The clock subsystem of {IDF_TARGET_NAME} is used to source and distribute system/module clocks from a range of root clocks. The clock tree driver maintains the basic functionality of the system clock and the intricate relationship among module clocks.
This document starts with the introduction to root and module clocks. Then it covers the clock tree APIs that can be called to monitor the status of the module clocks at runtime.
This section lists definitions of {IDF_TARGET_NAME}'s supported root clocks and module clocks. These definitions are commonly used in the driver configuration, to help select a proper source clock for the peripheral.
Root clocks generate reliable clock signals. These clock signals then pass through various gates, muxes, dividers, or multipliers to become the clock sources for every functional module: the CPU core(s), Wi-Fi, Bluetooth, the RTC, and the peripherals.
The about {IDF_TARGET_RC_FAST_ADJUSTED_FREQ} MHz signal output is also passed into a configurable divider, which by default divides the input clock frequency by 256, to generate a ``RC_FAST_D256_CLK``.
The exact frequency of ``RC_FAST_CLK`` cannot be computed in runtime through calibration, but it is still possible to get its frequency through an oscilloscope or a logic analyzer by routing the clock signal to a GPIO pin.
This RC oscillator generates a about {IDF_TARGET_RC_SLOW_VAGUE_FREQ}kHz clock signal output as the ``RC_SLOW_CLK``. The exact frequency of this clock can be computed in runtime through calibration.
The clock source for this ``XTAL32K_CLK`` can be either a 32 kHz crystal connecting to the ``32K_XP`` and ``32K_XN`` pins or a 32 kHz clock signal generated by an external circuit. The external signal must be connected to the ``32K_XN`` pin. Additionally, a 1 nF capacitor must be placed between the ``32K_XP`` pin and ground. In this case, the ``32K_XP`` pin cannot be used as a GPIO pin.
The clock source for this ``XTAL32K_CLK`` can be either a 32 kHz crystal connecting to the ``XTAL_32K_P`` and ``XTAL_32K_N`` pins or a 32 kHz clock signal generated by an external circuit. The external signal must be connected to the ``XTAL_32K_P`` pin.
A clock signal generated by an external circuit can be connected to {IDF_TARGET_OSC_SLOW_PIN} to be the clock source for the ``RTC_SLOW_CLK``. This clock can also be calibrated to get its exact frequency.
Typically, the frequency of the signal generated from an RC oscillator circuit is less accurate and more sensitive to the environment compared to the signal generated from a crystal. {IDF_TARGET_NAME} provides several clock source options for the ``RTC_SLOW_CLK``, and it is possible to make the choice based on the requirements for system time accuracy and power consumption. For more details, please refer to :ref:`rtc-clock-source-choice`.
{IDF_TARGET_NAME}'s available module clocks are listed in :cpp:type:`soc_module_clk_t`. Each module clock has a unique ID. You can get more information on each clock by checking the documented enum value.
The clock tree driver provides an all-in-one API to get the frequency of the module clocks, :cpp:func:`esp_clk_tree_src_get_freq_hz`. This function allows you to obtain the clock frequency at any time by providing the clock name :cpp:enum:`soc_module_clk_t` and specifying the desired precision level for the returned frequency value :cpp:enum:`esp_clk_tree_src_freq_precision_t`.
