Test points on an integrated circuit chip, especially points subject to IR voltage drop along power supply rails, are coupled to comparators controlled by an automatic test controller, all included on the chip. Each test point can have one or more comparators and one or more reference voltages over a testing range. A change of state at a comparator sets a latch that is read and reset by the on-chip automatic test controller during test intervals. The automatic test controller can coordinate with external automatic test equipment that applies stimulus signals to the chip during testing. The greatest voltage drop during a test interval is determined from the latched output of the switched comparator coupled to the lowest reference voltage. The setting and resetting of the latch can be gated through a selectable delay so as to discriminate for excursions that persist for a longer or shorter time.