STM32F103xC, STM32F103xD, STM32F103xE
Electrical 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) (2)
Table 24. LSE oscillator characteristics (fLSE = 32.768 kHz)
Symbol
Parameter
Feedback resistor
Recommended load capacitance
Conditions
Min
Typ Max Unit
RF
5
MΩ
CL1, CL2 versus equivalent serial
resistance of the crystal (RS)
RS = 30 kΩ
15
pF
I2
LSE driving current
VDD = 3.3 V, VIN = VSS
1.4
µA
gm
Oscillator transconductance
5
µA/V
TA = 50 °C
TA = 25 °C
TA = 10 °C
TA = 0 °C
1.5
2.5
4
6
VDD is
stabilized
(3)
tSU(LSE)
Startup time
s
TA = -10 °C
TA = -20 °C
TA = -30 °C
TA = -40 °C
10
17
32
60
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. 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 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
C
is the pin capacitance and board or trace PCB-related capacitance. Typically, it is
stray
between 2 pF and 7 pF.
Caution:
To avoid exceeding the maximum value of C and C (15 pF) it is strongly recommended
L1
L2
to use a resonator with a load capacitance C ≤ 7 pF. Never use a resonator with a load
L
capacitance of 12.5 pF.
Example: if you choose a resonator with a load capacitance of C = 6 pF, and C
= 2 pF,
59/130
L
stray
then C = C = 8 pF.
L1
L2
Doc ID 14611 Rev 8
STMICROELECTRONICS [ ST ]
