Past exam of the year 1 of the computer science course of the University of Oxford / 2022 / Digital systems, linear algebra, introduction to formal proof resit / 2 / c / Solution

A 32-bit free-running timer operates modulo $M=2^{32}$. If its previous reading is $p$, and the actual elapsed count is $d$ with $0\le d<M$, the new reading is $(p+d)\mathrm{mod}M$. The modulo-counter elapsed-time identity therefore givesUnsigned integer arithmetic in C reduces the subtraction modulo $2^{32}$, so the identity remains valid when a wrap occurs between pulses. For example, readings $2^{32}-8000$ and $8000$ give 16,000 ticks, hence one millisecond, rather than a negative interval.^[ref]

At the specified clock rate, the true wrap period is $2^{32}/16,000,000=268.435456$ seconds. The printed 256 seconds is inconsistent with the other two specifications. The implementation uses the stated 32-bit width and 16 MHz rate. Assuming consecutive handled pulses are less than 256 seconds apart is a sufficient conservative bound; the exact allowable bound is less than 268.435456 seconds.

This calculation requires less than one entire timer modulus of elapsed ticks between successive stored timestamps. If a whole wrap or several wraps elapse, identical register readings cannot reveal how many occurred; an overflow counter or an independent longer-range clock would be needed. Also assume pulse events are not lost and interrupt latency is constant or sufficiently small for the required accuracy: timestamps read in software measure handler-to-handler intervals, not perfect hardware capture times. Conversion to milliseconds occurs after modular subtraction, so separately rounded absolute timestamps cannot corrupt the wrap calculation.