GD32F403xx User Manual
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to indicate if the internal 48M RC oscillator is stable. An interrupt can be generated if the
related interrupt enable bit, IRC48MSTBIE, in the RCU_ADDINT register, is set when the
IRC48M becomes stable. The IRC48M clock is used for the clocks of USBFS.
The f requency accuracy of the IRC48M can be calibrated by the manuf acturer, but its
operating frequency is still not enough accurate because the USB need the f requency must
between 48MHz with 500ppm accuracy. A hardware automatically dynamic trim performed in
CTC unit adjust the IRC48M to the needed frequency.
Phase locked loop (PLL)
There are three internal phase locked loop, the PLL, PLL1 and PLL2.
The PLL can be switched on or of f by using the PLLEN bit in the RCU_CTL register. The
PLLSTB f lag in the RCU_CTL Register will indicate if the PLL clock is stable. An interrupt can
be generated if the related interrupt enable bit, PLLSTBIE, in the RCU_INT register, is set as
the PLL becomes stable.
The PLL1 can be switched on or off by using the PLL1EN bit in the RCU_CTL register. The
PLL1STB f lag in the RCU_CTL register will indicate if the PLL1 clock is stable. An interrupt
can be generated if the related interrupt enable bit, PLL1STBIE, in the RCU_INT register, is
set as the PLL1 becomes stable.
The PLL2 can be switched on or off by using the PLL2EN bit in the RCU_CTL register. The
PLL2STB f lag in the RCU_CTL register will indicate if the PLL2 clock is stable. An interrupt
can be generated if the related interrupt enable bit, PLL2STBIE, in the RCU_INT register, is
set as the PLL2 becomes stable.
The three PLLs are closed by hardware when entering the Deepsleep/Standby mode or
HXTAL monitor fail when HXTAL used as the source clock of the PLLs.
Low speed crystal oscillator (LXTAL)
The low speed external crystal or ceramic resonator oscillator, which has a f requency of
32,768 Hz, produces a low power but highly accurate clock source f or 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 f lag 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.
Select external clock bypass mode by setting the LXTALBPS and LXTALEN bits in the backup
domain control register (RCU_BDCTL). The CK_LXTAL is equal to the external clock which
drives the OSC32IN pin.
Internal 40K RC oscillator (IRC40K)
The internal RC oscillator has a f requency of about 40 kHz and is a low power clock source
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