AN2867 datasheet(13/41 Pages) STMICROELECTRONICS

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DocID15287 Rev 9

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AN2867

Pierce oscillator design

3.3

CL load capacitance

The load capacitance is the terminal capacitance of the circuit connected to the crystal

oscillator. This value is determined by the external capacitors CL1 and CL2 and the stray

capacitance of the printed circuit board and connections (Cs). The CL value is specified by

the crystal manufacturer. Mainly, for the frequency to be accurate, the oscillator circuit has to

show the same load capacitance to the crystal as the one the crystal was adjusted for.

Frequency stability mainly requires that the load capacitance be constant. The external

capacitors CL1 and CL2 are used to tune the desired value of CL to reach the value specified

by the crystal manufacturer.

The following equation gives the expression of CL:

Example of CL1 and CL2 calculation:

For example if the CL value of the crystal is equal to 15 pF and, assuming that Cs = 5 pF,

then:

. That is:

3.4

Oscillator transconductance

Theoretically, to make the oscillation start and build up until it reaches a stable oscillation

phase, the oscillator should provide sufficient gain that at the same time compensates for

the oscillation loop losses and provide the energy that makes the oscillation build up. When

the oscillation becomes stable, the equality between the oscillator provided power and the

oscillation loop dissipated power is achieved.

Practically speaking and due to tolerances on passive component values and their

dependency on environmental parameters (e.g. temperature), a ratio of x1 between the

oscillator gain and the oscillation loop critical gain is not recommended. This will induce a

too long oscillator startup time and might even prevent the oscillator from starting up.

This section describes the two approaches that can be used to check if an STM32 oscillator

can be paired with a given resonator in order to ensure that the oscillation is started and

maintained under the specified conditions for both resonator and oscillator. The approach

depends on how the oscillator parameters are specified in the microcontroller datasheet:

•

If the oscillation loop maximal critical gain parameter (gm_crit_max) is specified, it is

important to make sure that the oscillation loop critical gain (gmcrit) is smaller than the

specified parameter.

•

If the oscillator transconductance parameter (gm) is specified, make sure that the gain

margin ratio (gainmargin) is bigger than x5. Below the calculation formulas for both gmcrit

and gainmargin.

where:

gm is the oscillator transconductance specified in the microcontroller datasheet. Note

that the HSE oscillator transconductance is in the range of a dozens of mA/V while LSE

CL1 CL2

--------------------------

CL Cs

–

CL1 CL2

--------------------------

10 pF

CL1

CL2

20 pF

gainmin

arg

gmcrit

---------------

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