Doc ID 15058 Rev 3
For CL1 and CL2, 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 19). 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 (10 pF
can be used as a rough estimate of the combined pin and board capacitance) when sizing
CL1 and CL2. Refer to the application note AN2867 “Oscillator design guide for ST
microcontrollers” available from the ST website www.st.com.
Typical application with an 8 MHz crystal
REXT value depends on the crystal characteristics.
Low-speed external clock generated from a crystal/ceramic resonator
The low-speed external (LSE) clock can be supplied with a 32.768 kHz crystal/ceramic
resonator oscillator. All the information given in this paragraph are based on characterization
results obtained with typical external components specified in Table 22. 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
HSE 4-16 MHz oscillator characteristics(1)(2)
Resonator characteristics given by the crystal/ceramic resonator manufacturer.
Based on characterization, not tested in production.
Recommended load capacitance
versus equivalent serial
resistance of the crystal (RS)
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.
RS = 30
HSE driving current
VDD = 3.3 V, VIN = VSS
with 30 pF load
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
VDD is stabilized
8 MH z