Clocks; Hse Osc Clock; Figure 18. Hse External Clock; Figure 19. Hse Crystal/Ceramic Resonators - ST STM32F4 Series Getting Started

Clocks

4 Clocks
Three different clock sources can be used to drive the system clock (SYSCLK):
•
HSI oscillator clock (high-speed internal clock signal)
•
HSE oscillator clock (high-speed external clock signal)
• PLL clock
The devices have two secondary clock sources:
•
32 kHz low-speed internal RC (LSI RC) that drives the independent watchdog and,
optionally, the RTC used for Auto-wakeup from the Stop/Standby modes.
• 32.768 kHz low-speed external crystal (LSE crystal) that optionally drives the real-time
clock (RTCCLK)
Each clock source can be switched on or off independently when it is not used, to optimize
the power consumption.
Refer to the reference manual for the description of the clock tree.
4.1

HSE OSC clock

The high-speed external clock signal (HSE) can be generated from two possible clock
sources:
•
HSE user external clock (see
• HSE external crystal/ceramic resonator (see

Figure 18. HSE external clock

1. The value of R
(resonator series resistance).
2. Load capacitance C
capacitance and board or trace PCB-related capacitance. Typically, it is between 2 pF and 7 pF. Please
refer to
26/44
depends on the crystal characteristics. Typical value is in the range of 5 to 6 R
EXT
has the following formula: C
L
Section 7: Recommendations on page 35
DocID026304 Rev 3
Figure 18)
Figure
Figure 19. HSE crystal/ceramic
= C x C / (C
L L1 L2
to minimize its value.
19)
resonators
+ C ) + C where: C
L1 L2 stray stray
AN4488
S
is the pin