Understanding Smart Sensors - Nomads.usp

102 Understanding Smart Sensorsinstructions following an A/D conversion of a pressure sensor with a 500-Hzbandwidth. The MC68HC05P8 modes of operation allow several approaches,as shown in Figure 5.4 [7]. Assuming a typical CPU cycle of 1 ms (1-MHz bus),the MC68HC05P8 will require 32 ms for an A/D conversion and an additional300 ms for the digital procedure (approximately three cycles per instruction). Atotal system process time is 332 ms and would produce a 6X sampling of a500-Hz signal.If the MC68HC05P8 in Figure 5.4 is operated at a 1-MHz bus speed at5V dc over the temperature range of −40°Cto+85°C, continuous conversionsand processing will typically consume an average of 2.38 mA of supply current.However, if the device is only allowed to sample, convert, and process at twotimes the Nyquist criteria of once every millisecond, it could sample andprocess in 332 ms and then go into the WAIT mode. The on-chip timer wouldwake it up each millisecond. The resulting supply current would drop to anaverage of about 1.32 mA, as shown in Figure 5.4. Another alternative to savepower would slow the system clock to process a sample every millisecond. Inthat case, the CPU clock could be dropped to 3 ms (333-kHz bus) while thesupply current to the MC68HC05P8 would drop to about 0.79 mA, as shownin Figure 5.4. Other combinations of bus speed and WAIT times wouldrequire average supply currents from 0.79 to 1.32 mA.The STOP mode is useful in cases in which the oscillator’s restart time oftypically 4064 CPU cycles is not a significant portion of the sampling/processSupplycurrent(mA)2.5 Continuous processingat 1-MHz bus speed(average Idd = 2.38 mA)2.01.51.00.500 200 400 600 800 1,000Processing time (microseconds)Processing at 1-MHz bus speedthen WAIT for rest of 1 mSec(average Idd = 1.32 mA)Continuous processingat 333-kHz bus speed(average Idd= 0.79 mA)Processor STOPPED(average Idd = 0.18 mA)Figure 5.4 Processing mode effects on power consumption.