ST STM32F101 Series Reference Manual page 55

RM0008
Bit 18 HSEBYP External High Speed clock Bypass
Set and reset by software in debug for bypassing oscillator with external clock. This bit can be
written only if the external 1-25 MHz oscillator is disabled.
0: external 1-25 MHz oscillator not bypassed
1: external 1-25 MHz oscillator bypassed with external clock
Bit 17 HSERDY External High Speed clock ready flag
Set by hardware to indicate that external 1-25 MHz oscillator is stable. This bit needs 6 cycles of
external 1-25 MHz oscillator clock to fall down after HSEON reset.
0: external 1-25 MHz oscillator not ready
1: external 1-25 MHz oscillator ready
Bit 16 HSEON External High Speed clock enable
Set and reset by software.
Reset by hardware to stop the external 1-25MHz oscillator when entering in Stop and Standby
mode. This bit can not be reset if the external 1-25 MHz oscillator is used directly or indirectly as
system clock or is selected to become the system clock.
0: HSE oscillator OFF
1: HSE oscillator ON
Bits 15:8 HSICAL[7:0] Internal High Speed clock Calibration
These bits are initialized automatically at startup.
Bits 7:3 HSITRIM[4:0] Internal High Speed clock trimming
These bits provide an additional user-programmable trimming value that is added to the
HSICAL[7:0] bits. It can be programmed to adjust to variations in voltage and temperature that
influence the frequency of the internal HSI RC.
The default value is 16, which, when added to the HSICAL value, should trim the HSI to 8 MHz at T
= 25 °C. The HSI RC frequency increases when the HSICAL value increases and decreases when
the HSICAL value decreases. The trimming step is 40 kHz between two consecutive HSICAL steps.
Bit 2 Reserved, always read as 0.
Bit 1 HSIRDY Internal High Speed clock ready flag
Set by hardware to indicate that internal 8 MHz RC oscillator is stable. This bit needs 6 cycles of the
internal 8 MHz RC oscillator clock to fall down after HSION reset.
0: internal 8 MHz RC oscillator not ready
1: internal 8 MHz RC oscillator ready
Bit 0 HSION Internal High Speed clock enable
Set and reset by software.
Set by hardware to force the internal 8 MHz RC oscillator ON when leaving Stop and Standby mode
or in case of failure of the external 1-25 MHz oscillator used directly or indirectly as system clock.
This bit can not be reset if the internal 8 MHz RC is used directly or indirectly as system clock or is
selected to become the system clock.
0: internal 8 MHz RC oscillator OFF
1: internal 8 MHz RC oscillator ON
Reset and clock control (RCC)
A
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