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 Table 17. 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).
(1)
Table 17. HSE 4-16 MHz oscillator characteristics
Symbol
Parameter
Conditions
Min
Typ
Max
Unit
fOSC_IN Oscillator frequency
4
8
16
MHz
RF
Feedback resistor
200
kΩ
Recommended load capacitance
versus equivalent serial
CL1
RS = 30 Ω
30
pF
(2)
CL2
resistance of the crystal (RS)(3)
VDD= 3.3 V
VIN=VSS with 30 pF
load
i2
HSE driving current
1
mA
gm
Oscillator Transconductance
startup time
Startup
25
mA/V
ms
(4)
tSU(HSE)
VSS is stabilized
2
1. Resonator characteristics given by the crystal/ceramic resonator manufacturer.
2. For CL1 and CL2 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. CL1 and CL2, are usually the same size. The crystal manufacturer typically specifies a load
capacitance which is the series combination of CL1 and CL2. PCB and MCU pin capacitance must be
included when sizing CL1 and CL2 (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. tSU(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
C
L1
f
OSC_IN
HSE
Bias
controlled
gain
8 MHz
resonator
R
F
STM32F103xx
OSC_OUT
(1)
R
EXT
C
L2
ai14145
1. REXT value depends on the crystal characteristics. Typical value is in the range of 5 to 6RS.
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STMICROELECTRONICS [ ST ]
