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A sneak preview

July 08, 2013

JackGanssle-July 08, 2013

"Useful work" translates into a lot of things (clock rate, instruction set efficiency, etc), but is ultimately bounded by how much current the system (MCU and other components) can consume while awake. It's is the budget a design engineer has to work with, and cannot be exceeded (on average).

Doing the math, I came up with the following curve (Figure 3), which assumes a ten year battery life. It shows the number of mA available while awake, as a function of time spent sleeping and amount of current consumed while sleeping.

Figure 3. Milliamperes (mA) available as a function of sleep time and sleep current.
Here's the key takeaway: Sleep currents are almost irrelevant. Take two MCUs, one that needs 20 nA while sleeping and another that consumes 200 nA. The available awake current is just about the same in both cases. Even one that sucks a microamp while sleeping gives results not much different from an MCU needing an order of magnitude less.

Every MCU has vastly different mixes of features. Some wake up quickly. Others execute at great speed so wake times are minimized. Some preserve registers and memory while asleep; others don't. Brown-out and watchdog circuits may or may not be viable options while sleeping if maximum battery life is desired. So making comparisons is difficult. Even the sleep current numbers are frustratingly difficult to parse as not all vendors give worst-case values.

This article is titled "A Sneak Preview" because it's just the tip of the iceberg. I've been running experiments for 6 months to gain a deeper understanding about building ultra-long-lived battery-powered systems, and will be reporting more results soon.

Some of my experiments are quantifying the behavior of the components we use. For instance, there's very little known about how a CR2032 discharges in these ultra-low-sleep current applications, and I've amassed a vast amount of data using some custom tools, like that in the following photograph below. The results are surprising, and lead me to doubt that even a ten year life is attainable in a real system. 


Figure 4. Nine-cell battery profiler using an mbed ARM controller board.

Transistors switch different loads on each battery to run various current and time profiles. Loads are low tempco, 1% resistors. An A/D reads battery voltages and Vce of the transistors. A precision reference and software calibrates the entire analog path.

Vendors have been very generous in offering their tools and support; for instance, I've evaluated several tools for measuring current, including:

* IAR's I-Jet
* The Real-Time Current Monitor
* The uCurrent

Microchip just sent me their latest current-measuring tool, which I hope to dig into between trips to the Middle East and ESC-India this month. Agilent is sending their very interesting new N2820A current probe, which has a claimed 3 MHz bandwidth and 86 dB dynamic range. I'll review them both, and other similar tools as they become available.

Stay tuned!


Jack G. Ganssle is a lecturer and consultant on embedded development issues. He conducts seminars on embedded systems and helps companies with their embedded challenges, and works as an expert witness on embedded issues. Contact him at jack@ganssle.com. His website is www.ganssle.com.

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