Understanding Smart Sensors - Nomads.usp

Using MCUs/DSPs to Increase Sensor IQ 103time and data acquisition, and processing is requested by an external source.For example, that occurs if a serial signal is transmitted every second following atemperature reading. Using the STOP mode, the supply current for the previousexample would drop from about 2.38 mA for continuous 1-MHz busoperation to an average current near 0.18 mA. The sample, convert, process,and send sequence occurs at 1 MHz, then stops and waits for another externalrequest to restart the sequence almost a second later.5.3.6 Local Voltage or Current RegulationOnboard voltage or current regulation is important to sensors that are not ratiometric,because the variation in supply voltage over a –40°Cto+125°C operatingrange can be greater than ±5%. The availability of analog control circuitrywith 40V standoff capability allows the integration of a series pass 5V regulatoron the MCU (e.g., the MC68HC705V8) [7]. Availability of a similar shuntregulator allows a two-wire self-protected and self-powered system to bedesigned using only a sensor and an MCU. At higher levels of integration, suchanalog voltage and/or current regulation can both reduce component countand improve accuracy. Furthermore, those regulation schemes can be dynamicallyaltered to improve functionality or reduce power consumption.5.3.7 Modular MCU DesignA methodology has been developed that allows custom MCUs to be designedto specifically address the requirements of a particular application [7]. Thecustomer-specified integrated circuit (CSIC) approach differs from an ASIC inthe performance and density that can be achieved. Typical ASIC chip solutionsutilize (1) a family of basic elements designed to handle a variety of requirementsand (2) automated design tools to reduce the design cycle time. Both ofthose attributes contribute to a larger die size. The automated chip assemblymethods may also compromise analog performance when low-level signals areinvolved.CSICs utilize standard functional subsystems that have a proven field historyin applications, including the demanding environment of automobiles.The chassis consists of pretested reusable blocks based on existing 68HC05MCUs. Those modular blocks include RAM, ROM, EPROM, and EEPROMfrom existing modules as well as serial communications modules, a variety ofdisplay drivers such as liquid crystal display (LCD), vacuum fluorescent display(VFD), or light-emitting diodes (LED), timers, and A/D and D/A converters(Figure 5.5). These modules can be modified, or existing modules from previouslydeveloped products can be utilized. Also, new modules can be designed to