Datasheet

Table Of Contents
DocID022152 Rev 8 101/202
STM32F405xx, STM32F407xx Electrical characteristics
Figure 31. Low-speed external clock source AC timing diagram
High-speed external clock generated from a crystal/ceramic resonator
The high-speed external (HSE) clock can be supplied with a 4 to 26 MHz crystal/ceramic
resonator oscillator. All the information given in this paragraph are based on
characterization results obtained with typical external components specified in
Table 32. 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 25 pF range (typ.), designed for high-frequency applications, and selected to match
the requirements of the crystal or resonator (see
Figure 32). 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 32. HSE 4-26 MHz oscillator characteristics
(1)
1. Guaranteed by design.
Symbol Parameter Conditions Min Typ Max Unit
f
OSC_IN
Oscillator frequency - 4 - 26 MHz
R
F
Feedback resistor - - 200 - kΩ
G
m
Oscillator transconductance
Startup
5--
mA/V
G
mcritmax
Maximum critical crystal G
m
--1
t
SU(HSE)
(2)
2. Guaranteed by characterization. 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 can vary significantly with the crystal manufacturer
Startup time V
DD
is stabilized - 2 - ms
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