Datasheet

DocID027590 Rev 4 125/227
STM32F745xx STM32F746xx Electrical characteristics
195
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 32). 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 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 40. 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 40. LSE oscillator characteristics (f
LSE
= 32.768 kHz)
(1)
Symbol Parameter Conditions Min Typ Max Unit
I
DD
LSE current consumption
LSEDRV[1:0]=00
Low drive capability
-250-
nA
LSEDRV[1:0]=10
Medium low drive capability
-300-
LSEDRV[1:0]=01
Medium high drive capability
-370-
LSEDRV[1:0]=11
High drive capability
-480-
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