Use ws2812 leds on imxrt1062 boards

• For usage with smart-leds
• Implements the SmartLedsWrite trait

WARNING

WS2812s operate at 5v. IMXRT1062 pads operate at 3v and are not 5v tolerant. You will need a logic-level converter and some care in wiring.

Why?

Because I'm apparently too stupid to get a periodic timer working with ws2812-timer-delay.

How?

This library uses a simple for loop with no-ops inside. How many loops is determined by a magic number.

The default magic_number is 50.

WS2812s read inputs with cycles about 333ns long. Write 3 bytes (GRB | Green, Red, Blue) per LED, then "latch" (set low) for 6us to 250us depending on your model. Writing a single bit entails a three-bit message that looks like [1, x, 0], where x is the bit you want to write. That all is to say, to write a bit of 1 to an LED you must send [1, 1, 0], waiting 333ns between each bit.

As for the magic number, 600MHz a cycle should take 1.667 nanoseconds. The loop inside takes about 4-6 cycles, depending on your magic number (whether the loop is unrolled). This could be improved to be invariable by hand-rolling a loop in assembly.

In theory that means 333ns should be about 50 unrolled loops.

WS2812's have a pretty wide range of timing constraints, so this will work fine even with a clock rate of 720MHz.

Calculate your own magic number: 333/((1000/CLOCK_SPEED_MHZ) * 4) e.g.: 333/((1000/600) * 4) = ~50

Find out more about the timing constraints of WS2812s.

Example using teensy4-bsp

```rust

[cortexmrt::entry]

fn main() -> ! { ...

let mut data: [RGB8; 3] = [RGB8::default(); 3];
let empty: [RGB8; 3] = [RGB8::default(); 3];

data[0] = RGB8 {
    r: 0x10,
    g: 0x00,
    b: 0x00,
};
data[1] = RGB8 {
    r: 0x00,
    g: 0x10,
    b: 0x00,
};
data[2] = RGB8 {
    r: 0x00,
    g: 0x00,
    b: 0x10,
};

configure(&mut pins.p2,{
    Config::zero()
        .set_drive_strength(DriveStrength::R0_7)
        .set_speed(Speed::Max)
        .set_slew_rate(SlewRate::Fast)
});
let pin = GPIO::new(pins.p2).output();

let mut ws = ws2812_nop_imxrt1062::Ws2812::new(pin);

loop {
    ws.write(data.iter().cloned()).unwrap();
    systick.delay_ms(500);

    ws.write(empty.iter().cloned()).unwrap();
    systick.delay_ms(500);
}

} ```