STM32L031 controlling a PL9823 LED using SPI

stm32l031_pl9823
The PL9823 is a smart LED much like the WS2812B. There are some slight timing differences and they can sometimes be found cheaper than an equivalent WS2812B. I got hold of a few from Aliexpress in a frosted 8mm package with 4 pins that allow them to be used in a breadboard.
In a previous post I outlined how SPI could be used with an STM32F042 to drive a WS2182b. Things were a little different in this case as the STM32L031 has fewer options with the SPI clock and the slightly different timing requirements of the PL9823. pl9823_mosi
The image above shows the control from the MOSI signal for a nearly white colour on the LED. The signal is shows a 12 byte sequence which is interpreted as a 3 byte RGB sequence by the PL9823. A PL9823 logic ‘1’ is sent by sending 3 SPI ‘1’s followed by a zero. This takes 2 microseconds. A logic ‘0’ is sent by sending a single SPI ‘1’ and 3 SPI ‘0’s. The sequence is generated using the following function

void writePL9823(uint32_t Colour)
{
    // each colour bit should map to 4 SPI bits.
    // Format of Colour (bytes) 00RRGGBB
    uint8_t SPI_Output[12];
    int SrcIndex = 0;
    int DestIndex = 0;
    for (DestIndex = 0; DestIndex < 12; DestIndex++)
    {
        if (Colour & (1 << 23))
        {
            SPI_Output[DestIndex] = 0xe0;
        }
        else
        {
            SPI_Output[DestIndex] = 0x80;
        }
        Colour = Colour << 1;
        if (Colour & (1 << 23))
        {
            SPI_Output[DestIndex] |= 0xe;
        }
        else
        {
            SPI_Output[DestIndex] |= 0x8;
        }
        Colour = Colour << 1;
    }
    for (int i=0;i<12;i++)
    {
        transferSPI(SPI_Output[i]);
    }
}

The code expands a 3 byte colour sequence into a 12 byte one and sends it over an SPI link running at 2MHz. This is sufficient to match the timing requirements of the PL9823.
A demo program which cycles through various colours can be found over on github.

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