STM32F101x4, STM32F101x6
Electrical characteristics
resonator manufacturer for more details on the resonator characteristics (frequency,
package, accuracy).
Table 22. LSE oscillator characteristics (fLSE = 32.768 kHz)(1) (2)
Symbol
Parameter
Conditions
Min Typ Max Unit
RF
Feedback resistor
5
MΩ
Recommended load capacitance
C
versus equivalent serial
resistance of the crystal (RS)
RS = 30 KΩ
15 pF
I2
LSE driving current
VDD = 3.3 V
VIN = VSS
1.4 µA
gm
Oscillator transconductance
5
µA/V
TA = 50 °C
1.5
TA = 25 °C
2.5
TA = 10 °C
4
tSU(LSE)(3) Startup time
VDD is
stabilized
TA = 0 °C
TA = -10 °C
6
s
10
TA = -20 °C
17
TA = -30 °C
32
TA = -40 °C
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:
Caution:
For CL1 and CL2 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. CL1 and CL2, are
usually the same size. The crystal manufacturer typically specifies a load capacitance which
is the series combination of CL1 and CL2.
Load capacitance CL has the following formula: CL = CL1 x CL2 / (CL1 + CL2) + Cstray where
Cstray 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 CL1 and CL2 (15 pF) it is strongly recommended
to use a resonator with a load capacitance CL ≤ 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 CL = 6 pF, and Cstray = 2 pF,
then CL1 = CL2 = 8 pF.
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