Datasheet
DocID13587 Rev 16 53/105
STM32F103x8, STM32F103xB Electrical characteristics
104
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 24). 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 24. 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 23. 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 22. 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
ai14145
OSC_OU T
OSC_IN
f
HSE
C
L1
R
F
STM32F103xx
8 MHz
resonator
R
EXT
(1)
C
L2
Resonator with
integrated capacitors
Bias
controlled
gain