Change CPU frequency

[pablotix20]

Add ESP library command for system_update_cpu_freq

[igrr]

CPU frequency is set automatically depending on what is chosen in Tools > CPU Frequency menu in the IDE. If you want to override this behaviour for some reasons, you may provide an alternative implementation for preloop_update_frequency function inside your sketch. This function is declared as weak, so you can override it without modifying the library.
Also if you need to call system_update_cpu_freq, just include user_interface.h and call it directly. I think this function doesn't need a wrapper.

[dadler]

I've noticed that after using the Arduino IDE to set rate to 160 MHZ and then back to 80 MHZ, it doesn't properly return to 80 MHZ until I cycle power. Recompiling properly rebuilds with F_CPU set to 80 MHZ but preloop_update_frequency doesn't clear the clock frequency bit. Note that it's a no-op when F_CPU isn't 160 MHZ:

void preloop_update_frequency() {
#if defined(F_CPU) && (F_CPU == 160000000L)
REG_SET_BIT(0x3ff00014, BIT(0));
ets_update_cpu_frequency(160);
#endif
}

That caused pulseIn() to return values twice as large as it should (until I power cycled).