Trigger uncertainty in period-to-code converters based on counters embedded in microcontrollers

Abstract

Abstract Microcontrollers with embedded counters offer a simple, compact interface for quasi-digital sensors whose output period depends on the measurand. However, as opposed to bench-top universal counters, manufacturers of microcontrollers do not specify the uncertainty inherent to measurements performed with embedded time-counters. This paper analyses the effects of input signal slew rate, signal-to-noise ratio and power supply noise on the uncertainty of periods measured with a PIC16F873 microcontroller. That uncertainty is evaluated by the standard deviation and histogram of the measured time periods. Two noise parameters have been considered: bandwidth (for Gaussian noise) or frequency (for sinusoidal interference), and intensity (rms amplitude). Trigger uncertainty increases for slow signal slew rate, and it is slightly larger when Gaussian noise or sine wave interference are added to the input signal than when they are added to the power supply. Interference whose frequency is close (but not equal) to that of the input signal yields larger trigger uncertainty than interference whose frequency is apart from that of the input signal. Trigger uncertainty resulting from Gaussian noise of a given rms power increases for reduced noise bandwidth.