This project uses the SPI peripheral in a STM32F4 to communicate with WS2811 RGB LEDs. The PLL on the STM32F4 is tuned to 408MHz, and a total divisor of 32 is applied before going to SPI1. This makes it so that 16bits from the SPI output matchup reasonably well with the expected bit period of 2.5us.
The update_string function outputs data to the WS2811 string by transmitting different numbers of sequential ones in order to generate the duty cycles that the WS2811 expects for a 'one' vs 'zero' bit.
8MHz Xtal --> Div/8 --> PLL x 408 --> Div/4 --> SYSCLK --> PPRE1/2 --> 51MHz for SPI1 (with DIV/4\ inside the SPI peripheral)
This gives us 12.75MHz for the SPI clock. Using 16 SPI bits per WS2811 bit period we get close to the WS2811's desired bit rate at 1.25us per bit period.
This code will work with other parts that use the same communication protocol such as, TM1809, and WS2812B aka the neopixel.