Appliance Design - October 2008 - (Page 34)

ELECTRONICS PWM ISR counter between 19 and 21 (20 x 0.5 ms giving 10 ms). Conversely, after 100 PWM interrupts, one must verify that there was between 4 and 6 heartbeat ISRs (100 x 0.5 ms = 50 ms = 5 x 10 ms); any other value outside these boundaries indicates an error to be handled. This method is rather simple and easy to implement, but it must be tailored case-bycase, depending on the number of ISRs to be served and on their frequency. For this reason, it is not included in the library. For the I/O periphery, Class B software must detect any malfunction on digital I/Os and on analog channels. This is covered by plausibility checks. For example, consider a heating element controlled with a triac and regulated with a NTC resistor connected to the A/D converter. If the temperature does not rise when the triac is switched ON, it indicates that the digital output or the triac is damaged, or that the ADC is not working properly. In any case, Class B software has to handle these error conditions and react accordingly. The firmware library is supported by documentation and a useful RS-232 based verbose mode. It has been proof-checked in real-condi- Fig. 2. Run-time transparent RAM test. will result in an MCU reset before proceeding with the MCU and peripherals initialization. The firmware library does not include all tests listed in the table H.11.12.7. Some are relevant only for systems having external memories (Section 5 for internal data path) or ASIC/gate arrays (Section 9 for custom chips). Some others checks are application dependent: Section 6 on external communication, Section 2 on interruptions, and Section 7 on I/O periphery. There are several things to consider for interrupts and I/Os. When it is necessary to monitor the interrupt rate and detect “no interrupt or too frequent interrupt,” one solution is to cross-check several independent time bases. For instance, in a system where there is a 10 ms heartbeat interrupt and a PWM update event every 500 microseconds, each interrupt can detect a malfunction with simple software counters. In the 10 ms ISR, one must read a Convenience Matters A. O. Smith Cares Are you looking for an electric motor supplier that can reduce assembly costs for your convection ovens? A. O. Smith provides convenience and added value by furnishing air-moving sub assemblies for convection fans and cooling blowers. A. O. Smith offers customized fan assemblies with mounting plates, heating elements and wiring harnesses to ﬁt your oven applications. Multiple bearing options are available for the best combination of cost and longevity of the motor Cross ﬂow blowers available in 60mm and 45mm wheel diameters Motors and fans designed to meet your convection oven’s airﬂow and noise speciﬁcations Contact A. O. Smith to see how we can meet your unique requirements. 531 N. Fourth St. • Tipp City, OH 45371 • Tel: (937) 667-2431 • Fax: (937) 667-5030 www.aosmithmotors.com 34 applianceDESIGN 1 October 2008 AD04084AOSmith.indd www.applianceDESIGN.com 3/12/08 2:55:59 PM http://www.aosmithmotors.com http://www.aosmithmotors.com http://www.applianceDESIGN.com

Table of Contents Feed for the Digital Edition of Appliance Design - October 2008

Appliance Design - October 2008 Contents Editorial Shipments/Forecasts News Watch Prototyping - Materials Play the Part Prototyping - Mix & Match Motors Electronics Coatings Design Marts Association Report: AHRI Advertiser's Index

Appliance Design - October 2008

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