Hse Clock; Figure 15. Hse/ Lse Clock Sources (Hardware Configuration) - ST STM32F446 Series Reference Manual

Reset and clock control (RCC)
The RCC feeds the external clock of the Cortex System Timer (SysTick) with the AHB clock
(HCLK) divided by 8. The SysTick can work either with this clock or with the Cortex clock
(HCLK), configurable in the SysTick control and status register.
FCLK acts as Cortex
®
Cortex -M4 with FPU technical reference manual.
6.2.1

HSE clock

The high speed external clock signal (HSE) can be generated from two possible clock
sources:
•
HSE external crystal/ceramic resonator
•
HSE external user clock
The resonator and the load capacitors have to be placed as close as possible to the
oscillator pins in order to minimize output distortion and startup stabilization time. The
loading capacitance values must be adjusted according to the selected oscillator.
External clock
Crystal/ceramic
resonators
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
120/1328
®
-M4 with FPU free-running clock. For more details, refer to the

Figure 15. HSE/ LSE clock sources (hardware configuration)

RCC clock control register
RCC clock control register (RCC_CR)
RM0390 Rev 4
OSC_OUT
(HI-Z)
External
source
OSC_IN OSC_OUT
C
L1
Load
capacitors
(RCC_CR). The external clock
Figure 15.
Figure 15. Refer to the electrical
RM0390
C
L2
indicates if the high-speed