AN2867 datasheet(23/41 Pages) STMICROELECTRONICS

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AN2867

Guidelines for selecting suitable crystal and external components

4.2

Detailed steps to select an STM32-compatible crystal

This section describes the procedure recommended to select suitable crystal/external

components. The whole procedure is divided into three main steps:

Step 1: Check the resonator compatibility with the selected STM32

microcontroller

To check the compatibility between the selected crystal and the STM32 microcontroller, first

identify which procedure has to be followed among the two procedures described in

Section 3.4: Oscillator transconductance. The decision should be made based on the

oscillator specification provided in the product datasheet:

•

If the oscillator transconductance parameter is specified, then the first procedure

should be applied. Ensure that the gain margin ratio is higher than five (x5) to make

sure that the crystal is compatible with the selected STM32 microcontroller.

•

If Gm_Crit_max is specified instead, make sure Gm_crit for the oscillation loop is smaller

than the specified Gm_Crit_max value.

Step 2: Determine the capacitance value of the load capacitors CL1 and CL2

To determine the right capacitance values for CL1 and CL2 load capacitors, apply the

formula specified in Section 3.3: CL load capacitance. The values obtained are

approximations of the exact capacitances to be used. In a second phase, to fine tune the

values of the load capacitors, a series of experimental iterations should be performed until

the right capacitance values are found.

During the experimental phase, use an etalon crystal. An etalon crystal is a characterized

crystal which PPM drift is well known when it is loaded by the crystal nominal load

capacitance (CL). This kind of crystals can be provided by the crystal manufacturer upon

request. After the etalon crystal has been chosen, calculate its oscillation frequency (Fetalon)

when the crystal is loaded by its nominal load capacitance. This frequency is given by the

formula:

where:

Fetalon is the etalon crystal oscillation frequency when the crystal is loaded by its

nominal load capacitance.

Fnominal is the oscillation nominal frequency specified in the crystal datasheet.

PPMetalon is the oscillation frequency drift of the etalon crystal as it was characterized

by the crystal manufacturer.

Fetalon Fnominal PPMetalon 10

⁄

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