Implementation of Vernier TDCs in 8-bit microcontrollers

This paper discusses the aspects of implementing a TDC with Vernier resolution in a microcontroller system. Results will show that the proposed solution have a potential time resolution corresponding exactly to the theoretically expected resolution (equal to the time difference in the Vernier clocks´ period), but also that there is an inherent sample-to-sample uncertainty due to the fact that microcontrollers cannot compare two (running) timer registers in hardware. The moment of coincidence of the Vernier clocks must be detected in software and that will generate an uncertainty in the coincidence detection that depends on the microcontroller architecture. In the design example proposed, a time resolution of 2 ns is achieved using a PIC microcontroller clocked with a 20 MHz. However, the proposed method is general and resolution is limited only to the frequency matching of the two Vernier clocks.