STM32F103xC, STM32F103xD, STM32F103xE
Electrical characteristics
same size. The crystal manufacturer typically specifies a load capacitance which is the
series combination of C and C . PCB and MCU pin capacitance must be included (10 pF
L1
L2
can be used as a rough estimate of the combined pin and board capacitance) when sizing
and C . Refer to the application note AN2867 “Oscillator design guide for ST
C
L1
L2
microcontrollers” available from the ST website www.st.com.
Figure 22. Typical application with an 8 MHz crystal
Resonator with
integrated capacitors
C
L1
f
OSC_IN
HSE
Bias
controlled
gain
8 MHz
resonator
R
F
STM32F103xx
OSC_OUT
(1)
R
EXT
C
ai14145
L2
1. REXT 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 24. 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).
(1)
Table 24. LSE oscillator characteristics (fLSE = 32.768 kHz)
Symbol
Parameter
Feedback resistor
Conditions
Min
Typ Max Unit
RF
5
M
Recommended load capacitance
versus equivalent serial
C(2)
RS = 30 k
15
pF
resistance of the crystal (RS)(3)
I2
LSE driving current
Oscillator transconductance
Startup time
VDD = 3.3 V, VIN = VSS
1.4
µA
µA/V
s
gm
5
(4)
tSU(LSE)
VDD is stabilized
3
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 RS value for example MSIV-TIN32.768kHz. Refer to crystal manufacturer for more details
4. tSU(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 and C , it is recommended to use high-quality ceramic capacitors in the 5 pF to
L1 L2
15 pF range selected to match the requirements of the crystal or resonator (see Figure 23).
and C are usually the same size. The crystal manufacturer typically specifies a load
C
L1
L2,
capacitance which is the series combination of C and C .
L1
L2
Load capacitance C has the following formula: C = C x C / (C + C ) + C where
L
L
L1
L2
L1
L2
stray
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STMICROELECTRONICS [ ST ]
