Datasheet
DocID13586 Rev 16 47/90
STM32F101x8, STM32F101xB Electrical characteristics
89
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 21). 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 21. 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 22. 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
Table 21. HSE 4-16 MHz oscillator characteristics
(1)(2)
1. Resonator characteristics given by the crystal/ceramic resonator manufacturer.
2. Based on characterization, not tested in production.
Symbol Parameter Conditions Min Typ Max Unit
f
OSC_IN
Oscillator frequency 4 8 16 MHz
R
F
Feedback resistor 200 k
C
Recommended load capacitance
versus equivalent serial
resistance of the crystal (R
S
)
(3)
3. The relatively low value of the RF resistor offers a good protection against issues resulting from use in a
humid environment, due to the induced leakage and the bias condition change. However, it is
recommended to take this point into account if the MCU is used in tough humidity conditions.
R
S
= 30 30 pF
i
2
HSE driving current
V
DD
= 3.3 V, V
IN
= V
SS
with 30 pF load
1mA
g
m
Oscillator transconductance Startup 25 mA/V
t
SU(HSE)
(4)
4. t
SU(HSE)
is the startup time measured from the moment it is enabled (by software) to a stabilized 8 MHz
oscillation is reached. This value is measured for a standard crystal resonator and it can vary significantly
with the crystal manufacturer
Startup time V
DD
is stabilized 2 ms
ai14128b
OSC_OU T
OSC_IN
f
HSE
C
L1
R
F
STM32F10xxx
8 MHz
resonator
Resonator with
integrated capacitors
Bias
controlled
gain
R
EXT
(1)
C
L2