Electrical characteristics
STM32F103x8, STM32F103xB
(1) (2)
Table 23. LSE oscillator characteristics (fLSE = 32.768 kHz)
Symbol
Parameter
Feedback resistor
Conditions
Min
Typ
Max
Unit
RF
5
MΩ
Recommended load capacitance
versus equivalent serial
resistance of the crystal (RS)
C
RS = 30 KΩ
15
pF
VDD = 3.3 V
VIN = VSS
I2
LSE driving current
1.4
µA
gm
Oscillator transconductance
5
µA/V
TA = 50 °C
TA = 25 °C
TA = 10 °C
TA = 0 °C
1.5
2.5
4
6
VDD is
stabilized
(3)
tSU(LSE)
Startup time
s
TA = -10 °C
TA = -20 °C
TA = -30 °C
TA = -40 °C
10
17
32
60
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. tSU(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 and C it is recommended to use high-quality ceramic capacitors in the 5 pF to
L1 L2
15 pF range selected to match the requirements of the crystal or resonator. C and C are
L1
L2,
usually the same size. The crystal manufacturer typically specifies a load capacitance which
is the series combination of C and C .
L1
L2
Load capacitance C has the following formula: C = C x C / (C + C ) + C where
L
L
L1
L2
L1
L2
stray
C
is the pin capacitance and board or trace PCB-related capacitance. Typically, it is
stray
between 2 pF and 7 pF.
Caution:
To avoid exceeding the maximum value of C and C (15 pF) it is strongly recommended
L1
L2
to use a resonator with a load capacitance C ≤ 7 pF. Never use a resonator with a load
L
capacitance of 12.5 pF.
Example: if you choose a resonator with a load capacitance of C = 6 pF, and C
= 2 pF,
L
stray
then C = C = 8 pF.
L1
L2
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Doc ID 13587 Rev 15
STMICROELECTRONICS [ ST ]
