Low Frequency Clocks

The ring oscillator (RNG_OSC) clock is a low-frequency (256 kHz) and ultra-low power clock that is ON as long as the chip is enabled. This clock is used to clock the power sequencers at both the cold and warm boots of the SoC.

• 256 kHz nominal frequency
• Frequency division to 32 kHz
• Low current consumption
• Mapping clock to a pin
• Online calibration

The RNG_OSC is mainly used for clocking power sequencers during boot process and clocking the AON domain. The RNG_OSC can be enabled or disabled through the MCU_CLK_CTRL.RNG_2MHZ_CLK_EN register. When the RNG_OSC is disabled, the AON domain needs other slow clock source. The slow clock source of the AON domain can be selected through the MCU_CLK_CTRL.SLOW_CLK_SEL register. Developer is able to switch the slow clock source of the AON domain to RNG_32K in order to reduce the power consumption during sleep. However, the system wakeup and response speed will be reduced due to lower clock frequency.

By default, the system uses hardware to control signals. In order to improve the flexibility, the SoC provides overriding registers to allow software to control signals. The current state of RNG_OSC can be controlled by the overriding registers.

• Set status to the PMU_INTF_OVR_VAL_0. RNG_OSC_CLK_bit.
• Enable override function by setting PMU_INTF_OVR_EN_0. RNG_OSC_CLK_EN = 1.
• The status of RNG_OSC can be obtained by reading the PMU_INTF_OVR_RD0. RNG_OSC_CLK_EN bit.

The RNG_OSC cannot be regulated.

The SoC supports mapping the RNG_OSC or the RNG_32K from the RNG_OSC on pin. For detailed description of the mapping, refer to "Clock Mapping on Pin".

The SoC supports online calibration of the RNG_32K from the RNG_OSC through high precision crystal oscillator. Through the calibration module, developers can obtain the real frequency of the current RNG_32K clock. For detailed description of the calibration, refer to "Clock Calibration".

LFXO_32K clock is a crystal-based accurate clock (32.768 kHz) that is more stable than the low-frequency RC oscillator (LFRC_32K) across voltage and temperature. This clock is used for accurate sleep and wakeup.

In some cost-sensitive scenarios, the external crystal can be removed. In this situation, the SoC will not have LFXO_32K as the clock source.

• High precision clock
• Functional in sleep mode
• Mapping clock to a pin
• Online calibration

LFXO_32K can be enabled or disabled through the RF_REG_0[7] bit. The clock should not be used immediately after enabling. Sufficient time should elapse to wait for the LFXO_32K to stabilize.

LFXO_32K can be trimmed by adjusting the tuning capacitor through the RC_RTC_REG_0[29:24] bits. Due to the parasitic capacitance in the circuit, the real frequency of LFXO_32K may not be the nominal frequency (32.768kHz). Usually the system does not need to adjust the LFXO_32K to the nominal frequency. The system only needs to obtain the current frequency of the LFXO_32K. The SoC supports online calibration of the LFXO_32K through high frequency and precision crystal oscillator. Through the calibration module, developers can obtain the real frequency of the current LFXO_32K clock. For detailed description of the calibration, refer to "Clock Calibration".

The SoC supports mapping the LFXO_32K on pin. For detailed description of the mapping, refer to "Clock Mapping on Pin".

The low-frequency RC oscillator (LFRC_32K) is an internal 32 kHz clock used for generating various slow AON timer module clocks. In applications where no high-precision clock is required, the low-frequency RC oscillator (LFRC_32K) can replace the low-frequency crystal (LFXO_32K).

• 32 kHz low frequency RC oscillator
• Low frequency-temperature coefficient with automatic adaptation
• Low current consumption
• Fast startup
• Mapping clock to a pin
• Online calibration

LFRC_32K can be enabled or disabled through the RC_RTC_REG_0[15] bit. LFRC_32K has a fast startup time. It is not like LFXO_32K which needs to wait for stabilization after enabling. Since the RC oscillator is greatly affected by the environment, the actual frequency of LFRC_32K will fluctuate within a range.

LFRC_32K can be trimmed by adjusting the RC_RTC_REG_0[10:8] bits. Usually the system does not need to adjust the LFRC_32K to the nominal frequency (32kHz). The system only needs to obtain the current frequency of the LFRC_32K. The SoC supports online calibration of the LFRC_32K through high frequency and precision crystal oscillator. Through the calibration module, developers can obtain the real frequency of the current LFRC_32K clock. For detailed description of the calibration, refer to "Clock Calibration".

The SoC supports mapping the LFRC_32K on pin. For detailed description of the mapping, refer to "Clock Mapping on Pin".