Datasheet

Electrical characteristics STM32F103x8, STM32F103xB
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Note: For C
L1
and C
L2
it is recommended to use high-quality ceramic capacitors in the 5 pF to
15 pF range selected to match the requirements of the crystal or resonator. 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
.
Load capacitance C
L
has the following formula: C
L
= C
L1
x C
L2
/ (C
L1
+ C
L2
) + C
stray
where
C
stray
is the pin capacitance and board or trace PCB-related capacitance. Typically, it is
between 2 pF and 7 pF.
Caution: 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.
Example: if you choose a resonator with a load capacitance of C
L
= 6 pF, and C
stray
= 2 pF,
then C
L1
= C
L2
= 8 pF.
Figure 21. Typical application with a 32.768 kHz crystal
5.3.7 Internal clock source characteristics
The parameters given in Ta ble 2 3 are derived from tests performed under ambient
temperature and V
DD
supply voltage conditions summarized in Table 8.
Table 22. LSE oscillator characteristics (f
LSE
= 32.768 kHz)
(1)
1. Based on characterization, not tested in production.
Symbol Parameter Conditions Min Typ Max Unit
R
F
Feedback resistor 5 MΩ
C
L1
C
L2
(2)
2. Refer to the note and caution paragraphs above the table.
Recommended load capacitance
versus equivalent serial
resistance of the crystal (R
S
)
(3)
3. The oscillator selection can be optimized in terms of supply current using an high quality resonator with
small R
S
value for example MSIV-TIN32.768kHz. Refer to crystal manufacturer for more details
R
S
= 30 kΩ 15 pF
I
2
LSE driving current V
DD
= 3.3 V, V
IN
= V
SS
1.4 µA
g
m
Oscillator Transconductance 5 µA/V
t
SU(LSE)
(4)
4. 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
startup time V
DD
is stabilized 3 s
ai14146
OSC32_OU T
OSC32_IN
f
LSE
C
L1
R
F
STM32F103xx
32.768 kHz
resonator
C
L2
Resonator with
integrated capacitors
Bias
controlled
gain