stm32/spi: add new_txonly_nosck constructor, for neopixels, with an example in the stm32g0 directory.

embassy-stm32/src/spi/mod.rs

@@ -177,6 +177,23 @@ impl<'d, T: Instance, Tx, Rx> Spi<'d, T, Tx, Rx> {
         )
     }
 
+    pub fn new_txonly_nosck(
+        peri: impl Peripheral<P = T> + 'd,
+        mosi: impl Peripheral<P = impl MosiPin<T>> + 'd,
+        txdma: impl Peripheral<P = Tx> + 'd,
+        rxdma: impl Peripheral<P = Rx> + 'd, // TODO: remove
+        freq: Hertz,
+        config: Config,
+    ) -> Self {
+        into_ref!(mosi);
+        unsafe {
+            mosi.set_as_af_pull(mosi.af_num(), AFType::OutputPushPull, Pull::Down);
+            mosi.set_speed(crate::gpio::Speed::Medium);
+        }
+
+        Self::new_inner(peri, None, Some(mosi.map_into()), None, txdma, rxdma, freq, config)
+    }
+
     /// Useful for on chip peripherals like SUBGHZ which are hardwired.
     /// The bus can optionally be exposed externally with `Spi::new()` still.
     #[allow(dead_code)]

examples/stm32g0/src/bin/spi_neopixel.rs

@@ -0,0 +1,101 @@
+#![no_std]
+#![no_main]
+#![feature(type_alias_impl_trait)]
+
+use defmt::*;
+use embassy_executor::Spawner;
+use embassy_stm32::dma::word::U5;
+use embassy_stm32::dma::NoDma;
+use embassy_stm32::spi::{Config, Spi};
+use embassy_stm32::time::Hertz;
+use embassy_time::{Duration, Timer};
+use {defmt_rtt as _, panic_probe as _};
+
+const NR_PIXELS: usize = 15;
+const BITS_PER_PIXEL: usize = 24; // 24 for rgb, 32 for rgbw
+const TOTAL_BITS: usize = NR_PIXELS * BITS_PER_PIXEL;
+
+struct RGB {
+    r: u8,
+    g: u8,
+    b: u8,
+}
+impl Default for RGB {
+    fn default() -> RGB {
+        RGB { r: 0, g: 0, b: 0 }
+    }
+}
+pub struct Ws2812 {
+    // Note that the U5 type controls the selection of 5 bits to output
+    bitbuffer: [U5; TOTAL_BITS],
+}
+
+impl Ws2812 {
+    pub fn new() -> Ws2812 {
+        Ws2812 {
+            bitbuffer: [U5(0); TOTAL_BITS],
+        }
+    }
+    fn len(&self) -> usize {
+        return NR_PIXELS;
+    }
+    fn set(&mut self, idx: usize, rgb: RGB) {
+        self.render_color(idx, 0, rgb.g);
+        self.render_color(idx, 8, rgb.r);
+        self.render_color(idx, 16, rgb.b);
+    }
+    // transform one color byte into an array of 8 byte. Each byte in the array does represent 1 neopixel bit pattern
+    fn render_color(&mut self, pixel_idx: usize, offset: usize, color: u8) {
+        let mut bits = color as usize;
+        let mut idx = pixel_idx * BITS_PER_PIXEL + offset;
+
+        // render one bit in one spi byte. High time first, then the low time
+        // clock should be 4 Mhz, 5 bits, each bit is 0.25 us.
+        // a one bit is send as a pulse of 0.75 high -- 0.50 low
+        // a zero bit is send as a pulse of 0.50 high -- 0.75 low
+        // clock frequency for the neopixel is exact 800 khz
+        // note that the mosi output should have a resistor to ground of 10k,
+        // to assure that between the bursts the line is low
+        for _i in 0..8 {
+            if idx >= TOTAL_BITS {
+                return;
+            }
+            let pattern = match bits & 0x80 {
+                0x80 => 0b0000_1110,
+                _ => 0b000_1100,
+            };
+            bits = bits << 1;
+            self.bitbuffer[idx] = U5(pattern);
+            idx += 1;
+        }
+    }
+}
+
+#[embassy_executor::main]
+async fn main(_spawner: Spawner) {
+    let p = embassy_stm32::init(Default::default());
+    info!("Start test using spi as neopixel driver");
+
+    let mut spi = Spi::new_txonly_nosck(p.SPI1, p.PB5, p.DMA1_CH3, NoDma, Hertz(4_000_000), Config::default());
+
+    let mut neopixels = Ws2812::new();
+
+    loop {
+        let mut cnt: usize = 0;
+        for _i in 0..10 {
+            for idx in 0..neopixels.len() {
+                let color = match (cnt + idx) % 3 {
+                    0 => RGB { r: 0x21, g: 0, b: 0 },
+                    1 => RGB { r: 0, g: 0x31, b: 0 },
+                    _ => RGB { r: 0, g: 0, b: 0x41 },
+                };
+                neopixels.set(idx, color);
+            }
+            cnt += 1;
+            // start sending the neopixel bit patters over spi to the neopixel string
+            spi.write(&neopixels.bitbuffer).await.ok();
+            Timer::after(Duration::from_millis(500)).await;
+        }
+        Timer::after(Duration::from_millis(1000)).await;
+    }
+}