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Volume 65 (Apr 13, 2007)

A top-caliber ASIC core: SSCG macro for EMI noise reduction in electronic devices

We live in a world in which we are surrounded by electromagnetic waves. For example, the visible light originating from sunlight is actually electromagnetic wave energy, and there are electromagnetic waves that occur as natural phenomena. Other electromagnetic waves include infrared and ultraviolet rays. On the other hand, radio waves used for TV, radio, transmission, mobile phones, and other products are artificially produced electromagnetic waves. Even in our day-to-day lives, we encounter electromagnetic waves produced by electronic devices, including refrigerators, microwave ovens, PCs, printers and digital cameras. Since electromagnetic waves from electronic devices like these can cause transmission interruptions and malfunctions in electronic devices, standard values for electromagnetic waves are being strengthened in countries around the world as regulations.

For instance, if a product does not clear the noise value stipulated by CISPR^*1, which is the representative standard for EMI noise regulation, the electronics manufacturer must completely start the development process over from scratch. Moreover, due to advancements being made in the functionality of electronic devices, there has been a tendency toward increases in EMI noise.

The SSCG circuit and its contribution to EMI noise reduction

The most common methods for reducing EMI noise produced by electronic devices are the mounting of noise suppression components such as capacitors and ferrite beads or a spread-spectrum clock generator (SSCG) circuit on a printed circuit board, or the enclosing of the noise source with a steel plate. Among these, SSCG circuits are attracting a great deal of attention.

An SSCG circuit is a clock-generation circuit capable of lowering the spectrum peak value by modulating the clock signal frequencies of ASIC and other products, and thereby reducing EMI levels. With the frequency modulation method, the frequency multiplication ratio is changed at fixed intervals and the frequency spectrum is dispersed, thus lowering the peak value (Figures 1 and 2). EMI noise can be reduced through the mounting of this SSCG circuit. However, if the SSCG circuit is attached externally, there remain problems such as higher costs due to the increase in the number of components and a larger circuit board area. In addition, since noise measurement tests are conducted after the SSCG circuit has already been mounted on the circuit board, noise readings are not necessarily identical to those measured for the circuit alone. To take the concept of EMI noise suppression one step further, NEC Electronics came up with the idea of incorporating a noise reduction function into the LSI itself; that is, the development of the SSCG circuit (SSCG macro) for on-chips.

Advantages of the SSCG macro for on-chips

By equipping our company's proven ASIC products with the SSCG macro, we have achieved even greater EMI noise reduction. Since our development of an LSI chip for printer use in 1997, we have carried out developmental research on SSCG circuits for on-chips based on the concept of incorporating noise reduction functions into an LSI itself. Using this circuit technology, we have developed an SSCG macro for 0.35µm cell-based ICs, as well as 0.25µm CB-10 and 0.15µm CB-12 macros to keep pace with advancements made in processes, and many ASIC/ASSP products are now equipped with these macros. In addition, in 2005, we developed an SSCG macro for the 90-nanometer (nm) CB-90, stabilizing modulation characteristics and using a new method (Figure 3) with little dispersion in EMI characteristics for the avoidance of degradation due to the conditions of the degree of peak reduction. In this way, we have been carrying out SSCG macro development activities (Table 1) for the purpose of coping with increased EMI noise resulting from advancements in the functionality of electronic devices.

Incorporating the SSCG makes EMI characteristics more favorable than when the SSCG is attached externally (Figure 4). For example, users can expect significant improvements in EMI noise reduction, including a greater degree of modulation and no need for external wiring for the output chip. Moreover, in addition to realizing low power consumption, there is a greater degree of freedom in settings since the multiplication ratio, modulation frequency and degree of modulation are programmable.

In this way, ASIC products such as cell-based ICs can be equipped with the SSCG as an ASIC macro without raising costs. In addition, this can contribute to the shortening of the EMI noise evaluation period. That is why we offer solutions to printer, digital camera, digital TV and other electronics manufacturers that enable both reduced burden in terms of EMI noise suppression and higher functionality.

Working toward even further noise reduction

As we move toward even higher functionality in the future, advancements in the increase of frequencies and miniaturization of processes—from 90 nm to 55 nm—are being made. Since this will of course lead to increases in EMI noise and thus serious problems, noise suppression will become necessary for all sorts of electronic devices. NEC Electronics will remain a step ahead by continuing to contribute to the reduction of EMI noise in electronic devices by engaging in the development of both ASIC products that realize high functionality and SSCG macros with characteristics that are even more superior.

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Developer comments

What is the best way to suppress EMI noise? To find a solution to this problem, we put ourselves in the place of device manufacturers was and decided to incorporate an SSCG macro in ASIC products. We concluded that equipping ASICs with these SSCG macros would not only contribute to EMI noise reduction, but also shorten the evaluation period, reduce the number of components and lower costs. Although there is pressure in knowing that if EMI characteristics are unsatisfactory, ASIC products themselves will be questioned, we as developers are using our knowledge and knowhow to proceed with product development in cooperation with our in-house ASIC development department. We also feel a sense of satisfaction when chips equipped with SSCG macros are highly praised by our customers. It is our hope that we contribute to our customer by reducing EMI noise in chips and also we offer comfortably utilize sets to user.

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