Appliance Design - October 2008 - (Page 32)

ELECTRONICS (registers and program counter), interrupts (handling and execution), clock frequency monitoring, checks on variable memory (RAM) and invariable memory (Flash, EEPROM), external communications and peripherals, and a few others besides these main ones. These tests can be carried out directly with the microcontroller internal hardware or indirectly by software means. The STM32 has builtin hardware features to monitor CPU’s program counter stuck-at faults, abnormal clock frequency, and Flash memory content integrity. The norm proposes an “independent time-slot monitoring” to prevent any CPU run-away in case of a PC malfunction: this is the duty of the watchdog timer, which is embedded in most MCUs. Nonetheless, the norm states that it must be independent. For this reason, the STM32 has two watchdogs: a regular window watchdog running on the main clock source, and a second watchdog, using an independent internal oscillator and started with an option byte located in Flash memory. This ensures that at least one watchdog will be active in case of crystal failure and whatever the clock circuitry configuration. Fig. 1 shows an overview of the STM32 clock circuitry, which shows the various clock sources and their connections to the rest of the chip. This diagram shows that the real-time clock can be supplied by the LSI (Low Speed Internal) RC oscillator. This provides a means to monitor the external clock by comparing the expected external frequency with an internal one, as requested for the clock component. At the system level, this saves the cost of a 50/60 Hz mains zero-crossing detection circuitry, unless this is requested by the application to fire a triac. Finally, the STM32 embeds a 32-bit hardware CRC calculation unit, which significantly speeds-up flash content integrity check and reduces the related CPU load (spent during run-time) to a negligible value. Most of the other tests require a non-negligible silicon area to be implemented by hardware. This is the case for single-bit fault detection in RAM, for instance, which implies an addition of a parity bit on all data words. This option was discarded because it would have impacted the cost-effectiveness of the controller, and it can easily be implemented by software. For this reason, STMicroelectronics freely provides a self-diagnostic firmware library, which contains C modules for the following tests: CPU registers test. Clock monitoring. RAM functional check. ROM checksum integrity check. Watchdog self-test and stack overflow monitoring. (Even if not explicitly asked by the norm, this improves overall fault coverage). Looking at one of these modules, the RAM functional test, will help with understanding Class B software constraints. The standard asks to periodically test the RAM for DC faults, such as stuck-at or coupling faults. The March C- algorithm was chosen for its ability to detect all these faults, with a limited number of passes. However, to be efficient, it requires taking into account the physical implementation of the memory. In the STM32, there is a logical vs. physical address scrambling. The large register set of the Cortex-M3 core allows the descrambling to be included in the algorithm without using RAM memory cells (the test is destructive and must verify the whole memory array). As an additional requirement, this test must Our pumps for more efficiency! Give free rein to your ideas. Your requirements are our drive – and have been for more than 60 years. HANNING's sophisticated and customized pumps and drive systems in synchronous technology result from a close contact with our worldwide customers. If you wish highest efficiency and lowest ppm rates come to HANNING. Challenge us! HANNING ELEKTRO-WERKE GmbH & Co. KG · Holter Straße 90 D-33813 Oerlinghausen · Tel +49 (5202) 707-0 · Fax +49 (5202) 707-301 info@hew.hanning.com · www.hew.hanning.com 32 applianceDESIGN AD10084Hann.indd 1 October 2008 9/3/08 9:13:11 AM www.applianceDESIGN.com http://www.hew.hanning.com http://www.hew.hanning.com http://www.applianceDESIGN.com

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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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