AN3060

Oscillator circuit

Doc ID 16253 Rev 2

7/23

Effective load capacitance value versus internal capacitance value

Occasionally, the reader will see a reference to the individual load capacitors. There are two

used inside the RTC - one on each leg of the crystal. The actual value of each is twice the

rated or effective load value - that is, twice the load value specified in the data sheet. This is

because the internal load capacitors add in series. In Figure 2, the effective load

capacitance is:

capacitors built into its oscillator circuit. The reader is encouraged to be aware of this.

Furthermore, the reader should keep in mind that the crystal specifications should match

the rated value - in this case 12.5 pF - and not the individual values.

2

L

1

L

2

L

1

L

2

L

1

L

L

C

C

C

C

C

with

series

in

C

C

+

•

=

=