STM32L071KB datasheet(77/136 Pages) STMICROELECTRONICS

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STM32L071xx

Electrical characteristics

107

High-speed external clock generated from a crystal/ceramic resonator

The high-speed external (HSE) clock can be supplied with a 1 to 25 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 44. 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 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 23). 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.

Figure 23. HSE oscillator circuit diagram

Table 44. HSE oscillator characteristics(1)

1. Guaranteed by design.

Symbol

Parameter

Conditions

Min Typ Max Unit

fOSC_IN Oscillator frequency

MHz

Feedback resistor

200



Maximum critical crystal

transconductance

Startup

700

µA

tSU(HSE)

(2)

2. Guaranteed by characterization results. 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.

Startup time

VDD is stabilized

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