Conformity Magazine - September 2007 - (Page 45)

should be 10% of the maximum clock period. This can also be implemented in hardware using a delayed latch trigger, with the data ready signal coming from the delayed latch set condition (Figure 11). It is generally easier to give these examples in assembler or in a flow diagram as it is easier to understand what any target microprocessor is expected to do, but the concepts are not exclusive to assembler and should, in most parts, be able to be coded in any higher level language. While it is true that some compilers may make some of these possible methods difficult, others can be available as a compiler option (certainly a feature such as fill unused memory with NOP can be found in many modern compilers). Nothing Comes for Free Many think software changes and improvements are free, but this is simply not true. While these techniques have obvious overhead costs in thinking about the issues, and will possibly slow code development down while they are implemented, there will also be some increase in code size and hence memory requirements. The magnitude of the increase required for each technique is difficult to specify with any great accuracy, as it will depend on the code structure, complexity and even function to some extent. However, Table 1 gives a “rough guide” to what impact in code size terms the techniques discussed here might have. “EMC is Not a Software Issue” This is a response that often prevents these techniques from being implemented, and certainly few software engineers will accept any responsibility for EMC. Many of the defensive techniques are often seen as “hacks” for poor hardware implementation, but the evidence at the start of this article for the impact of software on EMC is clear. This will not make it any easier to get your software people to accept these methods, or to recognize that they can impact the EMC of a product at all. The greatest problem is the difficulty in quantifying the benefits, as these will vary from product to product. For example, I can not suggest that using a tri-state intermediate condition will reduce your emissions by 20dB, since there is simply no easy way to put a figure on this and, if your emissions are very low anyway, the chances are there would be no noticeable impact. But it is better to start with the best conditions possible than to wait until the test results come back negative and have to search for ideas to improve the product with a launch date looming. On the other side of the coin, if results are close to the limit, a software change may be a lower cost solution than filters and a new hardware revision. Martin O’Hara runs the Automotive EMC Network (www.autoemc.net), and can be reached at news@autoemc.net. Figure 11: Delayed port read is equivalent to a software low pass filter. Technique Unused Memory Fill Token Passing Port Delay Port Refresh Oversampling Ghost Coding Table 1 Code Increase 0% 5-15% 1% 2-5% 2-5% 10-30% SAY GOODBYE TO EMI Differentiate your products by combining the functionality of EMI shielding with decorative paint or chrome plating ﬁnishes. Cybershieldʼs high performance processes utilizes Electroless Plating, Electro-Plating and Conductive Paint to apply coatings to plastics. We assist with the design and manufacture of your custom EMI Shielding system, and provide mechanical assembly services to streamline your supply chain. FAST Link www.conformity.com/0960 214-227-3680 • www.cybershieldinc.com • sales@cybershieldinc.com September 2007 Conformity 45 http://www.cybershieldinc.com http://www.autoemc.net http://www.conformity.com/0960 http://www.cybershieldinc.com

Table of Contents Feed for the Digital Edition of Conformity Magazine - September 2007

Contents Editor’s Note FCC Extends Disability Access Requirements to VOIP Commission Fines Importer $3 Million Over Non-Compliant TVs FCC Gets Tough on Retailers Over Analog TV Tuner Labeling Analog Cellular Service Sunset Date Reaffirmed Commission Adopts Recommendations of Katrina Panel Free-Space Antenna Factors Through the Use of Time-Domain Signal Processing Editorial Correction ESD Open Forum An Update on Revisions to IEC 61000-4-2, the Basic Standard on ESD Practical Advice for Meeting the DFS Testing Requirements of the FCC The EMC Impact of Embedded Software ESD Design Buyer's Guide Tate Renominated to the FCC EPA Releases Report on U.S. Electronic Waste Additional Standards For The EU’s Toy Directive Medical Device Manufacturer Enters Consent Order Updated Guidance Document on Laser Products Issued FDA Offers Advice on Defective Heating Pads CPSC Issues China Product Safety Program Report Other CPSC Actions In The News IEC Standards Update Product Reviews Product News UL Standards Update From Our “You Can’t Make This Stuff Up” Department Looking Back: Items from Past Issues of Conformity Advertisers

Conformity Magazine - September 2007

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