Hsi Clock; Figure 10. Hse/ Lse Clock Sources - ST STM32F207 Series Reference Manual

RM0033
External clock
Crystal/ceramicr
esonators
External source (HSE bypass)
In this mode, an external clock source must be provided. You select this mode by setting the
HSEBYP and HSEON bits in the
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
External crystal/ceramic resonator (HSE crystal)
The HSE 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).
5.2.2

HSI clock

The HSI clock signal is generated from an internal 16 MHz RC oscillator and can be used
directly as a system clock, or 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.

Figure 10. HSE/ LSE clock sources

Hardware configuration
External
source
C
L1
RCC clock control register
Figure 10.
RCC clock control register (RCC_CR)
RM0033 Rev 9
Reset and clock control (RCC)
OSC_OUT
(HiZ)
OSC_IN OSC_OUT
C
L2
Load
capacitors
(RCC_CR). The external clock
Figure 10. Refer to the electrical
indicates if the high-speed
RCC clock interrupt register
RCC clock control
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