Electrical characteristics
STM32F103xx
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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
. 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).
Figure 14.
Typical application with a 32.768 kHz crystal
Table 18.
LSE oscillator characteristics (f
LSE
= 32.768 kHz)
Symbol
Parameter
Conditions
Min
Typ
Max
Unit
R
F
Feedback resistor
5
M
Ω
C
L1
C
L2
Recommended load capacitance
versus equivalent serial
resistance of the crystal (R
S
)
(1)
1.
The oscillator selection can be optimized in terms of supply current using an high quality resonator with
small R
S
value for example MSIV-TIN32.768kHz. Refer to crystal manufacturer for more details
R
S
= 30 k
Ω
15
pF
I
2
LSE driving current
V
DD
= 3.3 V
V
IN
= V
SS
1.4
µA
g
m
Oscillator Transconductance
5
µA/V
t
SU(LSE)
(2)
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
startup time
V
SS
is stabilized
3
s
ai14146
OSC32_OU T
OSC32_IN
fLSE
CL1
RF
STM32F103xx
32.768 kH z
resonator
CL2
Resonator with
integrated capacitors
Bias
controlled
gain