Lse Osc Clock; External Source (Lse Bypass); External Crystal/Ceramic Resonator (Lse Crystal); Figure 8. External Clock - ST STM32F10 Series Application Note

AN2586
2.2

LSE OSC clock

The low-speed external clock signal (LSE) can be generated from two possible clock
sources:
●
LSE external crystal/ceramic resonator (see
●
LSE user external clock (see
Figure 8.
Note: 1 "External clock" figure:
To avoid exceeding the maximum value of C
to use a resonator with a load capacitance C
capacitance of 12.5 pF
2 "External clock" and "crystal/ceramic resonators" figures:
OSC32_IN and OSC_OUT pins can be used also as GPIO, but it is recommended not to
use them as both RTC and GPIO pins in the same application
3 "Crystal/ceramic resonators" figure:
The value of R
would not be optimal. Typical value is in the range of 5 to 6 R
To fine tune R
2.2.1

External source (LSE bypass)

In this mode, an external clock source must be provided. It can have a frequency of up to
1 MHz. The external clock signal (square, sine or triangle) with a duty cycle of about 50%
has to drive the OSC32_IN pin while the OSC32_OUT pin must be left high impedance (see
Figure 9
2.2.2

External crystal/ceramic resonator (LSE crystal)

The LSE crystal is a 32.768 kHz low-speed external crystal or ceramic resonator. It has the
advantage of providing a low-power, but highly accurate clock source to the real-time clock
peripheral (RTC) for clock/calendar or other timing functions.
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.
External clock
Hardware configuration
OSC32_IN OSC32_OUT
(Hi-Z)
External source
depends on the crystal characteristics. A 0
EXT
value refer to AN2867 - Oscillator design guide for ST microcontrollers.
S
and Figure 8).
Doc ID 13675 Rev 7
Figure
Figure 8)
Figure 9.
ai14371
and C
L1 L2
≤
7 pF. Never use a resonator with a load
L
9)
Crystal/ceramic resonators
Hardware configuration
STM32F10xxx
OSC32_IN OSC32_OUT
R
C C
L1
(15 pF) it is strongly recommended
Ω
resistor would work but
(resonator series resistance).
S
Clocks
(3)
EXT
L2
ai14372c
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