Impedance sensor for ophthalmic device using shared antenna electrode

1. A contact lens system, comprising:

a shared antenna electrode;

an accommodation actuator to provide variable optical power; and

a controller coupled to the accommodation actuator and the shared antenna electrode, the controller including logic that when executed by the controller causes the contact lens system to perform operations comprising:

arbitrating access to the shared antenna electrode between an impedance sensor and a communication circuit;

selectively establishing an oscillator with the impedance sensor and the shared antenna electrode, wherein an impedance across the shared antenna electrode varies based upon an amount an eyelid overlaps the contact lens system when the contact lens system is worn on an eye;

correlating an oscillation condition of the oscillator to an accommodation setting; and

adjusting the variable optical power of the accommodation actuator based upon the accommodation setting.

2. The contact lens system of claim 1 , wherein arbitrating access to the shared antenna electrode between the impedance sensor and the communication circuit comprises:

monitoring the shared antenna electrode for wireless data and asserting a data detect signal in response to detecting a presence of the wireless data;

asserting an active signal when the impedance sensor is coupled to the shared antenna electrode to establish the oscillator;

signaling an enable signal to the impedance sensor to establish the oscillator with the shared antenna electrode in response to an assertion of the data detect signal; and

masking the data detect signal so as not to assert the enable signal while the active signal is asserted.

3. The contact lens system of claim 1 , wherein selectively establishing the oscillator with the impedance sensor and the shared antenna electrode comprises:

close circuiting three switches that interconnect components of the oscillator to form a Colpitts oscillator between the impedance sensor and the shared antenna electrode while the impedance sensor is accessing the shared antenna electrode.

4. The contact lens system of claim 3 , wherein the controller includes further logic that when executed by the controller causes the contact lens system to perform further operations comprising:

adjusting a bias setting of a variable current source in the impedance sensor;

resetting a voltage on a terminal of the shared antenna electrode with a fourth switch while the three switches are close circuited;

monitoring the oscillator for an onset of the oscillation condition; and

repeating the adjusting, the resetting, and the monitoring.

5. The contact lens system of claim 4 , wherein correlating the oscillation condition of the oscillator to the accommodation setting comprises:

correlating the bias setting of the variable current source upon the onset of the oscillation condition to the accommodation setting.

6. The contact lens system of claim 1 , wherein the shared antenna electrode includes a first terminal end coupled to a fixed voltage potential and a second terminal end shared between the impedance sensor and the communication circuit, wherein the oscillator established between the impedance sensor and the shared antenna electrode comprises a single-ended oscillator.

7. The contact lens system of claim 1 , wherein the controller includes further logic that when executed by the controller causes the contact lens system to perform further operations comprising:

harvesting power from a wireless signal incident upon the shared antenna electrode with a power rectifier;

decoupling a load of the power rectifier from the shared antenna electrode in an absence of detecting the wireless signal incident upon the shared antenna electrode.

8. The contact lens system of claim 7 , wherein decoupling the load of the power rectifier from the shared antenna electrode comprises:

rectifying the wireless signal;

low pass filtering the rectified wireless signal to generate a power switch enable signal; and

decoupling the load based upon the power switch enable signal.

9. The contact lens system of claim 8 , wherein decoupling the load based upon the power switch enable signal comprises:

open circuiting a power switch coupled between the shared terminal and the power rectifier responsive to the power switch enable signal.

10. The contact lens system of claim 1 , wherein the shared antenna electrode comprises an inductive loop that encircles the accommodation actuator.