How to select a crystal for the microcontroller?

 

When designing a microcontroller-based system, one of the critical components to select is the crystal oscillator. The crystal determines the clock frequency, which influences the microcontroller’s timing, performance, and power consumption. This blog will walk you through the key factors to consider when selecting a crystal for your microcontroller, including frequency value, size, manufacturer considerations, layout guidelines, and additional factors that might impact your design.

1. Crystal Frequency Value

The first and most crucial step in selecting a crystal is determining the appropriate frequency value. The crystal's frequency directly influences the microcontroller's clock speed, which in turn affects the processing speed and timing of the system. Here are some considerations:

• Microcontroller Requirements: Check the datasheet of the microcontroller for the acceptable range of crystal frequencies. Some microcontrollers can operate at multiple frequencies, while others are optimized for a specific range.
• Performance vs. Power Consumption: Higher frequencies result in faster processing but can increase power consumption. If you're designing a low-power application, consider using a lower frequency crystal.
• System Requirements: Consider the timing requirements of your system. For applications that require precise timing, such as real-time clocks (RTC), a 32.768 kHz crystal is often used.

2. Package Size and Form Factor

Crystals come in various package sizes and shapes, which should align with your PCB design and application constraints. The most common packages include:

• HC-49: Available in both through-hole and surface-mount versions, HC-49 is one of the most commonly used crystal packages. However, it is relatively large, making it less ideal for compact designs.
• SMD Packages (e.g., 3225, 2520, 2016): Surface-mount device (SMD) packages are more compact and suitable for modern, miniaturized designs. The smaller the package, the more challenging it becomes to handle and solder, but it saves space on the PCB.
• Tolerance and Stability: Smaller packages may offer less stability and higher frequency tolerance variations, which could impact precision. Always verify these parameters in the crystal's datasheet.

3. Manufacturer and Quality Considerations

Choosing a reliable manufacturer is crucial for ensuring the long-term reliability of your crystal. Here’s what to look for:

• Reputation and Reviews: Opt for manufacturers with a proven track record. Brands like Epson, NDK, and Citizen are well-known for producing high-quality crystals.
• Certification and Standards: Ensure the crystal meets industry standards (e.g., AEC-Q200 for automotive applications) and has the necessary certifications for your application.
• Availability and Lead Time: Check for the availability of the crystal and the lead times. Some crystals may have long lead times, which could delay your project.

4. Electrical Parameters

Several electrical parameters must be considered to ensure compatibility and optimal performance:

• Load Capacitance (C_L): This is the capacitance that the crystal requires to oscillate at its specified frequency. It’s essential to match the load capacitance of the crystal with the capacitance provided by the microcontroller's oscillator circuit. Incorrect matching can lead to frequency drift or unstable oscillations.
• Equivalent Series Resistance (ESR): ESR impacts the start-up time and stability of the oscillator circuit. Crystals with high ESR may not start reliably, especially in low-power circuits. Always check that the ESR is within the microcontroller’s specified range.
• Drive Level: This parameter defines the power level at which the crystal operates. Exceeding the maximum drive level can damage the crystal, while too low a drive level can result in poor oscillation performance.

5. PCB Layout Considerations

Proper PCB layout is vital for ensuring the crystal operates correctly. Here are some layout tips:

• Proximity to the Microcontroller: Place the crystal as close as possible to the microcontroller’s oscillator pins to minimize trace inductance and reduce noise pickup.
• Minimize Trace Lengths: Shorter traces reduce parasitic capacitance and inductance, which can adversely affect the crystal's performance.
• Ground Planes: Implement a ground plane beneath the crystal and its associated components to shield them from noise. However, avoid placing the ground plane directly under the oscillator traces to prevent unwanted parasitic capacitance.
• Isolation from High-Frequency Signals: Keep the crystal and its traces away from high-speed data lines and power lines to reduce the risk of interference.

6. Environmental Considerations

The operational environment can impact the crystal’s performance and longevity:

• Temperature Range: Ensure that the crystal's operational temperature range aligns with your application's requirements. Crystals often have a specified frequency stability over temperature, so choose one that remains within acceptable limits for your design.
• Vibration and Shock: For applications exposed to high levels of vibration or shock, such as automotive or industrial settings, select a crystal with appropriate environmental ratings or consider using a MEMS oscillator as an alternative.
• Aging: Crystals can exhibit frequency drift over time due to aging. Review the manufacturer’s specifications for aging rates and choose a crystal with minimal long-term drift if your application requires precise timing over extended periods.

7. Cost vs. Performance Trade-offs

Finally, consider the cost-performance trade-offs:

• Application Sensitivity: For critical applications where precise timing is essential (e.g., communication systems), investing in a high-quality, low-tolerance crystal is justified.
• Budget Constraints: For less critical applications, where timing is not as stringent, a more cost-effective crystal with standard tolerance and stability may be sufficient.
• Volume Production: If your design is intended for mass production, consider the overall cost impact of the crystal on the bill of materials (BOM).

Conclusion

Selecting the right crystal for your microcontroller involves balancing several factors, including frequency, size, electrical parameters, and layout considerations. By carefully analyzing these aspects, you can ensure that your design is robust, reliable, and optimized for the specific needs of your application. Remember, the crystal is a small but vital component, and its proper selection is crucial for the overall performance of your system.