Electrical characteristics
Table 23.
Symbol
R
F
C
STM32F103x8, STM32F103xB
LSE oscillator characteristics (f
LSE
= 32.768 kHz)
(1) (2)
Parameter
Feedback resistor
Recommended load capacitance
versus equivalent serial
resistance of the crystal (R
S
)
LSE driving current
Oscillator transconductance
T
A
= 50 °C
T
A
= 25 °C
T
A
= 10 °C
R
S
= 30 KΩ
V
DD
= 3.3 V
V
IN
= V
SS
5
1.5
2.5
4
6
s
10
17
32
60
Conditions
Min
Typ
5
15
Max
Unit
MΩ
pF
I
2
g
m
1.4
µA
µA/V
t
SU(LSE)(3)
Startup time
V
DD
is
stabilized
T
A
= 0 °C
T
A
= -10 °C
T
A
= -20 °C
T
A
= -30 °C
T
A
= -40 °C
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.
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 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.
Caution:
54/105
Doc ID 13587 Rev 15
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