Battery Life

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  • It could make sense. However I'm not sure how much it would save. SPI flash can momentarily draw like 15mA (see e.g. table on page 83 for similar chip here­25q128fv%20rev.m%2005132016%20kms.pdf#pa­ge=83 ) while the CPU being 100% busy draws like 5mA (table here­ps_nrf52840/_tmp/nrf52840/autodita/CURRE­NT/parameters.i_cpu.html?cp=4_0_0_4_1_0_­2 ) however there is some cache buffer implemented for reading JS data from SPI flash so it is not read all the time and may actually sleep most of the time with short bursts of activity. Interpreting JS code makes the CPU busy almost all the time as long as your redrawing code runs so while CPU draws less than SPI flash it may consume majority of the power.

    There is also internal flash directly on the nrf52 chip, bangle 2 has like 500KB free, would be interesting to have test with some longer busy looping JS code running from storage how much loading code from external SPI would add vs running same code from internal flash (or from "ram" which should be similar)

  • So would it make sense to load redrawing code into "ram"?

    True - it would help power usage (and speed)

  • But also the screen draws power when it is redrawn, the larger the area redrawn the more power is required?

  • Screen data transfer over SPI of course draws something too but I'd guess it is minimal when compared to CPU usage needed for typical javascript redraw routine (as the javascript interpreter may need to run thousands of cycles per each trivial JS line). Tens or hundreds of milliseconds of 100% cpu usage at ~4mA is much more than few miliseconds of SPI transfer and LCD screen pixel changes. As an example see this­376504/#16554635 - burning 15ms of cpu for base64 decoding and heatshrink decompression over and over again probably draws much more than pushing pixels to the display in the end. But still maybe those 15 ms per icon are not worth optimizing if drawn only once per minute.

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Battery Life

Posted by Avatar for user143850 @user143850