检测电路保持您的微处理器控制-Supervisory Cir

文章摘要：A companion feature to battery-backup switchover is chip-enable write protection or chip-enable gating (Active-low CE gating). The Active-low CE line from µP or address-decode logic, which normally goes to the SRAM, is routed instead through the supervisory chip to the SRAM. This signal normal

检测电路保持您的微处理器控制-Supervisory Cir,标签：计算机控制技术,工厂电气控制技术,http://www.88dzw.com

A companion feature to battery-backup switchover is chip-enable write protection or chip-enable gating (Active-low CE gating). The Active-low CE line from µP or address-decode logic, which normally goes to the SRAM, is routed instead through the supervisory chip to the SRAM. This signal normally passes through the chip unaltered. During reset, however, the supervisory chip forces Active-low CE high, disabling access to the memory and thereby protecting the SRAM contents from errant writes by a µP that has temporarily lost its mind.

Most recent supervisory chips are relatively simple devices that reside in small packages such as the SOT, but some offer additional features. The MAX818, for example, provides basic power-monitoring and watchdog capability, along with battery-backup switchover and CE gating in an eight-pin µMAX package (Figure 6). The battery-backup switchover circuit in this part also provides a "battery freshness seal" that prevents the discharge of batteries installed in products before shipment.


Figure 6. This supervisory device includes a watchdog timer, battery-backup switchover, and chip-enable gating, along with basic supply-voltage monitoring.

The freshness seal is enabled during production of the product: With the battery installed, the test equipment forces the Active-low CE OUT line to ground, applies VCC, and then removes VCC after the reset timeout period has expired. The internal battery-backup circuit keeps the battery and the load disconnected even when the supervisory chip is powered off. The chip then returns to normal operation the next time VCC is applied (without externally holding Active-low CE OUT low).

Another feature offered in more complex devices is low-line output. This binary output is triggered by an internal comparator that monitors the supply voltage with a threshold slightly above the reset threshold. By monitoring this pin via an interrupt, the µP gets an advance warning of any impending reset due to a voltage sag.

Some devices provide an internal "power-fail" comparator, with one input connected to the internal reference and the other input (and output) uncommitted. This arrangement allows the designer to detect any desired voltage level using an external voltage divider. It is often used to detect raw voltage from a battery or line-derived source applied to the VCC regulator. The power-fail output informs the µP when the input voltage is getting close to the minimum allowed for proper regulation. This early warning can allow the system to perform an orderly shutdown before the power fails. Examples of such complex, multifunction supervisory circuits include the DS1236, the MAX793, and the MAX807.

The requirements of certain complex, mission-critical applications, however, are beyond the capability of any single chip, including these multifunction supervisors. A good example is the high-end multi-axis motion controllers from Motion Engineering Inc. (Santa Barbara, CA). These systems (the XMP family) implement a unique, comprehensive protection scheme using a combination of standard supervisors and some minimal external logic (Figure 7). The initial members of this family (a PCI version and a Compact PCI (CPCI) version) provide 150+ MFLOPS of DSP power and control up to 16 axes; i.e., 16 motors in close synchronization.

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