Datasheet

Electrical characteristics STM32F405xx, STM32F407xx
102/202 DocID022152 Rev 8
Note: 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.
Figure 32. Typical application with an 8 MHz crystal
1. R
EXT
value depends on the crystal characteristics.
Low-speed external clock generated from a crystal/ceramic resonator
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 33. 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).
Note: 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.
Table 33. LSE oscillator characteristics (f
LSE
= 32.768 kHz)
(1)
1. Guaranteed by design.
Symbol Parameter Conditions Min Typ Max Unit
f
OSC_IN
Oscillator frequency - - 32.768 - MHz
R
F
Feedback resistor - - 18.4 - MΩ
I
DD
LSE current consumption - - - 1 µA
G
m
Oscillator transconductance
Startup
2.8 - -
µA/V
G
mcritmax
Maximum critical crystal G
m
- - 0.56
t
SU(LSE)
(2)
2. Guaranteed by characterization. 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
DD
is stabilized - 2 - s
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