STM32F217RGT7 查看數據表（PDF） - STMicroelectronics
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STM32F217RGT7 Datasheet PDF : 173 Pages
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STM32F21xxx
Electrical characteristics
Table 28. LSE oscillator characteristics (fLSE = 32.768 kHz) (1)
Symbol
Parameter
Conditions
Min Typ Max Unit
RF Feedback resistor
- 18.4 - MΩ
IDD LSE current consumption
-
-
1
µA
gm Oscillator Transconductance
tSU(LSE)(2) startup time
2.8
-
VDD is stabilized
-
2
- µA/V
-
s
1. Guaranteed by design, not tested in production.
2. 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:
Note:
Caution:
For CL1 and CL2 it is recommended to use high-quality external ceramic capacitors in the
5 pF to 15 pF range selected to match the requirements of the crystal or resonator (see
Figure 30). 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.
For information on electing the crystal, refer to the application note AN2867 “Oscillator
design guide for ST microcontrollers” available from the ST website www.st.com.
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.
Figure 30. Typical application with a 32.768 kHz crystal
Resonator with
integrated capacitors
CL1
32.768 kH z
resonator
CL2
OSC32_IN
RF
OSC32_OU T
Bias
controlled
gain
fLSE
STM32F
ai17531
Doc ID 17050 Rev 8
83/173
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