APM32F103CBT6-R datasheet(33/78 Pages) GEEHY

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P a g e 32

5.5

Clock

5.5.1

Characteristics of external clock source

5.5.1.1

A high-speed external clock generated crystal/ceramic resonator

A high-speed external clock (HSECLK) can be generated using a 4~16MHz crystal/ceramic

resonator. The following table shows the information of evaluated external devices. In the

applications, the resonator and load capacitor must be as close to the oscillator pins as

possible to reduce distortion and stable time of startup. For detailed parameters (frequency,

package, precision, etc.) of crystal resonator, please consult the corresponding manufacturer.

Table19 HSECLK4~16MHz Oscillator Characteristics (1) (2)

Symbol

Parameter

Condition

Minimum

value

Typical

value

Maximum

value

Unit

fOSC_IN

Oscillator frequency

MHz

Feedback resistance

400

kΩ

CL1&

CL2(3)

Recommended load

capacitance and

corresponding crystal

serial impedance (RS) (4)

= 30kΩ

HSECLK drive current

VDD=3.3V，VIN=VSS

30pF load

1.1

Transconductance of

oscillator

Start

mA/V

tSU(HSECLK)(5)

Start Time

VDD is stable

1.33

The characteristic parameters of the resonator are given by the manufacturer of crystal/ceramic

resonator.

The above is obtained through comprehensive evaluation.

It is recommended that high-quality ceramic capacitors between 5pF and 25pF (typical value)

designed for high frequency applications should be used for CL1 and CL2, and the capacitance

value selected should meet the requirements of crystals or resonators. Typically, CL1 and CL2

have the same parameters. The load capacitance parameters usually given by crystal

manufacturers are the serial combination values of CL1 and CL2. When selecting CL1 and CL2, the

capacitive reactance of PCB and MCU pins shall be considered (usually the capacitance is

estimated at 10pF).

Relatively low RF resistance value should be adopted when it is used in wet environment.

However, if the MCU is used in a harsh humid environment, attention should be paid to protection

during design.

tSU(HSECLK) is the startup time, which defines the time when HSECLK is enabled by software to the

time when stable oscillation at 8MHz is obtained. This value is measured using a standard crystal

resonator, which may vary greatly due to different crystal manufacturers.

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