Datasheet

Electrical characteristics STM32F103xx
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High-speed external clock
The high-speed external (HSE) clock can be supplied with a 4 to 16 MHz crystal/ceramic
resonator oscillator. All the information given in this paragraph are based on characterization
results obtained with typical external components specified in Tabl e 21 . 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).
Figure 20. Typical application with a 8-MHz crystal
1. R
EXT
value depends on the crystal characteristics. Typical value is in the range of 5 to 6R
S
.
Table 21. HSE 4-16 MHz oscillator characteristics
(1)
1. Resonator characteristics given by the crystal/ceramic resonator manufacturer.
Symbol Parameter Conditions Min Typ Max Unit
f
OSC_IN
Oscillator frequency 4 8 16 MHz
R
F
Feedback resistor 200 kΩ
C
L1
C
L2
(2)
2. For C
L1
and C
L2
it is recommended to use high-quality ceramic capacitors in the 5 pF to 25pF range (typ.),
designed for high-frequency applications, and selected to match the requirements of the crystal or
resonator. 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 when sizing C
L1
and C
L2
(10 pF can be used as a rough estimate of the combined pin and board
capacitance).
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 Ω 30 pF
i
2
HSE driving current
V
DD
= 3.3 V
V
IN
=V
SS
with 30 pF
load
1mA
g
m
(4)
4. Based on characterization results, not tested in production.
Oscillator transconductance Startup 25 mA/V
t
SU(HSE)
(5)
5. 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_OUT
OSC_IN
f
HSE
C
L1
R
F
STM32F103xx
8 MHz
resonator
R
EXT
(1)
C
L2
Resonator with
integrated capacitors
Bias
controlled
gain