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Issue #228 July 2009
LiOn King
A Look at “Battery-in-a-Chip” Technology
by Tom Cantrell
Start | Energy In A Chip | Charge It | UPS-Lite | Dust Storm | Tips & Tricks | Harvest Time | Sources & PDF
TIPS & TRICKS
By now most designers are familiar with the typical low-power design techniques (i.e., sleep mode, powering down unused logic, up/down-shifting the clock rate, and so on). But getting on the energy-harvesting bandwagon requires taking low-power design techniques even further. When you’re talking millionths of an amp for a power budget, every little bit adds up.
For example, when was the last time anyone really thought much about all those lowly pull-up resistors littering most designs? Well, think again. Consider the typical 100-kW pull-ups inside most MCUs, not just on the I/O lines, but also on the control inputs such as interrupts and reset. The bad news is that a 100-kW pull-up at 3.3 V burns 33 µA just sitting there. We always new that, but just didn’t care. Now we do.
So, for example, you don’t want to leave the pull-ups on your software-scanned matrix keypad enabled all the time. Instead they should only be powered during the active scan. Indeed, where possible (i.e., external pull-ups), use a higher value resistor (e.g., 1 MW). But pay close attention to your rise and fall times since chips burn more power during the time an input transitions through the “floating” region between rails.
Similarly, be on the lookout for subtle leakage paths between chips. For instance, an RTC powered by a battery can leak power through its pins to an attached MCU, even if the MCU is powered off. Use diodes and transistors as hose clamps and valves to seal even the tiniest leaks.
The cyclic nature of the capacitor discharge power supply poses all manner of creative challenges for designers. You no longer have the luxury of consuming all the clock cycles you want whenever you want them. Instead your hardware and software design has to deal with the reality that the power supply drives the schedule. Imagine how this complicates an already tricky and timing-sensitive task like wireless networking. The MCU may have to “stairstep” its way through complex procedures one short burst of energy at a time.
With all the starting and stopping, you even need to pay attention to the energy overhead of waking up and shutting down. After all, if your workday was only 10 minutes long, how fast you tie your shoes would suddenly matter a lot.
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