STM32F103xC, STM32F103xD, STM32F103xE
Electrical characteristics
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 22. Typical application with an 8 MHz crystal
Resonator with
integrated capacitors
CL1
OSC_IN
8 MH z
resonator
REXT(1)
OSC_OU T
RF
Bias
controlled
gain
STM32F103xx
ai14145
fHSE
CL2
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
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 24.
Symbol
R
F
C
(2)
I
2
g
m
LSE oscillator characteristics (f
LSE
= 32.768 kHz)
(1)
Parameter
Feedback resistor
Recommended load capacitance
versus equivalent serial
resistance of the crystal (R
S
)
(3)
LSE driving current
Oscillator transconductance
V
DD
is stabilized
R
S
= 30 k
V
DD
= 3.3 V, V
IN
= V
SS
5
3
Conditions
Min
Typ
5
15
1.4
Max
Unit
M
pF
µA
µA/V
s
t
SU(LSE)(4)
Startup time
1. Based on characterization, not tested in production.
2. Refer to the note and caution paragraphs below the table, and to the application note AN2867 “Oscillator
design guide for ST microcontrollers”.
3. 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
4.
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
Note:
For C
L1
and C
L2
, it is recommended to use high-quality ceramic capacitors in the 5 pF to
15 pF range selected to match the requirements of the crystal or resonator (see
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
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