Datasheet

Electrical characteristics STM32F303x6/x8
68/121 DocID025083 Rev 7
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 16). 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 16. 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 design
simulation results obtained with typical external components specified in
Table 41. In the
application, the resonator and the load capacitors have to be placed as close as possible to
the oscillator pins 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 41. LSE oscillator characteristics (f
LSE
= 32.768 kHz)
Symbol Parameter Conditions
(1)
Min.
(2)
Typ. Max.
(2)
Unit
I
DD
LSE current consumption
LSEDRV[1:0]=00
lower driving capability
-0.50.9
µA
LSEDRV[1:0]=10
medium low driving
capability
--1
LSEDRV[1:0]=01
medium high-driving
capability
--1.3
LSEDRV[1:0]=11
higher-driving capability
--1.6
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