How to Specify a Crystal's Load Capacitance for an Oscillator

1. Identify the Crystal Load Definition

A crystal requirement needs more than frequency and a familiar case size. Identify the exact resonator and the oscillator stage it serves. Locate the load-capacitance entry in the crystal manufacturer's information, keeping its units and reference document. If the original code is unknown, preserve that gap rather than borrowing the CL value of a similar-looking item.

ST's AN2867 explains load capacitance as the terminal capacitance presented by the circuit connected to the crystal. The crystal manufacturer specifies the load associated with its nominal frequency. This provides the central distinction for purchasing: the resonator's specified CL and the values of individual capacitors on a schematic are related quantities, not interchangeable labels.

Ask the oscillator designer which load requirement belongs to the released configuration. Keep the approved crystal and circuit references together. The application note covers its stated ST device context; it does not supply a universal selection rule for every oscillator. This guide gives no numerical crystal choice or frequency-adjustment example.

2. Record Circuit Contributions to the Load

The AN2867 load discussion identifies the external load capacitors and the stray capacitance of the board and connections as contributors. Preserve the circuit context when the requirement is reviewed. Reading only one capacitor marking would leave out the relationship the designer needs to assess.

Suggest a review record that names each contribution, its evidence and any uncertainty. The engineer can distinguish documented component values from an estimated parasitic contribution or an input awaiting confirmation. Do not set an unknown contribution to zero simply to complete the table. Nor should a capacitance value be copied from a different layout without explaining its applicability.

Keep a proposed component change visible at the circuit level. If a capacitor, crystal or layout reference changes, ask the design owner whether the prior load assessment still applies. This is a review question, not a prediction of the resulting frequency. No board capacitance was measured and no oscillator equation was evaluated for this article.

3. Preserve the Crystal and Host References

Retain two separate source references: the exact crystal information and the host oscillator guidance. The crystal source identifies its own characteristics, while the host document supplies the applicable oscillator context. A general application note cannot fill missing fields in an unidentified crystal's ordering description.

Check document applicability before quoting a recommendation. Record the host device or oscillator block, document revision and the scope of any manufacturer example. If a listed combination differs from the customer's hardware, keep it as a reference requiring review. Do not describe membership in a recommendation table as approval of another board.

Other crystal and oscillator requirements should remain separate checks assigned by engineering. The narrow load-capacitance question does not settle every electrical, mechanical or assembly condition. Link the approved information rather than combining unrelated characteristics into one assertion that the crystal is compatible. Missing source data remains a reason to seek clarification.

4. Keep Tuning and Validation with Engineering

Ask the design owner to identify the evaluation needed to accept the load configuration. Keep selection, adjustment and verification as separate statuses. A documented target is an input to that process; it is not evidence that the actual oscillator has been tuned or tested.

Where an existing evaluation record is available, retain the circuit revision and conditions it covers. A result from an earlier layout or another host should not silently become the evidence for a new configuration. If no record is supplied, state that evaluation is pending instead of presenting an application-note illustration as a customer measurement.

For an ICULTRA sourcing discussion, provide the approved crystal code and the permitted technical references, with unresolved load or identity questions clearly separated. Keep any proposed alternative conditional until engineering completes its decision. The linked BOM guide handles revision controls, and the MLCC guide addresses a different capacitor operating-condition question. This article establishes no computed load, tuned frequency, compatibility or supply result.

Sources and further reading

• STMicroelectronics — AN2867 Oscillator Design Guidelines

ST AN2867 supports the qualitative load definition within its stated device scope. No crystal ordering example, capacitor calculation, diagram measurement or board-frequency validation is supplied.