File content as of revision 2:3c3c41774cdf:

#include <stdint.h>
#include "mbed.h"
#include "neopixel.h"

namespace neopixel
{

PixelArray::PixelArray(PinName out, ByteOrder byte_order)
    : spi_(out, NC, NC), byte_order_(byte_order)
{
    // WS281x bit encodings:
    //  '0': ----________
    //  '1': --------____
    // The period is 1.25us, giving a basic frequency of 800kHz.
    // Getting the mark-space ratio right is trickier, though. There are a number
    // of different timings, and the correct (documented) values depend on the
    // controller chip.
    //
    // The _real_ timing restrictions are much simpler though, and someone has
    // published a lovely analysis here:
    //   http://cpldcpu.wordpress.com/2014/01/14/light_ws2812-library-v2-0-part-i-understanding-the-ws2812/
    //
    // In summary:
    // - The period should be at least 1.25us.
    // - The '0' high time can be anywhere from 0.0625us to 0.5us.
    // - The '1' high time should be longer than 0.625us.
    //
    // These constraints are easy to meet by splitting each bit into three and packing them into SPI packets.
    //  '0': 100             mark: 0.42us, space: 0.83us
    //  '1': 110             mark: 0.83us, space: 0.42us

    spi_.frequency(2400000);  // 800kHz * 3
    spi_.format(12);          // Send four NeoPixel bits in each packet.
}

static void SendFourBits(BurstSPI& spi, uint32_t bits)
{
    // Encode '0' bits as 100 and '1' bits as 110.
    // We have this bit pattern: 00000000abcd
    // We want this bit pattern: 1a01b01c01d0
    uint32_t ac = (bits * 0x088) &        // 0abcdabcd000
                  0x410; // 0a00000c0000

    uint32_t bd = (bits * 0x022) &        // 000abcdabcd0
                  0x082; // 0000b00000d0

    static uint32_t const base = 04444;   // 100100100100

    spi.fastWrite(base | ac | bd);        // 1a01b01c01d0
}

void PixelArray::send_pixel(Pixel& pixel)
{
    // Pixels are sent as follows:
    // - The first transmitted pixel is the pixel closest to the transmitter.
    // - The most significant bit is always sent first.
    //
    // g7,g6,g5,g4,g3,g2,g1,g0,r7,r6,r5,r4,r3,r2,r1,r0,b7,b6,b5,b4,b3,b2,b1,b0
    // \_____________________________________________________________________/
    //                           |      _________________...
    //                           |     /   __________________...
    //                           |    /   /   ___________________...
    //                           |   /   /   /
    //                          GRB,GRB,GRB,GRB,...

    if (byte_order_ == BYTE_ORDER_RGB) {
        SendFourBits(spi_, (pixel.red >> 4) & 0xf);
        SendFourBits(spi_, (pixel.red >> 0) & 0xf);
        SendFourBits(spi_, (pixel.green >> 4) & 0xf);
        SendFourBits(spi_, (pixel.green >> 0) & 0xf);
        SendFourBits(spi_, (pixel.blue >> 4) & 0xf);
        SendFourBits(spi_, (pixel.blue >> 0) & 0xf);
    } else {
        SendFourBits(spi_, (pixel.green >> 4) & 0xf);
        SendFourBits(spi_, (pixel.green >> 0) & 0xf);
        SendFourBits(spi_, (pixel.red >> 4) & 0xf);
        SendFourBits(spi_, (pixel.red >> 0) & 0xf);
        SendFourBits(spi_, (pixel.blue >> 4) & 0xf);
        SendFourBits(spi_, (pixel.blue >> 0) & 0xf);
    }
}

void PixelArray::update(Pixel buffer[], uint32_t length)
{
    for (size_t i = 0; i < length; i++) {
        send_pixel(buffer[i]);
    }

    wait_us(latch_time_us_);
}

void PixelArray::update(PixelGenerator generator, uint32_t length, uintptr_t extra)
{
    for (size_t i = 0; i < length; i++) {
        Pixel out;
        generator(&out, i, extra);
        send_pixel(out);
    }

    wait_us(latch_time_us_);
}

}