ws2812.c

#include <xc.h>
#include <pic18.h>
#include <stdlib.h>
#include "ws2812.h"

// the pin that the LEDs are on. I don't like this being in here
// but timing is so tight, it is hard to put it elsewhere
#define PIN PORTBbits.RB1

unsigned long int bitflip(unsigned char b);

// transmit the ws2812 led
void ws2812_send(ws2812_ptr* led) {
    int j;
    long int val;
    // the WS2812 wants bits in the order of:
    // GGGGGGGGRRRRRRRRBBBBBBBB
    // but I want to work in the opposite order. so i'm going to flip
    // the bits around and do some shifting so my order is
    // BBBBBBBBRRRRRRRRGGGGGGGG
    // with the most significant bit on the far right. so the RGB value
    // of 128 64 32, which normally would be:
    // R : 0b10000000
    // G : 0b01000000
    // B : 0b00100000
    // will become:
    // BBBBBBBBRRRRRRRRGGGGGGGG
    // 000001000000000100000010
    val = (bitflip(led->b) << 16) + (bitflip(led->r) << 8) + (bitflip(led->g));

    // now begin shifting them over one at a time
    for(j = 0; j < 24; j++) {
        // depending on if the currently viewed bit is 1 or 0
        // the pin will stay high for different times

        if (val & 1 == 1) {
            // if it is a 1, let it stay higher a bit longer
            PIN = 1;
            NOP();
            NOP();
            NOP();
            PIN = 0;
        } else {
            // but a 0 should go high and then low as fast as possible
            PIN = 1;
            PIN = 0;
        }
        
        // and then right shift to get the next bit
        val = val >> (unsigned char)1;
    }
}

// reverse the bits in a char
unsigned long int bitflip(unsigned char b) {
   b = (b & 0xF0) >> 4 | (b & 0x0F) << 4;
   b = (b & 0xCC) >> 2 | (b & 0x33) << 2;
   b = (b & 0xAA) >> 1 | (b & 0x55) << 1;
   return (unsigned long int)b;
}