GD32F403xx User Manual
79
speed external crystal oscillator is stable. When the HXTAL is powered up, it will not be
released f or 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.
Select external clock bypass mode by setting the HXTALBPS and HXTALEN bits in the
control register RCU_CTL. During bypass mode, the signal is connected to OSCIN, and
OSCOUT remains in the suspended state, as shown in
Figure 5-4. HXTAL clock source
. The CK_HXTAL is equal to the external clock which drives the OSCIN
pin.
Figure 5-4. HXTAL clock source in bypass mode
OSCI N
OSCO UT
Internal 8M RC oscillators (IRC8M)
The internal 8M 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 RCU_CTL.
The IRC8MSTB f lag in the control register RCU_CTL is used to indicate if the internal 8M 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 clock interrupt register, RCU_INT, is set when the IRC8M becomes stable. The IRC8M
clock can also be used as the system clock source or the PLL input clock.
The f requency accuracy of the IRC8M can be calibrated by the manufacturer, but its operating
f requency 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.
Internal 48M RC oscillators (IRC48M)
The internal 48M RC oscillator, IRC48M, has a f ixed f requency of 48 MHz. The IRC48M
oscillator provides a lower cost type clock source as no external components are required
when USBFS used. The IRC48M RC oscillator can be switched on or off using the IRC48MEN
bit in the RCU_ADDCTL register. The IRC48MSTB flag in the RCU_ADDCTL register is used
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