Electrical characteristics
STM32F103x8, STM32F103xB
(1)
Table 23. LSE oscillator characteristics (fLSE = 32.768 kHz)
Symbol
Parameter
Feedback resistor
Conditions
Min
Typ
Max
Unit
RF
5
M
Recommended load capacitance
versus equivalent serial
C(2)
RS = 30 k
15
pF
resistance of the crystal (RS)(3)
I2
LSE driving current
Oscillator Transconductance
startup time
VDD = 3.3 V, VIN = VSS
1.4
µA
µA/V
s
gm
5
(4)
tSU(LSE)
VDD is stabilized
3
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 resonator 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
Figure 24. Typical application with a 32.768 kHz crystal
Resonator with
integrated capacitors
C
L1
f
OSC32_IN
LSE
Bias
controlled
gain
32.768 kHz
resonator
R
F
STM32F103xx
OSC32_OUT
C
L2
ai14146
5.3.7
Internal clock source characteristics
The parameters given in Table 24 are derived from tests performed under ambient
temperature and V supply voltage conditions summarized in Table 9.
DD
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Doc ID 13587 Rev 12
STMICROELECTRONICS [ ST ]
