STM32F103x8, STM32F103xB
Table 23.
Symbol
R
F
C
(2)
Electrical characteristics
LSE oscillator characteristics (f
LSE
= 32.768 kHz)
(1)
Parameter
Feedback resistor
Recommended load capacitance
versus equivalent serial
resistance of the crystal (R
S
)
(3)
LSE driving current
Oscillator Transconductance
V
DD
is stabilized
R
S
= 30 k
V
DD
= 3.3 V, V
IN
= V
SS
5
3
Conditions
Min
Typ
5
15
1.4
Max
Unit
M
pF
µA
µA/V
s
I
2
g
m
t
SU(LSE)(4)
startup time
1. Based on characterization, not tested in production.
2. Refer to the note and caution paragraphs below the table, and to the application note AN2867 “Oscillator
design guide for ST microcontrollers.
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
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
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.
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 23. Typical application with a 32.768 kHz crystal
Caution:
Resonator with
integrated capacitors
CL1
OSC32_IN
32.768 kH z
resonator
CL2
RF
OSC32_OU T
Bias
controlled
gain
STM32F103xx
fLSE
ai14146
5.3.7
Internal clock source characteristics
The parameters given in
are derived from tests performed under ambient
temperature and V
DD
supply voltage conditions summarized in
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