Electrical characteristics
STM32F100x4, STM32F100x6, STM32F100x8, STM32F100xB
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
(1)
Table 22. 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
CL1
RS = 30 K
15
pF
µA
(2)
CL2
resistance of the crystal (RS)(3)
VDD = 3.3 V
VIN = VSS
I2
LSE driving current
1.4
gm
Oscillator transconductance
Startup time
5
µA/V
s
(4)
tSU(LSE)
VDD is stabilized
3
1. Based on characterization, not tested in production.
2. Refer to the note and caution paragraphs above the table.
3. The oscillator selection can be optimized in terms of supply current using an high quality resonator with
small RS value for example MSIV-TIN32.768 kHz. 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
Figure 20. 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
STM32F10xxx
OSC32_OUT
C
L2
ai14129b
48/84
Doc ID 16455 Rev 2
STMICROELECTRONICS [ ST ]
