device of claim 6 , wherein the reference current generator comprises:

a differential amplifier including a first input to receive a reference voltage, a second input to receive feedback from a second node, and an output; and

a transistor including a first terminal coupled to the second node, a control terminal coupled to the output, and a second terminal coupled to the voltage supply terminal.

9. The powered device of claim 8 , wherein the differential amplifier applies the reference voltage to the first node to produce a reference current related to the external resistor.

10. The powered device of claim 9 , further comprising a current mirror to mirror the reference current.

11. A circuit device comprising:

a first node responsive to an external circuit element;

a detection controller to detect a first process and to apply a first voltage to the first node responsive to detection of the first process;

a reference current generator to apply a second voltage to the first node to generate a reference current related to the external circuit element after the first process is completed; and

a switch including a first terminal coupled to the first node, a second terminal coupled to a voltage supply terminal, and a control terminal coupled to the detection controller;

wherein the detection controller comprises an open drain pull-down circuit coupled to the control terminal of the switch.

12. The circuit device of claim 11 , wherein the first node, the detection controller and the reference current generator are fabricated on an integrated circuit.

13. The circuit device of claim 11 , wherein the first process comprises a power over Ethernet powered device detection process.

14. The circuit device of claim 11 , wherein the external circuit element comprises a passive circuit element.

15. A power over Ethernet powered device comprising:

a dual-use pin responsive to an external circuit component;

first logic to apply a first voltage to the dual-use pin during a first process; and

second logic to apply a second voltage to the dual-use pin after the first process is complete;

wherein the first process comprises a powered device detection process and wherein the first logic applies the first voltage to the pin to generate a detection signature.

16. The power over Ethernet powered device of claim 15 , wherein the second voltage is applied to the dual-use pin to generate a reference current related to the external circuit component.

17. The power over Ethernet powered device of claim 15 , wherein the external circuit component comprises a passive circuit element.

18. The power over Ethernet powered device of claim 17 , wherein the external circuit component is comprises a resistor.

19. The power over Ethernet powered device of claim 15 , further comprising:

a diode to isolate the second logic from the pin during the first process.