Electrical characteristics STM32F038x6
54/102 DocID026079 Rev 3
High-speed external clock generated from a crystal/ceramic resonator
The high-speed external (HSE) clock can be supplied with a 4 to 32 MHz crystal/ceramic
resonator oscillator. All the information given in this paragraph are based on design
simulation results obtained with typical external components specified in
Table 31. 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).
For C
L1
and C
L2
, it is recommended to use high-quality external ceramic capacitors in the
5
pF to 20 pF range (Typ.), designed for high-frequency applications, and selected to match
the requirements of the crystal or resonator (see
Figure 15). 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
.
Note: For information on selecting the crystal, refer to the application note AN2867 “Oscillator
design guide for ST microcontrollers” available from the ST website www.st.com.
Table 31. HSE oscillator characteristics
Symbol Parameter Conditions
(1)
1. Resonator characteristics given by the crystal/ceramic resonator manufacturer.
Min
(2)
Typ Max
(2)
2. Guaranteed by design, not tested in production.
Unit
f
OSC_IN
Oscillator frequency - 4 8 32 MHz
R
F
Feedback resistor - - 200 - kΩ
I
DD
HSE current consumption
During startup
(3)
3. This consumption level occurs during the first 2/3 of the t
SU(HSE)
startup time
-8.5
mA
V
DD
= 1.8 V,
Rm = 30 Ω,
CL = 10 pF@8 MHz
-0.4-
V
DD
= 1.8 V,
Rm = 45 Ω,
CL = 10 pF@8 MHz
-0.5-
V
DD
= 1.8 V,
Rm = 30 Ω,
CL = 5 pF@32 MHz
-0.8-
V
DD
= 1.8 V,
Rm = 30 Ω,
CL = 10 pF@32 MHz
-1-
V
DD
= 1.8 V,
Rm = 30 Ω,
CL = 20 pF@32 MHz
-1.5-
g
m
Oscillator transconductance Startup 10 - - 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
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