Datasheet

Electrical characteristics STM32F105xx, STM32F107xx
50/108 DocID15274 Rev 10
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
. 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
characterization results obtained with typical external components specified in Table 23. 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).
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Table 23. LSE oscillator characteristics (f
LSE
= 32.768 kHz)
(1)
Symbol Parameter Conditions Min Typ Max Unit
R
F
Feedback resistor - - 5 - MΩ
C
(2)
Recommended load capacitance
versus equivalent serial
resistance of the crystal (R
S
)
(3)
R
S
= 30 kΩ - - 15 pF
I
2
LSE driving current V
DD
= 3.3 V, V
IN
= V
SS
- - 1.4 µA
g
m
Oscillator Transconductance - 5 - - µA/V
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