GD32F3x0 User Manual
73
CK_PLL or the clock of IRC48M by CK48MSEL bit in RCU_ADDCTL register.
The FWDGT is clocked by IRC40K clock, which is forced on when FWDGT started.
If the APB prescaler is 1, the timer clock frequencies are set to AHB frequency divide by 1.
Otherwise, they are set to the AHB frequency divide by half of APB prescaler.
4.2.2. Characteristics
4 to 32 MHz high speed crystal oscillator (HXTAL)
Internal 8 MHz RC oscillator (IRC8M)
Internal 48 MHz RC oscillator (IRC48M)
Internal 28 MHz RC oscillator (IRC28M)
32.768 KHz low speed crystal oscillator (LXTAL)
Internal 40KHz RC oscillator (IRC40K)
PLL clock source can be HXTAL or IRC8M or IRC48M
HXTAL clock monitor
4.2.3. Function overview
High Speed Crystal Oscillator (HXTAL)
The high speed crystal oscillator (HXTAL), which has a frequency from 4 to 32 MHz,
produces a highly accurate clock source for use as the system clock. A crystal with a specific
frequency must be connected and located close to the two HXTAL pins. The external resistor
and capacitor components connected to the crystal are necessary for proper oscillation.
Figure 4-3. HXTAL clock source
The HXTAL crystal oscillator can be switched on or off using the HXTALEN bit in the Control
register0, RCU_CTL0. The HXTALSTB flag in Control register 0, RCU_CTL0 indicates if the
high-speed external crystal oscillator is stable. When the HXTAL is powered up, it will not be
released for use until this HXTALSTB bit is set by the hardware. This specific delay period is
known as the oscillator “Start-up time”. As the HXTAL becomes stable, an interrupt will be
generated if the related interrupt enable bit HXTALSTBIE in the Interrupt register RCU_INT
is set. At this point the HXTAL clock can be used directly as the system clock source or the
PLL input clock.
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