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GD32E23x User Manual
76
Internal 8 MHz RC Oscillator (IRC8M)
The internal 8 MHz RC oscillator, IRC8M, has a fixed frequency of 8 MHz and is the default
clock source selection for the CPU when the device is powered up. The IRC8M oscillator
provides a lower cost type clock source as no external components are required. The IRC8M
RC oscillator can be switched on or off using the IRC8MEN bit in the Control register 0,
RCU_CTL0. The IRC8MSTB flag in the control register 0, RCU_CTL0 is used to indicate if
the internal RC oscillator is stable. The start-up time of the IRC8M oscillator is shorter than
the HXTAL crystal oscillator. An interrupt can be generated if the related interrupt enable bit,
IRC8MSTBIE, in the Interrupt register, RCU_INT, is set when the IRC8M becomes stable.
The IRC8M clock can also be used as the PLL input clock.
The frequency accuracy of the IRC8M can be calibrated by the manufacturer, but its
operating frequency is still less accurate than HXTAL. The application requirements,
environment and cost will determine which oscillator type is selected.
If the HXTAL or PLL is the system clock source, to minimize the time required for the system
to recover from the Deep-sleep Mode, the hardware forces the IRC8M clock to be the
system clock when the system initially wakes-up.
Phase Locked Loop (PLL)
The internal Phase Locked Loop, PLL, can provide 16~72 MHz clock output which is 2 ~32
multiples of a fundamental reference frequency of 4 ~ 32 MHz.
The PLL can be switched on or off by using the PLLEN bit in the Control register 0,
RCU_CTL0. The PLLSTB flag in the Control register 0, RCU_CTL0 will indicate if the PLL
clock is stable. An interrupt can be generated if the related interrupt enable bit, PLLSTBIE, in
the Interrupt register, RCU_INT, is set as the PLL becomes stable.
Internal 28 MHz RC Oscillator (IRC28M)
The internal 28 MHz RC Oscillator, IRC28M, has a fixed frequency of 28 MHz and dedicated
as ADC clock. The IRC28M RC oscillator can be switched on or off using the IRC28MEN bit
in the control register 1 (RCU_CTL1). The IRC28MSTB flag in the control register 1
(RCU_CTL1) is used to indicate if the internal 28M RC oscillator is stable. An interrupt can
be generated if the related interrupt enable bit, IRC28MSTBIE, in the Interrupt register,
RCU_INT, is set when the IRC28M becomes stable.
Low Speed Crystal Oscillator (LXTAL)
The low speed crystal or ceramic resonator oscillator, which has a frequency of 32,768 Hz,
produces a low power but highly accurate clock source for the real time clock circuit. The
LXTAL oscillator can be switched on or off using the LXTALEN bit in the backup domain
control register(RCU_BDCTL). The LXTALSTB flag in the backup domain control
register(RCU_BDCTL) will indicate if the LXTAL clock is stable. An interrupt can be
generated if the related interrupt enable bit, LXTALSTBIE, in the interrupt register RCU_INT
is set when the LXTAL becomes stable.