Finding Workarounds for the Obsolete ATTINY15 1.5MHz Clock for FCC Exemption
The ATTINY15 is a popular microcontroller, known for its low cost and simplicity. It is frequently used in hobbyist projects, but a potential stumbling block for designers is the 1.5MHz clock frequency limitation. While this frequency falls within the FCC exemption for unintentional radiators, it can present difficulties when seeking FCC certification for products that utilize this microcontroller. This article explores the challenges and potential solutions for overcoming this limitation when designing with the ATTINY15.
Understanding FCC Regulations and the ATTINY15
The Federal Communications Commission (FCC) has regulations in place to minimize electromagnetic interference (EMI) and radio frequency interference (RFI) caused by electronic devices. These regulations often require devices to undergo testing and certification to ensure they meet specific emission limits.
The FCC has a specific exemption for devices operating below 150kHz, allowing these devices to be marketed without formal testing or certification. However, this exemption only applies to unintentional radiators – devices that are not designed to transmit radio signals.
The ATTINY15, operating at 1.5MHz, falls outside this 150kHz limit and is technically subject to FCC testing and certification. Although this is a standard operating frequency for the ATTINY15, it can complicate the certification process, especially when operating close to the 150kHz limit.
Challenges with the 1.5MHz Clock
The main challenge in designing with the ATTINY15 and aiming for FCC compliance lies in managing the inherent EMI generated due to its 1.5MHz operating frequency. This high-frequency operation can lead to unwanted emissions that might exceed FCC limits, potentially hindering the product's certification.
1. Frequency Range:
The ATTINY15's clock frequency, while within the FCC's exemption, is close enough to the 150kHz limit that even slight variations in the oscillator's frequency, temperature fluctuations, or power supply inconsistencies could potentially push the operating frequency over the limit. This would then require FCC testing and certification.
2. Noise Generation:
Higher-frequency operations can generate more noise, which might cause unwanted emissions. The ATTINY15, operating at 1.5MHz, could be susceptible to generating enough noise to create problems with FCC compliance.
3. Shielding:
Shielding is a crucial part of controlling EMI, and the ATTINY15's high-frequency operation might require more robust shielding compared to devices operating at lower frequencies. Effective shielding can be costly and might add complexity to the product design.
Workarounds and Solutions
While directly addressing the 1.5MHz clock frequency might seem impossible, several workarounds and solutions can be implemented to mitigate the potential issues for FCC compliance.
1. Frequency Reduction
One of the most straightforward solutions involves reducing the ATTINY15's operating frequency. This can be achieved through a combination of clock divider circuits and careful software design.
-
Clock Divider Circuit: Implementing a clock divider circuit can effectively reduce the clock frequency delivered to the ATTINY15. By carefully choosing the divider ratio, you can bring the operating frequency well below the 150kHz limit, ensuring FCC exemption.
-
Software Design: The software used to control the ATTINY15 plays a crucial role in minimizing EMI generation. Techniques like careful code optimization, minimizing clock cycles required for critical operations, and reducing switching times can help minimize unwanted emissions.
2. EMI Filtering
The use of effective EMI filters can significantly reduce the unwanted emissions generated by the ATTINY15. These filters are typically passive circuits designed to attenuate specific frequency bands, effectively blocking out EMI generated at the device's operating frequency.
-
Passive Filters: Various passive filters like low-pass filters, band-stop filters, and high-pass filters can be implemented based on the specific needs of the application. These filters can be effectively used to filter out the unwanted emissions generated by the ATTINY15, enhancing its FCC compliance.
-
Active Filters: More complex active filters using operational amplifiers can be employed for greater control over the frequency response and attenuation characteristics. These filters can be customized to effectively remove specific frequency bands of EMI, further reducing the need for FCC testing and certification.
3. Shielding and Grounding
Effective shielding and grounding are critical for reducing EMI emissions. Proper shielding and grounding practices can significantly contribute to FCC compliance.
-
Shielding: The ATTINY15 and its associated circuitry should be housed in a metal enclosure with appropriate shielding to minimize the escape of unwanted emissions.
-
Grounding: A dedicated ground plane should be used for the ATTINY15 and its circuitry, ensuring that all components are properly grounded to minimize noise propagation.
Best Practices for FCC Compliance
Beyond the workarounds mentioned, several best practices can be implemented to improve the chances of FCC compliance while using the ATTINY15.
-
Use of Low-Noise Components: Replacing components with low-noise alternatives can significantly minimize EMI generation. Using low-noise oscillators, power regulators, and other components can contribute to a cleaner signal and reduced emissions.
-
Component Placement: Careful placement of components on the PCB layout can help minimize EMI. By keeping components with high-frequency operation away from sensitive areas and using proper routing techniques, you can effectively reduce unwanted emissions.
-
PCB Layout Design: Using proper PCB layout design techniques like using ground planes, controlled impedance traces, and minimizing loop areas can significantly reduce EMI.
-
Testing and Verification: Testing and verifying the device's EMI emissions before submitting for FCC certification is crucial. Using test equipment like spectrum analyzers and EMI test chambers can help identify and address potential issues early on.
Conclusion
While the ATTINY15's 1.5MHz clock frequency presents a challenge for FCC compliance, it is not an insurmountable hurdle. By implementing the workarounds and best practices discussed in this article, designers can effectively mitigate EMI emissions and achieve FCC compliance. It is important to remember that thorough testing and verification are essential to ensure the product meets the required standards. The ATTINY15 remains a viable option for many projects, and with careful planning and execution, it can be used to create successful products that meet FCC requirements.