A hardware-based state machine is provided for use in a pacemaker or other implantable cardiac stimulation device for use in discriminating near field signals sensed in the atria from far field signals emanating from the ventricles. The state machine is interposed between atrial/ventricular channel sense amplifiers and a main microcontroller of the device. The state machine operates to quarantine each P-wave interrupt received from the atrial channel sense amplifier to determine whether the P-wave interrupt corresponds to a true P-wave (i.e. a near field P-wave) as opposed to a false P-wave (i.e. a far field R-wave). Interrupts corresponding to true P-waves are forwarded by the quarantine circuit to the microcontroller after a short time delay. Interrupts corresponding to false P-waves are not forwarded. Hence, the microcontroller receives only true P-wave interrupts and therefore does not need to devote processing resources to distinguishing between true P-waves and false P-waves.