Conformity Magazine - September 2007 - (Page 38)

different compiler settings. Due to confidentiality I am only allowed to show the magnitude of the difference between the 2 compiler options and not the absolute levels. However, this shows that even making the same software a little more compact (for the ever present “future expansion”) has an impact on the radiated emissions profile, with over ±10dB of difference that can be observed between the two compiler versions. Worse still, this type of impact is not only almost impossible to predict, but neither version offers significant benefit over the other, and both have equally positive and negative impacts on the EMC performance. Programming Issues for EMC A program within a system, microcontroller, or even a simple logic controller can have an influence on the immunity as well as the emissions of a system. Immunity is often easiest to consider, as many error checking and code validation routines already exist to ensure that corrupted transmissions are not accepted. Hence, many programmers are already aware of the issues of firmware immunity, although they may not have considered this as an EMC issue. On the emissions side, it is more likely to be a case of acknowledging the best practices for addressing and operating the hardware from firmware codes. For example, setting a UART to tri-state from input, then to outputs, may reduce the transient power demand, compared to a direct input-tooutput change of state. This is generally known as defensive programming. Many techniques, such as a watchdog function, do not actually improve emissions or immunity but simply provide a controlled method of recovery. Techniques involving simple known recovery states are usually less program intensive than performance improving techniques. The latter tend to have a higher programming overhead (i.e., require more code to execute) and more memory for storage. Immunity Issues in Programming It is much easier to understand how the firmware in a product impacts on immunity than emissions, but conversely it is relatively easy to prove the emissions impact of code, given adequate testing resources (see Figures 1 and 2). Often, the impact of immunity programming is difficult to quantify and determine effectiveness without expensive and time consuming testing, typically well beyond what most companies are prepared to invest in. Once a product meets its essential EMC requirements, few companies test to failure to determine the immunity safety margin. Watchdog Programming When incorporating a watchdog circuit into a design, some programming may be required for the timer reset of the watchdog. If possible, the code should poll the watchdog pin once per program cycle. This is not too much of a problem with short microcontroller code or sequential routines but, with general operation processors and long programs, this may be difficult. There may therefore need to be additional timed interrupts or hardware generated interrupts to address the watchdog (e.g. dedicated watchdog interrupt handling code, as shown in Figure 3). The watchdog timer must be short enough to ensure noncatastrophic failure of the circuit, and long enough not to Figure 2: Same hardware, same firmware, different compiler options alone has an impact on measured radiated emissions. 38 Conformity September 2007 Figure 3: Software reset watchdog routine

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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