AN3029 datasheet(11/42 Pages) STMICROELECTRONICS

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AN3029

Clock management

Crystal/ceramic resonator

•

Frequency range: 1 to 16 MHz

•

Stabilization time: Programmable from 1 to 4096 cycles

•

Oscillation mode: Preferred fundamental

•

Output duty cycle: Max 55/45%

•

I/Os: Standard I/O pins multiplexed with OSCIN and OSCOUT

•

Cload: 10 to 20 pF

•

Drive level maximum: at least 100 µW

The values of the load capacitors CL1 and CL2 are heavily dependent on the crystal type and

frequency. Refer to the datasheet of the crystal manufacturer to select the capacitances. For

best oscillation stability, CL1 and CL2 normally have the same value. Typical values are in

the range from below 20 pF up to 40 pF (cload: 10 to 20 pF). The parasitic capacitance of

the board layout also needs to be considered and typically adds a few pF to the component

values (refer to AN2867).

A clock security system prevents any CPU fatal error from a HSE failure, as it safely

switches to HSI.

Figure 4. HSE clock sources

Hardware configuration

1. The value of REXT depends on the crystal characteristics. A 0 Ω resistor works well with most oscillators

but, it is not optimal. A typical value is in the range 5 to 6 RS (resonator series resistance). To fine-tune the

REXT value, refer to AN2867 (Oscillator design guide for ST microcontrollers).

MS32524V1

OSCIN

OSCOUT

External source

STM8

(I/O available)

MS32529V1

Load capacitors

STM8

CL2

CL1

OSCIN

OSCOUT

REXT

(1)

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