AN2867 datasheet(24/41 Pages) STMICROELECTRONICS

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Guidelines for selecting suitable crystal and external components

AN2867

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DocID15287 Rev 9

When Fetalon is computed, execute the sequence below:

The first experimental iteration should be made with CL1 and CL2 capacitance values

determined by calculation:

–

If the oscillation frequency is equal to Fetalon, then CL1 and CL2 are the correct

capacitances. You can therefore skip sub-steps 2) and 3).

–

If the oscillation frequency is slower than Fetalon then go to sub-step 2).

–

Otherwise execute sub-step 3).

For this experimental iteration, decrease CL1 and CL2 capacitance values, measure

again the oscillation frequency and compare it to Fetalon:

–

If the oscillation frequency is slower than Fetalon, execute sub-step 2).

–

Otherwise execute sub-step 3).

–

If the oscillation frequency is almost equal to Fetalon then the latter CL1 and CL2

capacitance values should be used.

For this experimental iteration, increase CL1 and CL2 capacitance values, measure

again the oscillation frequency and compare it to Fetalon:

–

If the oscillation frequency is slower than Fetalon then execute sub-step 2).

–

Otherwise execute sub-step 3).

–

If the oscillation frequency is almost equal to Fetalon then the latter CL1 and CL2

capacitance values should be used.

Step 3: Check the Safety Factor of the oscillation loop

The safety factor should be assessed as described in Section 3.8: Safety factor to ensure a

safe oscillation of the oscillator under operating conditions.

Note:

Many crystal manufacturers can check microcontroller/crystal pairing compatibility upon

request. If the pairing is judged valid, they can provide a report including the recommended

CL1 and CL2 values as well as the oscillator negative resistance measurement. In this case

steps 2 and 3 can be skipped.

Step 4: Calculate the drive level and external resistor

Compute the drive level (DL) (see Section 3.5: Drive level (DL) and external resistor (RExt)

calculation) and check if it is greater or lower than DLcrystal:

•

If DL < DLcrystal, no need for an external resistor. Congratulations you have found a

suitable crystal.

•

If DL > DLcrystal, you should calculate RExt in order to have: DL < DLcrystal. You should

then recalculate the gain margin taking RExt into account.

If you find that gain margin > 5, congratulations, you have found a suitable crystal. If

not, then this crystal will not work and you have to choose another. Return to step 1 to

run the procedure for the new crystal.

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