ST STM32G0 1 Series Reference Manual page 179

RM0444
Bit 18 HSEBYP: HSE crystal oscillator bypass
Set and cleared by software to bypass the oscillator with an external clock. The external
clock must be enabled with the HSEON bit set, to be used by the device. The HSEBYP bit
can be written only if the HSE oscillator is disabled.
0: HSE crystal oscillator not bypassed
1: HSE crystal oscillator bypassed with external clock
Bit 17 HSERDY: HSE clock ready flag
Set by hardware to indicate that the HSE oscillator is stable.
0: HSE oscillator not ready
1: HSE oscillator ready
Note: Once the HSEON bit is cleared, HSERDY goes low after 6 HSE clock cycles.
Bit 16 HSEON: HSE clock enable
Set and cleared by software.
Cleared by hardware to stop the HSE oscillator when entering Stop, Standby, or Shutdown
mode. This bit cannot be reset if the HSE oscillator is used directly or indirectly as the system
clock.
0: HSE oscillator OFF
1: HSE oscillator ON
Bits 15:14 Reserved, must be kept at reset value.
Bits 13:11 HSIDIV[2:0]: HSI16 clock division factor
This bitfield controlled by software sets division factor of the HSI16 clock divider to produce
HSISYS clock:
000: 1
001: 2
010: 4
011: 8
100: 16
101: 32
110: 64
111: 128
Bit 10 HSIRDY: HSI16 clock ready flag
Set by hardware to indicate that HSI16 oscillator is stable. This bit is set only when HSI16 is
enabled by software by setting HSION.
0: HSI16 oscillator not ready
1: HSI16 oscillator ready
Note: Once the HSION bit is cleared, HSIRDY goes low after 6 HSI16 clock cycles.
Bit 9 HSIKERON: HSI16 always enable for peripheral kernels.
Set and cleared by software to force HSI16 ON even in Stop modes. The HSI16 can only
feed USART1, USART2, CEC and I2C1 peripherals configured with HSI16 as kernel clock.
Keeping the HSI16 ON in Stop mode allows avoiding to slow down the communication speed
because of the HSI16 startup time. This bit has no effect on HSION value.
0: No effect on HSI16 oscillator.
1: HSI16 oscillator is forced ON even in Stop mode.
RM0444 Rev 5
Reset and clock control (RCC)
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