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Bangle.js/SPIFLASH: improve SPI flash speed #1849

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2 of 6 tasks
gfwilliams opened this issue Jun 5, 2020 · 2 comments
Closed
2 of 6 tasks

Bangle.js/SPIFLASH: improve SPI flash speed #1849

gfwilliams opened this issue Jun 5, 2020 · 2 comments
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enhancement

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@gfwilliams
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gfwilliams commented Jun 5, 2020 •
edited

From http://forum.espruino.com/conversations/348553 - posting here because they'll get lost otherwise:

Some things that I think really might increase speed:

  • Create a spiFlashRead so that when you're reading data from flash you're not also writing - that would be a really easy win
  • Keep CS asserted on the flash chip after a read, and if the next byte(s) requested come right after the previous bytes requested then you don't need to send the 4 bytes of address again - you just clock out more data. That should be pretty easy as well.
  • Merging the writes - don't use OUTSET and OUTCLR but write directly to OUT - you may be able to change the clock and data at the same time on one of the clock edges which would help to overcome the 16Mhz limitation.
  • The original watch firmware actually used QSPI (so 4 data wires) but it did it in software so it was basically slower than just bit-banging with 1 data bit! It's a good example of why you need to actually benchmark before optimising :) With some thought you may be able to do it properly though - using multiply + shift (or just a lookup table) to get the bits in the correct places for their pins with only a few clock cycles (taking some inspiration from https://graphics.stanford.edu/~seander/bithacks.html#ReverseByteWith64Bits).
  • Using hardware SPI - you may be able to get this faster than bit-banged SPI, but supposedly the max bitrate is still 8Mbps
  • Preloading with hardware SPI and DMA - a lot of the time when executing from flash, Espruino will load data in chunks of 16 bytes. You could potentially pre-load the next 16 bytes using DMA while Espruino runs through the first 16 bytes
gfwilliams added a commit that referenced this issue Jun 5, 2020
@gfwilliams
Bangle.js: Improve SPI flash speed by with specific function for read…
49b4b52 
…ing and keeping CS asserted ( #1849)
@gfwilliams
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function mandelbrot() {
 var tm = getTime();
 var sizex=32,sizey=32;
 for (var y=0;y<sizey;y++) {
  var line="";
  for (var x=0;x<sizex;x++) {
   var Xr=0;
   var Xi=0;
   var Cr=(4.0*x/sizex)-2.0;
   var Ci=(4.0*y/sizey)-2.0;
   var i=0;
   while ((i<16) && ((Xr*Xr+Xi*Xi)<4)) {
    var t=Xr*Xr - Xi*Xi + Cr;
    Xi=2*Xr*Xi+Ci;
    Xr=t;
    i++;
   }
   line += (i&1) ? "*" : " ";
  }
  print(line);
 }
 print(getTime()-tm);
}
// save to flash and then read&eval, so it'll end up in flash
require("Storage").write("mandeltest","("+mandelbrot.toString()+")");
mandelbrot = eval(require("Storage").read("mandeltest"));

// execute from flash
mandelbrot(); // 8.79 sec before
mandelbrot(); // 8.52 sec with most recent changes

require("Storage").write("test","",0,240*240*2)

t=getTime();g.drawImage({width:240,height:240,bpp:16,buffer:require("Storage").read("test")});print(getTime()-t);
// 0.47 before
// 0.36 with most recent changes

t=getTime();E.sum(require("Storage").read("test"));print(getTime()-t);
// 0.80 before
// 0.67 with most recent changes

@gfwilliams
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Closing this as I think the initial changes did a pretty good job

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