Datasheet

STM32F405xx, STM32F407xx Electrical characteristics
Doc ID 022152 Rev 2 87/167
Note: 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 30). 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.
Caution: 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 30. Typical application with a 32.768 kHz crystal
Table 29. LSE oscillator characteristics (f
LSE
= 32.768 kHz)
(1)(2)
1. Based on characterization, not tested in production.
2. TBD stands for “to be defined”.
Symbol Parameter Conditions Min Typ Max Unit
R
F
Feedback resistor - TBD - MΩ
C
(3)
3. Refer to the note and caution paragraphs below the table, and to the application note AN2867 “Oscillator
design guide for ST microcontrollers”.
Recommended load capacitance
versus equivalent serial
resistance of the crystal (R
S
)
(4)
4. 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 kΩ --TBDpF
I
2
LSE driving current V
DD
= 3.3 V, V
IN
= V
SS
--TBDµA
g
m
Oscillator Transconductance TBD - - µA/V
t
SU(LSE)
(5)
5. 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 - TBD - s
ai17531
OSC32_OUT
OSC32_IN
f
LSE
C
L1
R
F
STM32F
32.768 kHz
resonator
Resonator with
integrated capacitors
Bias
controlled
gain
C
L2