Hsi Clock; Figure 12. Hse/ Lse Clock Sources - ST STM32F101 series Reference Manual

RM0008
Clock source
External clock
Crystal/ceramicr
esonators
External source (HSE bypass)
In this mode, an external clock source must be provided. It can have a frequency of up to
50 MHz. You select this mode by setting the HSEBYP and HSEON bits in the
register (RCC_CR). The external clock signal (square, sinus or triangle) with ~50% duty
cycle has to drive the OSC_IN pin while the OSC_OUT pin should be left hi-Z. See
Figure 12.
External crystal/ceramic resonator (HSE crystal)
The 3 to 25 MHz external oscillator has the advantage of producing a very accurate rate on
the main clock.
The associated hardware configuration is shown in
characteristics section of the datasheet for more details.
The HSERDY flag in the
external oscillator is stable or not. At startup, the clock is not released until this bit is set by
hardware. An interrupt can be generated if enabled in the
(RCC_CIR).
The HSE crystal can be switched on and off using the HSEON bit in the
register (RCC_CR).
8.2.2

HSI clock

The HSI clock signal is generated from an internal 8 MHz RC Oscillator and can be used
directly as a system clock or divided by 2 to be used as PLL input.
The HSI RC oscillator has the advantage of providing a clock source at low cost (no external
components). It also has a faster startup time than the HSE crystal oscillator however, even
with calibration the frequency is less accurate than an external crystal oscillator or ceramic
resonator.
Connectivity line devices: reset and clock control (RCC)

Figure 12. HSE/ LSE clock sources

Hardware configuration
External
source
C
L1
Clock control register (RCC_CR)
DocID13902 Rev 15
OSC_OUT
(HiZ)
OSC_IN OSC_OUT
C
L2
Load
capacitors
Figure 12. Refer to the electrical
indicates if the high-speed
Clock interrupt register
Clock control
Clock control
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