Electrical characteristics
STM32F105xx, STM32F107xx
resonator manufacturer for more details on the resonator characteristics (frequency,
package, accuracy).
(1)
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
C(2)
RS = 30 kΩ
15
pF
resistance of the crystal (RS)(3)
I2
LSE driving current
VDD = 3.3 V, VIN = VSS
1.4
µA
gm
Oscillator Transconductance
5
µA/V
TA = 50 °C
1.5
2.5
4
TA = 25 °C
TA = 10 °C
TA = 0 °C
6
(4)
tSU(LSE)
Startup time
VDD is stabilized
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. The oscillator selection can be optimized in terms of supply current using an high quality resonator with small RS value for example
MSIV-TIN32.768kHz. Refer to crystal manufacturer for more details
4. 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 external ceramic capacitors in the
L1 L2
5 pF to 15 pF range selected to match the requirements of the crystal or resonator (see
Figure 17). C and C are usually the same size. The crystal manufacturer typically
L1
L2,
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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