STM32L15XCC datasheet(74/136 Pages) STMICROELECTRONICS

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Electrical characteristics

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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 17). 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.

Table 29. HSE oscillator characteristics(1)(2)

Symbol

Parameter

Conditions

Min Typ

Max

Unit

fOSC_IN

Oscillator frequency

MHz

Feedback resistor

200

Recommended load

capacitance versus

equivalent serial

resistance of the crystal

(RS)(3)

RS = 30 Ω

-20

IHSE

HSE driving current

VDD= 3.3 V,

VIN = VSS with 30 pF

load

IDD(HSE)

HSE oscillator power

consumption

C = 20 pF

fOSC = 16 MHz

2.5 (startup)

0.7 (stabilized)

C = 10 pF

fOSC = 16 MHz

2.5 (startup)

0.46 (stabilized)

Oscillator

transconductance

Startup

3.5

mA /V

tSU(HSE)(4)

Startup time

VDD is stabilized

1. Resonator characteristics given by the crystal/ceramic resonator manufacturer.

2. Guaranteed by characterization results.

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.

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