Presumably someone working on an e-ink device at some point ran the same calculation, and decided to use the e-ink display. Which means that e-ink being more power hungry if you update it more than once per hour wouldn't have been true
People building e-paper devices after calculating their power usage definitely doesn't mean that my calculation is wrong. First, they started doing it 16 years ago, and things have changed since then, in ways that have made e-paper less popular; second, power usage is far from the only consideration in product design.
E-paper has some real advantages over memory-in-pixel LCDs: as I said in the other thread, the whites are whiter, it's not glossy, it can do grayscale, some versions can even do color, it's available in larger panel sizes, it existed in 02007 when Amazon launched the Swindle, and it can continue to display Amazon advertising even when the device is entirely powered off.
None of these are true of memory-in-pixel LCDs (though the Playdate does display advertising on its memory-in-pixel screen when it's "off").
Moreover, the computing power budget for the Swindle and similar e-readers is also measured in hundreds of milliwatts, just like the screen. There's not much reason to worry about whether your screen uses 100 microwatts or 100 milliwatts if your CPU is using 200 milliwatts. Reducing your power consumption by 50% or 5% is never a critical factor in the success or failure of your product, because you could just add another 50% in battery weight if it were.
Ambiq didn't even exist until 02010, and didn't sample the Apollo3 until 02018, so to get even a single order of magnitude improvement in CPU power usage, Amazon (or competing vendors) would have had to cut the compute budget for e-readers by an order of magnitude, probably resulting in major increases in space usage, reductions in rendering quality, sluggish interactions, or all three, and certainly increasing development cost. Ambiq's CPUs are fast enough, but you can't just drop them into a new model of Swindle or Kobo and keep the same software; they have tiny onboard RAM, little support for offboard RAM (which would make power consumption balloon anyway), and no MMUs, so they can't run either Linux or any of the other e-reader software that's been developed for existing devices—it needs way too much memory.
For what its worth, my math was ~1 update per 15-minutes was the crossover point between memory-lcd and eink.
There's a lot of different eink and memory-lcd screens that could explain the difference between your calculation and mine. But all else considered, 15-minutes vs 60-minutes isn't much of a difference at all.
For most applications of updates-per-second (or at worst, a few seconds per update), low-power monochrome seems best served by Sharp's memory-lcd.
Agreed. Also, my figures for eink power consumption are very uncertain; possibly you were able to take measurements or get good datasheets, but I couldn't.
A thing I didn't mention is that there are actually eink applications that have lower update rates than this. Store price tags, for example.
Pervasive Displays has an oscilloscope measurement of 3.75mA on their 5.9-second updates for their 1.54" eink screen. (Page 14). That's 22.125 mCoulombs of electrons.
Sharp 1.2" memory-LCD (a bit smaller) has 12uW to 50uW on static images. Assuming 3.3V, that's 15uA worst case (0.015mA).
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22.125 mCoulombs / 0.015mA == 1475 seconds per update break-even point, or ~24 minutes.
So yet another number, but yeah, memory-LCD is just really good. Remember that the 1.2" screen from Sharp is a bit smaller though, so its not a perfect apples-to-apples comparison.
Either way, its a much longer period between updates than I was expecting.
