WS2801 LED strip without library

hi,

i'm trying control ws2801 led stript without library. googling found following example: click me

this code working fine. but, don't idea of code, mean, don't understand why work. of code figured out, there 1 piece don't @ all. method post_frame not clear me.

here code i'm talking about:

code: [select]


void post_frame (void) {
  //each led requires 24 bits of data
  //msb: r7, r6, r5..., g7, g6..., b7, b6... b0
  //once 24 bits have been delivered, ic relays these bits neighbor
  //pulling clock low 500us or more causes ic post data.

  for(int led_number = 0 ; led_number < strip_length ; led_number++) {
    long this_led_color = strip_colors[led_number]; //24 bits of color data

    for(byte color_bit = 23 ; color_bit != 255 ; color_bit--) {
      //feed color bit 23 first (red data msb)
     
      digitalwrite(cki, low); //only change data when clock low
     
      long mask = 1l << color_bit;
      //the 1'l' forces 1 start 32 bit number, otherwise defaults 16-bit.
     
      if(this_led_color & mask)
        digitalwrite(sdi, high);
      else
        digitalwrite(sdi, low);
 
      digitalwrite(cki, high); //data latched when clock goes high
    }
  }


the first loop quite straightforward, loops on leds. second loop isn't clear me. reason start byte has value of 23 , stops, don't know why exactly. send data, clock has low first, send data, , set clock high. after 2 loops, there delay (500us) result 'showed'.

within second loop there long, called mask. don't havy idea why used. after that, there if else statement uses mask. , reason, somethimes sdi pin high, , low. don't have idea why should this.

could explain me piece of code?

mask allows isolate individual bits in variable using bitwise , (&)

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