Patent ID: 8115618

Claim:
An ingestible event marker comprising: an identifier operative to emit a signal when activated at a target site, wherein the identifier comprises a signal generation circuit comprising a radio frequency identification (RFID) antenna, wherein the signal generation circuit comprises a voltage controlled oscillator (VCO) and a digital control circuit coupled to the VCO, the digital control circuit assigned an address, the digital control circuit to control the VCO with a control signal associated with the address; wherein the digital control circuit further comprises: a digital counter circuit comprising an input portion coupled to the VCO and an output portion, wherein the VCO is operative to generate a first clock signal and a second clock signal, wherein the second clock signal is an inverted version of the first clock signal, and wherein the first and second clock signals are coupled to the digital counter circuit, wherein the digital counter circuit is operative to count a number of clock cycles received from the VCO, store the count in at least one register, and provide the count to the output portion of the digital counter circuit.