STM32F105xx, STM32F107xx
Electrical characteristics
resonator manufacturer for more details on the resonator characteristics (frequency,
package, accuracy).
(1)
Table 23. LSE oscillator characteristics (fLSE = 32.768 kHz)
Symbol
Parameter
Feedback resistor
Conditions
Min
Typ
Max
Unit
RF
5
MY
Recommended load capacitance
versus equivalent serial
resistance of the crystal (RS)(3)
C(2)
RS = 30 kY
15
pF
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 external ceramic capacitors in the
L2
L1
5 pF to 15 pF range selected to match the requirements of the crystal or resonator (see
Figure 16). C and C are usually the same size. The crystal manufacturer typically
specifies a load capacitance which is the series combination of C and C .
L1
L2,
L1
L2
Load capacitance C has the following formula: C = C x C / (C + C ) + C where
stray
L
L
L1
L2
is the pin capacitance and board or trace PCB-related capacitance. Typically, it is
L1
L2
C
stray
between 2 pF and 7 pF.
Caution:
To avoid exceeding the maximum value of C and C (15 pF) it is strongly recommended
L2
L1
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,
L
stray
then C = C = 8 pF.
L1 L2
Figure 16. Typical application with a 32.768 kHz crystal
Resonator with
integrated capacitors
C
L1
f
OSC32_IN
LSE
Bias
controlled
gain
32.768 KHz
resonator
R
F
STM32F10xxx
OSC32_OUT
C
L2
ai14129b
Doc ID 15274 Rev 4
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STMICROELECTRONICS [ ST ]
