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

Microcontrollers with embedded timers/counters offer a simple, compact, and low-cost interface to convert from time period to digital code. This makes them suitable to interface quasi-digital sensors that yield a signal whose period is modulated by the measurand. The accuracy, however, is limited by the uncertainty of the counting process, which depends on trigger uncertainty and other factors. This paper analyzes the effect that the counting program running in a PIC16F873 microcontroller has on the uncertainty of the measured time periods. Because different instructions imply different power consumption, when the microcontroller program involves a loop, the power supply rails display periodic transients corresponding to the more power-demanding instructions. These transients increase the internal noise at trigger inputs, and if they are spaced τ μs, the measured period when the input signal does not have a fast slew rate is also quantified to ± τ μs.