Datasheet

DS10198 Rev 8 155/270
STM32L476xx Electrical characteristics
238
High-speed external clock generated from a crystal/ceramic resonator
The high-speed external (HSE) clock can be supplied with a 4 to 48 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 57. 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 25). 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
.
Table 57. HSE oscillator characteristics
(1)
1. Guaranteed by design.
Symbol Parameter Conditions
(2)
2. Resonator characteristics given by the crystal/ceramic resonator manufacturer.
Min Typ Max Unit
f
OSC_IN
Oscillator frequency - 4 8 48 MHz
R
F
Feedback resistor - - 200 - k
I
DD(HSE)
HSE current consumption
During startup
(3)
3. This consumption level occurs during the first 2/3 of the t
SU(HSE)
startup time
--5.5
mA
V
DD
= 3 V,
Rm = 30 ,
CL = 10 pF@8 MHz
-0.44-
V
DD
= 3 V,
Rm = 45 ,
CL = 10 pF@8 MHz
-0.45-
V
DD
= 3 V,
Rm = 30 ,
CL = 5 pF@48 MHz
-0.68-
V
DD
= 3 V,
Rm = 30 ,
CL = 10 pF@48 MHz
-0.94-
V
DD
= 3 V,
Rm = 30 ,
CL = 20 pF@48 MHz
-1.77-
G
m
Maximum critical crystal
transconductance
Startup - - 1.5 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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