Datasheet

DS10690 Rev 6 81/144
STM32L071xx Electrical characteristics
111
Low-speed external clock generated from a crystal/ceramic resonator
The low-speed external (LSE) clock can be supplied with a 32.768 kHz crystal/ceramic
resonator oscillator. All the information given in this paragraph are based on
characterization results obtained with typical external components specified in Table 45. In
the application, 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. Refer to the crystal resonator manufacturer for more details on the resonator
characteristics (frequency, package, accuracy).
Note: For information on selecting the crystal, refer to the application note AN2867 “Oscillator
design guide for ST microcontrollers” available from the ST website www.st.com.
Figure 23. Typical application with a 32.768 kHz crystal
Note: An external resistor is not required between OSC32_IN and OSC32_OUT and it is forbidden
to add one.
Table 45. LSE oscillator characteristics
(1)
Symbol Parameter Conditions
(2)
Min
(2)
Typ Max Unit
f
LSE
LSE oscillator frequency - 32.768 - kHz
G
m
Maximum critical crystal
transconductance
LSEDRV[1:0]=00
lower driving capability
--0.5
µA/V
LSEDRV[1:0]= 01
medium low driving capability
- - 0.75
LSEDRV[1:0] = 10
medium high driving capability
--1.7
LSEDRV[1:0]=11
higher driving capability
--2.7
t
SU(LSE)
(3)
Startup time V
DD
is stabilized - 2 - s
1. Guaranteed by design.
2. Refer to the note and caution paragraphs below the table, and to the application note AN2867 “Oscillator design guide for
ST microcontrollers”.
3. Guaranteed by characterization results. t
SU(LSE)
is the startup time measured from the moment it is enabled (by software)
to a stabilized 32.768 kHz oscillation is reached. This value is measured for a standard crystal resonator and it can vary
significantly with the crystal manufacturer. To increase speed, address a lower-drive quartz with a high- driver mode.
MS30253V2
OSC32_IN
OSC32_OUT
Drive
programmable
amplifier
f
LSE
32.768 kHz
resonator
Resonator with integrated
capacitors
C
L1
C
L2
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