ST STM32F10 Series Application Note

Clocks
14/29 AN2586 Rev 8
3.2 LSE OSC clock
The low-speed external clock signal (LSE) can be generated from two possible clock
sources:
• LSE external crystal/ceramic resonator (see Figure 9)
• LSE user external clock (see Figure 8)
Note: 1 “External clock” figure:
To avoid exceeding the maximum value of C
L1
and C
L2
(15 pF), it is strongly recommended
to use a resonator with a load capacitance C
L
≤ 7 pF. Never use a resonator with a load
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
EXT
depends on the crystal characteristics. A 0 Ω resistor works, but it is not
optimal. Typical value is in the range of 5 to 6 R
S
(resonator series resistance). To fine tune
R
S
value refer to AN2867 - Oscillator design guide for ST microcontrollers.
3.2.1 External source (LSE bypass)
In this mode, an external clock source must be provided. It can have a frequency up to
1 MHz. The external clock signal (square, sine or triangle) with a duty cycle of about 50%
must drive the OSC32_IN pin, while the OSC32_OUT pin must be left high impedance (see
Figure 9 and Figure 8).
3.2.2 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.
Figure 8. External clock Figure 9. Crystal/ceramic resonators
OSC32_OUTOSC32_IN
External source
(Hi-Z)
ai14371
Hardware configuration
OSC32_OUTOSC32_IN
ai14372c
STM32F10xxx
C
L1
C
L2
Hardware configuration
R
EXT
(3)

ST STM32F10 Series Specifications

General

Brand	ST
Model	STM32F10 Series
Category	Microcontrollers
Language	English