CD00237391 Datasheet, PDF (84/177 Pages) STMicroelectronics – ARM-based 32-bit MCU, 150DMIPs, up to 1 MB Flash/128+4KB RAM

Electrical characteristics
STM32F20xxx
Table 29. LSE oscillator characteristics (f_LSE = 32.768 kHz) ⁽¹⁾
Symbol
Parameter
Conditions
Min Typ Max Unit
R_F Feedback resistor
- 18.4 - MΩ
I_DD LSE current consumption
-
-
1
µA
g_m Oscillator Transconductance
t_SU(LSE)(2) startup time
2.8
-
V_DD is stabilized
-
2
- µA/V
-
s
1. Guaranteed by design, not tested in production.
2. t_SU(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 C_L1 and C_L2 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 31). C_L1 and C_L2, are usually the same size. The crystal manufacturer typically
specifies a load capacitance which is the series combination of C_L1 and C_L2.
Load capacitance C_L has the following formula: C_L = C_L1 x C_L2 / (C_L1 + C_L2) + C_stray where
C_stray 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 C_L1 and C_L2 (15 pF) it is strongly recommended
to use a resonator with a load capacitance C_L ≤ 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 C_L = 6 pF, and C_stray = 2 pF,
then C_L1 = C_L2 = 8 pF.
Figure 31. 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
fL_SE
STM32F
ai17531
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Doc ID 15818 Rev 9