Understanding Smart Sensors - Nomads.usp

Using MCUs/DSPs to Increase Sensor IQ 101hardware-driven SAR, each conversion can be started and performed withoutdecreasing CPU processing power. Meanwhile, the CPU can process prior A/Dsamples until it is interrupted with a new conversion result. Some variations ofthis ADC on the higher performance 68HC11 family have hardware capableof automatically storing four consecutive readings into four separate resultsregisters.Another A/D consideration is the use of a sample-and-hold of the signalbefore the SAR operation begins. The ADC in the HC05 family performs thatfunction in typically 6–12 ms. If the input is sampled too long, the signal mustnot change more than one-half LSB during sampling to limit errors. Thatmaximum change of one-half LSB in the input signal during the sample timecan severely limit input bandwidth considerably more than the minimum 2Xsampling Nyquist criteria.A further advantage of an ADC design with an internal RC oscillator isthe ability to optimize power versus bandwidth. That is accomplished by providinga short A/D conversion cycle while the main processor is run at a muchlower bus frequency. This power-saving approach combined with others canincrease useful battery life in portable applications.5.3.5 Power-Saving CapabilityAnother advantage of MCU hardware and software is the variety of powersavingapproaches. Varying the processing speed or stopping processing altogethercan have a significant effect on overall power consumption. In addition,the ability to operate at lower voltages also reduces power consumption. Reducingpower consumption involves more than reducing the supply current whilerunning processor code.The 68HC05 MCU family has two additional modes of operation toreduce power consumption, called the WAIT and STOP modes. In the WAITmode, the processor bus is halted but the on-chip oscillator and internal timersare left in operation. The device returns to normal operation following anyinterrupt or an external reset. The WAIT mode in the MC68HC05P8 devicereduces current consumption by nearly a factor of 2 at any given processorspeed. In the STOP mode, both the processor and the on-chip oscillator arehalted. The device returns to normal operation only after an external interruptor reset. The STOP mode in the MC68HC05P8 reduces supply current consumptionto less than 180 mA with a 5V dc supply operating at temperaturesbelow 85°C.An example demonstrates the importance of modes of operation andADC performance versus power consumption of the signal processor. Considera signal processing example using a procedure that requires 100 CPU