The clock uses the main clock of a T4.x (600MHz) as time-base for a std::chrono compatible clock. Resolution is 1/600MHz = 1.66ns.
By default HiResClock::begin() synchronizes the clock with the RTC.
A few useless usage examples
#include "HiResClock.h"
using namespace std::chrono;
void setup()
{
HiResClock::begin(); // start the clock and sync to rtc (works with and without battery)
auto t0 = HiResClock::now();
delay(1234);
auto t1 = HiResClock::now();
nanoseconds delta_in_ns = duration_cast<nanoseconds>(t1 - t0);
Serial.printf("delta: %0.9fs\n", delta_in_ns.count() / 1E9);
// transform current time to standard time_t value
auto now = HiResClock::now();
time_t t = HiResClock::to_time_t(now);
Serial.print(ctime(&t));
}
auto then = HiResClock::now() + 10h;
void loop()
{
if (HiResClock::now() < then)
{
Serial.println("still waiting...");
};
constexpr unsigned delaytime = duration_cast<milliseconds>(0.34h + 2min + 3434ns).count(); // let the compiler calculate that
delay(delaytime);
}