Conformity Magazine - September 2007 - (Page 41)

located near the edge of the processor’s package (if included in the microprocessor itself), and may be connected directly to the external circuits. Consequently, these are highly likely to have noise on their lines. A simple way of minimizing this noise disturbing the data settings and propagating into the microprocessor IC or system is to refresh these registers regularly. The action of the microprocessor rewriting these data registers stabilizes the interfaced circuits and minimises the risk that noise on these ports is corrupting other internal registers. This is a simple task to perform regularly and involves minimal programming overhead, just occasional write commands to the ports and data registers. Some care does need exercising to ensure that writing the port status is appropriate to the program activity at each re-write command. It cannot be assumed that the value in the port status is the correct setting, so a read and then re-write could result in enforcing an erroneous setting. Polling Interface Ports and Oversampling Input and output to interfaced functions usually occurs at a much slower rate than the microprocessor clock. This enables the microprocessor to poll these pins several times to ensure that either the level is set for an output or that the incoming signal is stable. This is analogous with the types of over sampling used in digital audio circuits (e.g., CD players). The microprocessor can be programmed with an over sampling scheme, either to ignore the first and last and take the mean of a middle sample, for instance, or simple average. The actual scheme for polling can be left to the programmer and will depend on the type of interface being addressed (e.g., audio, keyboard, serial data link) and the processing rate. This type of programming has a code space and run time overhead, as it requires several repeats of an operation plus usually some mathematical interpretation. The result is an improved immunity to noisy input signals and less susceptible output ports. This may best be implemented using a dedicated input/output microcontroller with this type of software programmed onchip rather than as an auxiliary program for a main processor. Dedicated input/output interface controllers are quite popular in larger systems; hence adding local code to improve the immunity should be relatively easy. The same code could then be used for any system with that type of interface controller making the immunity improvement portable to other systems. Token Passing Token passing is a deliberate method of ensuring that the program is progressing in a controlled manner and has not been jumped into due to a code corruption. This requires additional programming steps at each critical subroutine or program block within the final program. Token passing requires a token (value) to be either updated in memory or in a register. As each routine is called, it checks to ensure that the call is from the previous set of code by comparing the token with a programmed value or memory location. If a jump has occurred prematurely, the token will not be set correctly; likewise, if the call has come from a random corruption, the token will be in the wrong state. On the discovery of a wrong token, a reset routine is initiated. The advantages of token passing over other methods are that the program continuously looks for errors, rather than waiting for an error to occur, and the token itself can be used to reset to a predefined location in the program. It may not be necessary to completely reset the system depending on the location of the error and the token value. This obviously saves program time but also gives a genuine level of immunity rather than a simple reset-on-error recovery. The extra programming overhead is in adding the compare program to Ferrites by Fair-Rite Products Three Key Elements: One Clear Choice Quality Engineering Service Fair-Rite offers a comprehensive line of ferrite components for EMI Suppression, Power Applications, and RFID Antennas. We have an experienced team of engineers to assist you with new designs. Although we are not in Hawaii with you, we are always available to support you. September 2007 Conformity 41 http://www.fair-rite.com http://www.fair-rite.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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