Conformity Magazine - September 2007 - (Page 42)

the start of each sub-routine and the token update to the end, plus any error handling software control if a system reset is to be avoided, as follows: Code Comment :sub21 start of sub routine LDA &token load accumulator with token LDB &value load correct token value CMP compare token and value BNE #reset jump to reset routine if token incorrect XXX otherwise proceed with program XXX LDA &token load token INC increment token value for next routine STA &token save token :end_sub21 end of sub routine program If attempting to use the token passing technique to recover to a known program position, more programming effort will be required and probably several tokens to ensure register and flag conditions are suitable for re-entry into areas of code. Using the token value alone is insufficient to reset routines where register or memory writes have occurred, as these will most likely be incorrectly set. Unused Memory Addresses In most applications it is impossible to exactly fill all memory locations, either ROM or RAM. These locations could be accessed due to a software or hardware addressing error caused by a corrupted code. These unused memory locations should therefore have a known operation placed in them, usually either a no operation (NOP) or an unconditional jump to a reset routine (JMP RESET), as follows: Address 0000 00FA 00FB 00FC 010F 0110 Code XXX XXX NOP NOP JMP RESET XXX Figure 6: AC coupled watchdog needs closer control of set/reset activity of watchdog line, hence has a higher code overhead. Comment functional program area end of program area start of unused memory block jump to reset routine functional program area Figure 4: A hardware watchdog based on low count line activity needs careful implementation. Where blocks of memory are unused, the jump to reset should be at the bottom of the memory with no operation commands above. These blocks may be scattered about the ROM/RAM locations and need to be carefully mapped if this type of feature is to be used. Writing these areas will be required by the initialization program if they are in RAM locations. There is of course the possibility that a corrupted write code could rewrite these NOP instructions in the RAM space, similarly with the program itself, but it is impossible to program for all eventualities and a reasonable trade-off between effectiveness and efficiency of programming has to be drawn. Figure 5: By using an AC coupled watchdog, static “reset” states are also captured. 42 Conformity September 2007 This technique requires only a small amount of additional programming to the start-up (boot) routine and should not require any run time program overhead. The filling of

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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