Datasheet

Electrical characteristics STM32L031x4/6
70/126 DS10668 Rev 6
High-speed external clock generated from a crystal/ceramic resonator
The high-speed external (HSE) clock can be supplied with a 1 to 25 MHz crystal/ceramic
resonator oscillator (LQFP48 package only). All the information given in this paragraph are
based on characterization results obtained with typical external components specified in
Table 39. 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).
For C
L1
and C
L2
, it is recommended to use high-quality external ceramic capacitors in the
5 pF to 25 pF range (typ.), designed for high-frequency applications, and selected to match
the requirements of the crystal or resonator (see Figure 22). C
L1
and C
L2
are usually the
same size. The crystal manufacturer typically specifies a load capacitance which is the
series combination of C
L1
and C
L2
. PCB and MCU pin capacitance must be included (10 pF
can be used as a rough estimate of the combined pin and board capacitance) when sizing
C
L1
and C
L2
. Refer to the application note AN2867 “Oscillator design guide for ST
microcontrollers” available from the ST website www.st.com.
Figure 22. HSE oscillator circuit diagram
Table 39. HSE oscillator characteristics
(1)
1. Guaranteed by design.
Symbol Parameter Conditions Min Typ Max Unit
f
OSC_IN
Oscillator frequency - 1 25 MHz
R
F
Feedback resistor - - 200 - kΩ
G
m
Maximum critical crystal
transconductance
Startup - - 700
µA
/V
t
SU(HSE)
(2)
2. Guaranteed by characterization results. t
SU(HSE)
is the startup time measured from the moment it is
enabled (by software) to a stabilized 8 MHz oscillation is reached. This value is measured for a standard
crystal resonator and it can vary significantly with the crystal manufacturer.
Startup time V
DD
is stabilized - 2 - ms
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