Patent ID: 7672722

Claim:
A system for use in an implantable cardiac stimulation device, said system comprising: sensing circuitry operative to sense electrical cardiac signals indicative of possible first-chamber cardiac depolarization events and possible second-chamber cardiac depolarization events, to generate corresponding first-chamber event interrupts and second chamber event interrupts in response thereto, and to output second chamber event interrupts without delay, the sensing circuit comprising atrial channel sensing circuitry for sensing atrial channel signals indicative of possible atrial depolarization events corresponding to first-chamber cardiac depolarizations and generating atrial depolarization event interrupts corresponding to first-chamber event interrupts in response thereto; and ventricular channel sensing circuitry for sensing ventricular channel signals indicative of possible ventricular depolarization events corresponding to second-chamber cardiac depolarizations and generating ventricular depolarization event interrupts corresponding to second-chamber event interrupts in response thereto; a microcontroller operative to control cardiac stimulation therapy in response to the interrupts; and a quarantine circuit interposed between the sensing circuitry and the microcontroller and operative to delay receipt of a first-chamber event interrupt by the microprocessor for at least a predetermined time period and to prevent receipt of the first-chamber event interrupt by the microprocessor if a second-chamber event interrupt is generated during the time period, the quarantine circuit comprising a timer for timing the predetermined time period; and a hardware state machine operative, in response to receipt of an atrial depolarization event interrupt, to forward the atrial interrupt to the microcontroller after the timer expires so long as no ventricular depolarization event interrupt is received before the timer expires; wherein the microcontroller is further operative to process the first-chamber event interrupt and the second-chamber event interrupt, taking into account the predetermined time period, to determine a cardiac signal interval and the state machine is operative to selectively transition among an idle state, a quarantine state, a true atrial depolarization event state, and a false atrial depolarization event state.