STM32F103xx
Electrical characteristics
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
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 17.
Symbol
f
OSC_IN
R
F
C
L1
C
L2(2)
i
2
g
m
HSE 4-16 MHz oscillator characteristics
(1)
Parameter
Oscillator frequency
Feedback resistor
Recommended load capacitance
versus equivalent serial
resistance of the crystal (R
S
)
(3)
HSE driving current
Oscillator Transconductance
R
S
= 30
Ω
V
DD
= 3.3 V
V
IN
=V
SS
with 30 pF
load
Startup
V
SS
is stabilized
25
2
Conditions
Min
4
Typ
8
200
30
Max
16
Unit
MHz
kΩ
pF
1
mA
mA/V
ms
t
SU(HSE)(4)
startup time
1. Resonator characteristics given by the crystal/ceramic resonator manufacturer.
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).
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.
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
Figure 13. Typical application with a 8-MHz crystal
Resonator with
integrated capacitors
CL1
OSC_IN
8 MH z
resonator
REXT(1)
OSC_OU T
RF
Bias
controlled
gain
STM32F103xx
fHSE
CL2
ai14145
1. R
EXT
value depends on the crystal characteristics. Typical value is in the range of 5 to 6R
S
.
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