Conformity Magazine - September 2007 - (Page 44)

values, as follows: Code Comment :sub32 start of sub routine LDA &code load accumulator with code value LDB &ghost load ghost value CMP compare value and ghost BNE #error jump to error routine if values do not match XXX otherwise proceed with program section). If a setting of the port from logical outputs to inputs causes a large switch in internal states of an IC, resulting in a large current demand, an intermediate state may be available that has only a fraction of the transient current demand when switched (Figure 7). Using the intermediate state between transitions requires only one additional code instruction and will reduce transient supply demand and hence conducted noise levels. The intermediate state transition technique may be applicable to other circuits, such as clearing selective flags prior to rewriting, and may save the number of actual data or address lines driven, thereby again reduce transient demand. Actual transient savings in many circuits will be negligible, and the idea is best reserved for those functions that are known to require larger current supply (e.g., line drivers, interface circuits and bidirectional ports). Another technique that can help with interfaces is to use a coding scheme, such as Manchester coding, which has only a few frequencies in the transmission. In Manchester coding, only 2 frequencies are used: a ‘1’ is indicated by a single transition, and a ‘0’ by a double transition within the clock cycle (Figure 8). The signal is therefore always changing state, and a latched condition or end of transmission are also easy to detect. The receiving circuit can potentially determine if the sender is experiencing EMC problems. With standard non-return to zero (NRZ) coding, the frequencies present in the transmission can be from the clock frequency to whatever the maximum bit code length period is (i.e., 1/8 the clock frequency for an 8-bit code, as shown in Figure 9). Manchester coding makes filtering easier, if required, and allows for a simpler method of triggering on levels rather than edges that will also improve immunity. The clock of the transmission circuit is derivable from the signal; hence, using a fixed offset delay, a level triggered receive circuit is easy to implement in either software (using a delay routine) or in hardware (using a delay line). Converting standard edge triggered circuits to level triggered is possible at specific ports by using a software code delay to allow the value to settle prior to reading. The length of delay will depend on the function, but should allow the ringing or overshoot to settle and consequently not affect the value read at the port (Figure 10). A typical settling time This method is a memory implementation of the oversampling technique and could be extended if necessary, although at some cost if there is a lot of code (i.e., 3 or 4 copies could be used for comparison). If multiple ghost values are used, a mathematical method of selecting the correct value could be applied (e.g., numerical digital average of each bit). The method can also be selectively applied to code or data values from a known problematic location or EMC critical area of the system or program. Other Techniques There are other techniques that are more specific to the hardware being used and the best methods of operating interfaces and associated circuits (for example, the interface with a tristate setting mentioned in the first part of this Surge protection for: • AC Power UL 1449 2nd Edition • Data lines like T1/ E1, RS232, RS422, RS485, POE and HDSL • COAXIAL lines N, 7/16 DIN, TNC, SMA Ideal for Radio, Wireless and Cellular base stations. Surge Arrester Gas Tubes for PCB. Available in surface mount and through hole. Any voltage from 75V to 2500V. (305) 621-0022 • Fax (305) 621-0766 • e-mail: citel@citelprotection.com 1-800-CITEL-4U www.citelprotection.com Figure 10: Setting the port prior to reading can flush any noise built on the terminal pins when in a high impedance state. 44 Conformity September 2007 http://www.citelprotection.com http://www.citelprotection.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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