Oscillator circuit

AN3060

6/23

Doc ID 16253 Rev 2

2

Oscillator circuit

2.1

Besides the crystal, what other components do I need to add

to make my RTC oscillator work correctly?

None. Nothing else is required. ST's real-time clocks have built-in load capacitors.

External load capacitors are not needed, nor are external resistors. All that is needed is the

crystal. It must be a 32,768 Hz watch crystal and be rated for the load capacitance built into

the RTC.

Figure 2.

Built-in load capacitors

No other components are required to make the oscillator run. Adding any external

components will only serve to reduce the accuracy and reliability of the oscillator. Extra load

capacitors will tend to slow the clock. At low temperatures, such capacitance, or any

external resistance, may keep the oscillator from starting.

Matching load capacitance

Each RTC comes with built-in load capacitors. These are usually specified as 12.5 pF, or in

the case of the M41T6x family, 6 pF. The crystal selected for use with the RTC should have

this same rating. Mismatched crystals will tend to run slightly off frequency. For example, a

crystal rated for 12.5 pF expects to see that load. If it is used with an RTC rated for 6 pF, the

crystal will only see 6 pF instead of the specified 12.5 pF and will thus run fast.

XI

XO

CRYSTAL

INTERNAL RTC

CIRCUITRY

EXTERNAL LOAD CAPACITORS

NOT NEEDED

THEY ARE BUILT INTO RTC

EXTERNAL RESISTOR

NOT NEEDED

CHOOSE CRYSTAL TO MATCH RTC’S RATED

LOAD CAPACITANCE - eg, 6pF OR 12.5pF

AM04720v1