3.6 Implementation Details

To minimize power consumption, the AVR EA is configured to stay in Power-Down sleep mode whenever a measurement is not in progress. In this sleep mode, AVR EA consumption was measured as approximately 0.9 μA (with V_DD = 3.3V). The PIT (Periodic Interrupt Timer), a part of the RTC (Real Time Counter), is set up to periodically generate an interrupt to bring the device out of sleep mode. When this happens, the DAC is enabled to produce an output voltage of 1.8V, and the ADC is re-enabled. The ADC is commanded to start a differential conversion immediately. While the ADC conversion is in progress, the CPU performs the calculations necessary for converting the previous ADC value into resistance and temperature. As soon as the ADC conversion is complete and the result is saved, DAC and ADC are disabled, and the device is put back to sleep.

The DAC and ADC are enabled after the device comes out of sleep. The DAC output stabilizes before the ADC is ready to start its first conversion, so there is no need for additional delays in the software.

Various strategies were tested to minimize power consumption (higher/lower CPU and ADC clock speeds, PGA on/off with less/more conversions). But in this case, the overriding issue is the fact that the DAC must supply nearly 1 mA of current to the RTD sensor while AD conversions are in progress. Therefore the best strategy is to run both the CPU and ADC as fast as possible (10 MHz and 5 MHz clocks, respectively) with maximum PGA gain so the time that the DAC must supply 1 mA is minimized. During the Power-Down Sleep mode, the 10 MHz clock source is disabled, and only the internal 32 kHz oscillator and the RTC clock source are running. With this configuration, a burst of 16 ADC conversions takes only 155 µs.

During that 155 microseconds of conversion time with DAC enabled, the microcontroller supply current measured 4.7 mA (this includes what the DAC needed to drive the RTD, with V_DD = 3.3V). If there is one conversion per second, the average current will be (155 µs/1s) × 4.7 mA = 1.55e-4 × 4.7 mA = 0.73 µA.

Average current consumption is thus expected to be the sleep current of 0.9 μA + (0.73 μA) × n, where n is the number of RTD temperature measurements per second: