Oscillator

AN2353

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1

Oscillator

The ST10F27x can run with an external clock connected to the XTAL1 input pin of the

oscillator inverter or with a clock signal generated by a resonator connected to the XTAL1 /

XTAL2 pins. According to the device, two kinds of oscillators have been implemented:

●

ST10F276E, ST10F275E and ST10F273E: Wide-swing oscillator

●

ST10F272E/B and ST10F271E/B: Low-power oscillator

Moreover, the ST10F27x provides a new feature when the Real Time Clock module is used

and a reference clock is needed in Power Down mode. In this case, two possible

configurations may be selected by the user application according to the desired level of

power reduction:

●

A 32 kHz crystal is connected to the on-chip 32 kHz oscillator (pins XTAL3 / XTAL4)

and running. In this case, the main oscillator is stopped when Power Down mode is

entered, while the Real Time Clock continues counting using the 32 kHz clock signal as

reference.

●

Only the main oscillator is running (XTAL1 / XTAL2 pins). In this case, the main

oscillator is not stopped when Power Down is entered and the Real Time Clock

continues counting using the main oscillator clock signal as reference.

Refer to the ST10F27x datasheet for the possible combinations. This chapter provides

detailed information on the use of the on-chip oscillator in conjunction with an external

resonator.

1.1

Oscillator characteristics

Although simple to implement, using an external resonator (crystal or ceramic resonator)

requires a few basic precautions. Referring to the schematic of the on-chip oscillator

(

Figure 1), the key items are described in the following section.

Figure 1.

Oscillator characteristics

The resonator component can be a crystal or a ceramic resonator. It is represented as a

capacitances and the stray capacitance of the resonator.

On-chip Oscillator circuit

Resonator

XTAL1

XTAL2

Equivalent circuit

Resonator