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Suppose you are building a device that needs a small CPU, it's running on battery power so you want to minimize the energy consumption, and you only need a small amount of computation. What's the most energy-efficient CPU?

My first thought would be that an 8-bit CPU would use the least energy. But on thinking about it further, maybe something a bit more powerful would be more efficient. For example, a 6502 has only a few thousand transistors, an ARM-1 has an order of magnitude more - but the ARM-1 delivers more than an order of magnitude more computing power, so it might overall save energy. Is that reasoning correct?

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    \$\begingroup\$ The only correct reasoning would be to define your functional requirements, narrow down a list using them and then look at the specs of the resulting list and see the power consumption. \$\endgroup\$ – Eugene Sh. Dec 8 '17 at 19:11
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    \$\begingroup\$ Some MCUs from STM32L series are optimized for battery-powered applications. Maybe a good option. \$\endgroup\$ – Rohat Kılıç Dec 8 '17 at 19:29
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    \$\begingroup\$ It all boils down to the workload. If its only a bit, then having a high quiescent current low power cpu might be worse than spending lots of energy for a much shorter time \$\endgroup\$ – PlasmaHH Dec 8 '17 at 19:52
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    \$\begingroup\$ Don't forget leakage, which can be considerable and depends on which peripherals you have and which are switched on. \$\endgroup\$ – pjc50 Dec 8 '17 at 20:57
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    \$\begingroup\$ been playing with these a lot lately, battery and solar. every year someone makes a new "best" mcu. arm based, msp430 have some nice solutions. depending on your application you may or may not care about what it takes to get started, this defeated me initially with solar. can get the part down to 1.8 or so volts in a dozen or two microamps, but it needs a bunch more to get started up then software slows it down and turns stuff off. some others dont have this problem but may not get as low, etc. Eugene Sh. basically answered this question. \$\endgroup\$ – old_timer Dec 8 '17 at 21:23
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That old 6502 is far outclassed by modern low-power microcontrollers. Some considerations that affect power consumption of modern microcontrollers:

  • Supply voltage: modern chips operate at low DC voltages.

  • uA/MHz: supply current is proportional to speed... modern:100-200uA/MHz

  • latency coming out of sleep: how fast does it wake up?

  • Interrupts can wake up a sleeping CPU?...then waking up to poll isn't necessary.

  • subsection sleep: some unused peripherals can be shut down, saving power.

Writing efficient code also helps: wake up, do a task, then go back to sleep. If the task takes less time, you save power.
To take advantage of the interrupt-while-asleep ability of a microcontroller, you may have to re-write some legacy code. Same is true of putting peripheral sub-sections to sleep while not required.

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It is possible to benchmark an MCU for power consumption if you know what tasks you wish to perform. For example, an MSP430 might have a uA range current consumption, but it might require more instructions than an ARM M0 for the same task, and then, assuming their clocks are identical, you can estimate which one consumes less power. There are many other parameters which affect the solution's total power consumption.

Of course, if you have the hardware, you can compare the actual measurements.

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I recently worked on a similar design and choose NXP Kinetis L series which is ARM based. Since we already were using these parts in other designs (not low power), we have design tools and experience with them which made it an easier decision. It can be very low power or high performance by using different power modes. In low power mode the power can be very low, less than 1 mA. The cost is also quite low.

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