MHZ macro

THIS FILE IS CONSIDERED AS A PRIVATE HEADER FILE **************** IT IS NOT RECOMMENDED TO USE THE APIS IN THIS FILE DIRECTLY IN APPLICATIONS *** @file rtc.h Low-level RTC power, clock, and sleep functions. Functions in this file facilitate configuration of ESP32's RTC_CNTL peripheral. RTC_CNTL peripheral handles many functions: - enables/disables clocks and power to various parts of the chip; this is done using direct register access (forcing power up or power down) or by allowing state machines to control power and clocks automatically - handles sleep and wakeup functions - maintains a 48-bit counter which can be used for timekeeping These functions are not thread safe, and should not be viewed as high level APIs. For example, while this file provides a function which can switch CPU frequency, this function is on its own is not sufficient to implement frequency switching in ESP-IDF context: some coordination with RTOS, peripheral drivers, and WiFi/BT stacks is also required. These functions will normally not be used in applications directly. ESP-IDF provides, or will provide, drivers and other facilities to use RTC subsystem functionality. The functions are loosely split into the following groups: - rtc_clk: clock switching, calibration - rtc_time: reading RTC counter, conversion between counter values and time - rtc_sleep: entry into sleep modes - rtc_init: initialization