Patent ID: 7385491

Claim:
A tamper monitor circuit, comprising: a reference oscillator operable to supply a reference signal having a reference frequency; a detection oscillator configured to operate at a circuit temperature and to be energized with a supply voltage, the detection oscillator operable to supply a detection signal having a frequency that varies with variations in the circuit temperature, variations in the supply voltage, or both; and a comparison circuit coupled to receive the reference signal and the detection signal and operable, in response to the reference signal, to (i) selectively determine the frequency of the detection signal, (ii) determine a frequency difference between two or more of the determined frequencies, and (iii) supply a tamper detect signal if the determined frequency difference exceeds a predetermined difference threshold; wherein the comparison circuit includes a detection counter coupled to receive the detection signal and operable to generate a detection count value representative of a number of detection signal periods; and a detection count capture circuit coupled to the detection counter and configured to receive a count capture command, the count capture circuit operable, in response to the count capture command, to capture the detection count value.