Conformity Magazine - September 2007 - (Page 40)

interfere with functionality of the program (durations between 10ms and 2s are typical for watchdog timers). The trade-off in timing can be difficult to gauge correctly for some coding. Processors with a Harvard bus structure (independent data and instruction busses) are easier in this respect, as each instruction is known to require a single clock cycle. Therefore code timing is simple—just count the instructions and divide by the clock frequency: watchdog time out = number of instruction cycles watchdog clock frequency A potential problem could occur if the watchdog code is not included within the sleep cycle for those processors with low power sleep modes. When the processor goes into sleep mode and the internal clock frequency is reduced, unless the watchdog interrupt or poll function is similarly adjusted, a reset could be implemented simply because the sleep mode was activated. (I have seen this more frequently than I like to admit, even when accompanied by the common solution of removing the watchdog to solve the problem.) Watchdog programming has memory overhead since it requires additional program routines or interrupts. This may not be possible with fixed memory microcontrollers and a hardware solution may be required. Likewise, in real time operating systems (RTOS), interrupts or software-based polling may not be possible due to their effect on program functionality. One simple hardware solution is to monitor lines that are known to change frequently, such as the lowest order address or data line, and use this for the watchdog polling line (Figure 4). The biggest problem here is ensuring that these lines will always change with variable code and within the watchdog timing regime. The advantage of hardware is that it requires no programming, and therefore less memory. The disadvantage is the increase in component count to implement the function. It is most likely to cost more in development time to implement the watchdog function in software than in hardware, but code is generally the best implementation, especially as even a crashed program could continue to toggle the lines a hardware based watchdog system is monitoring. The most effective reset coding for a watchdog timer is to dedicate an output pin of the microprocessor to the watchdog (Figure 5). This pin should be set and cleared alternatively as the program routines are executed (Figure 6). By using an AC coupled watchdog set and reset, if the code loops within a single routine that includes a watchdog set command, the watchdog is still effective and this single port condition status and will operate the reset. This effectively doubles the robustness of the watchdog with only a small code overhead (alternating port set and port clear commands). Refresh Port Connections The data direction registers and the input/ output port data registers are usually 40 Conformity September 2007 http://www.techtoursite.com http://www.techtoursite.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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