Datasheet

DocID027107 Rev 6 105/202
STM32F446xC/E Electrical characteristics
175
For C
L1
and C
L2
, it is recommended to use high-quality external ceramic capacitors in the
5
pF to 25 pF range (typ.), designed for high-frequency applications, and selected to match
the requirements of the crystal or resonator (see
Figure 25). 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
. PCB and MCU pin capacitance must be included (10 pF
can be used as a rough estimate of the combined pin and board capacitance) when sizing
C
L1
and C
L2
.
Note: For information on selecting the crystal, refer to the application note AN2867 “Oscillator
design guide for ST microcontrollers” available from the ST website www.st.com.
Figure 25. 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 40. In
the application, the resonator and the load capacitors have to be placed as close as
Table 39. HSE 4-26 MHz oscillator characteristics
(1)
1. Guaranteed by design.
Symbol Parameter Conditions Min Typ Max Unit
f
OSC_IN
Oscillator frequency - 4 - 26 MHz
R
F
Feedback resistor - - 200 - kΩ
I
DD
HSE current consumption
V
DD
=3.3 V,
ESR= 30 Ω,
C
L
=5 pF@25 MHz
- 450 -
µA
V
DD
=3.3 V,
ESR= 30 Ω,
C
L
=10 pF@25 MHz
- 530 -
ACC
HSE
(2)
2. This parameter depends on the crystal used in the application. The minimum and maximum values must
be respected to comply with USB standard specifications.
HSE accuracy - -500 - 500 ppm
G
m
_crit_max Maximum critical crystal g
m
Startup - - 1 mA/V
t
SU(HSE
(3)
3. t
SU(HSE)
is the startup time measured from the moment it is enabled (by software) to a stabilized 8 MHz
oscillation is reached. This value is Guaranteed based on test during characterization. It is measured for a
standard crystal resonator and it can vary significantly with the crystal manufacturer.
Startup time V
DD
is stabilized - 2 - ms
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