Reference timer for frequency measurement in a microprocessor

Abstract

A timer on a microprocessor includes a vibrator formed by a comparator, a capacitor, three reference voltages, and switched current sources, which charge and discharge the capacitor. The vibrator oscillates at two different amplitudes to generate two timing windows, one at high amplitude and the other at low amplitude. A counter counts incoming clocks and times out after a fixed number of vibrator oscillations. Logic starts the timing windows and subtracts incoming clock measurements taken during the two timing windows. The logic subtraction cancels errors accumulated from the multiple ramps of the capacitor in the vibrator. The subtraction allows more precise measurement of incoming clocks. If the clocks counted exceed a threshold value, the microprocessor shuts down due to over clocking.