Hse Osc Clock; External Source (Hse Bypass); External Crystal/Ceramic Resonator (Hse Crystal); Figure 7. External Clock - ST STM32L152-EVAL Application Note

AN3216
3.2

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 7.
1. The value of R
(resonator series resistance).
2. Load capacitance, C
pin capacitance and board or trace PCB-related capacitance. Typically, it is between 2 pF and 7 pF. Please
refer to
3.2.1

External source (HSE bypass)

In this mode, an external clock source must be provided. It can have a frequency of up to
32 MHz.
The external clock signal (square, sine or triangle) with a duty cycle of about 50%, has to
drive the OSC_IN pin while the OSC_OUT pin must be left in the high impedance state (see
Figure 7
3.2.2

External crystal/ceramic resonator (HSE crystal)

The external oscillator frequency ranges from 1 to 24 MHz.
The external oscillator has the advantage of producing a very accurate rate on the main
clock. The associated hardware configuration is shown in
The resonator and the load capacitors have to be connected as close as possible to the
oscillator pins in order to minimize output distortion and startup stabilization time. The load
capacitance values must be adjusted according to the selected oscillator.
For C and C
L1
pF range (typical), designed for high-frequency applications and selected to meet the
requirements of the crystal or resonator. C
crystal manufacturer typically specifies a load capacitance that is the series combination of
C and C
L1
C (10 pF can be used as a rough estimate of the combined pin and board capacitance).
L2
Refer to the electrical characteristics sections in the datasheet of your product for more
details.
External clock
Hardware configuration
OSC_IN OSC_OUT
(Hi-Z)
External source
depends on the crystal characteristics. A typical value is in the range of 5 to 6 R
EXT
, has the following formula: C
L
Section 6: Recommendations on page 23
and Figure 8).
it is recommended to use high-quality ceramic capacitors in the 5 pF to 25
L2
. The PCB and MCU pin capacitances must be included when sizing C
L2
Doc ID 17496 Rev 5
Figure 7)
Figure
Figure 8.
ai14369
= C x C
L L1 L2
to minimize its value.
and C
L1 L2,
8)
Crystal/ceramic resonators
/ (C + C ) + C where: C
L1 L2 stray
Figure 8.
are usually the same value. The
Clocks
S
is the
stray
and
L1
15/30