Patent ID: 7652578

Claim:
A circuit for detecting the proximate location of an external communication circuit with a near field communication device, the circuit comprising: a communication coil configured for near field communication at a communication frequency; a communication circuit coupled to the communication coil for transmitting data at the communication frequency, the communication circuit having at least a communication mode when transmitting data an idle mode when not communicating; a control circuit; a capacitor configured to be selectively coupled by the control circuit with the communication coil; an oscillator, responsive to the control circuit, configured to selectively apply a probing signal to the communication coil at a probing frequency; and a comparator coupled to the communication coil for sensing the probing signal across the capacitor; wherein the control circuit is configured to couple the capacitor with the communication coil and to cause the oscillator to apply the probing signal when the communication circuit is in the idle mode; wherein the comparator comprises an output coupled to the control circuit, the output configured to transition from a first state to a second state when a inductively coupled object is proximately located with the communication coil; wherein the presence of an inductively coupled object causes the probing signal across the capacitor to change by at least a predetermined amount thereby indicating the presence of the inductively coupled object.