Electrical characteristics
STM32F103xx
Low-speed external clock
The low-speed external (LSE) clock can be supplied with a 32.768 kHz crystal/ceramic
resonator oscillator. All the information given in this paragraph are based on characterization
results obtained with typical external components specified in Table 18. In the application,
the resonator and the load capacitors have to be placed as close as possible to the oscillator
pins in order to minimize output distortion and startup stabilization time. Refer to the crystal
resonator manufacturer for more details on the resonator characteristics (frequency,
package, accuracy).
Table 18. 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
CL1
CL2
RS = 30 kΩ
15
pF
µA
resistance of the crystal (RS)(1)
VDD = 3.3 V
VIN = VSS
I2
LSE driving current
1.4
gm
Oscillator Transconductance
startup time
5
µA/V
s
(2)
tSU(LSE)
VSS is stabilized
3
1. 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
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
Figure 14. 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
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STMICROELECTRONICS [ ST ]
