drivers: led_strip: ws2812_spi: Ensure MOSI is low during reset

Some platforms, such as STM32, do not guarantee that the MOSI line
is held low when the SPI peripheral is disabled. This can prevent
WS2812-based LEDs from resetting correctly, as they require a low
signal for a specific duration to trigger a reset.

This commit introduces a Kconfig option to actively drive the MOSI
line low during the reset period by sending zero-byte frames via SPI.
This ensures the reset condition is reliably met across all hardware
platforms.

Signed-off-by: Arthur Gay <[email protected]>

Author: arthur-mmlw

drivers/led_strip/Kconfig.ws2812

@@ -26,6 +26,13 @@ config WS2812_STRIP_SPI_FORCE_NOCACHE
 	  For SPI communication using DMA, place the WS2812 SPI buffer in the __nocache section to
 	  ensure it is stored in uncached memory, as DMA usually requires access to uncached regions.
 
+config WS2812_STRIP_SPI_RESET_DELAY_SEND_NOP
+	bool "Send reset delay using NOP frames"
+	help
+	  Use SPI NOP frames to generate the reset delay before and after sending pixel data. This
+	  ensures the MOSI line is held low during the reset period, which may not be guaranteed on
+	  some platforms such as STM32.
+
 endif
 
 config WS2812_STRIP_UART

drivers/led_strip/ws2812_spi.c

@@ -126,19 +126,41 @@ static int ws2812_strip_update_rgb(const struct device *dev,
 	const size_t total_bits = num_pixels * cfg->num_colors *
 				  BITS_PER_COLOR_CHANNEL * bits_per_symbol;
 	const size_t buf_len = DIV_ROUND_UP(total_bits, SPI_FRAME_BITS);
-	struct spi_buf buf = {
-		.buf = cfg->px_buf,
-		.len = buf_len,
-	};
+	struct spi_buf buffers[COND_CODE_0(CONFIG_WS2812_STRIP_SPI_RESET_DELAY_SEND_NOP, (1), (3))];
+	struct spi_buf *buf = &buffers[0];
 	const struct spi_buf_set tx = {
-		.buffers = &buf,
-		.count = 1
+		.buffers = buffers,
+		.count = ARRAY_SIZE(buffers),
 	};
+	uint32_t reset_delay_cycles;
 	uint8_t *px_buf = cfg->px_buf;
 	uint8_t bit_mask = BIT(SPI_FRAME_BITS - 1);
 	size_t i;
 	int rc;
 
+	if (IS_ENABLED(CONFIG_WS2812_STRIP_SPI_RESET_DELAY_SEND_NOP)) {
+		/*
+		 * Convert the reset delay from micro seconds to a number of cycles of the SPI clock.
+		 * The bus frequency is a maximum and may not be the actual frequency used. In that
+		 * case, the reset delay will be longer than expected but that should not be an
+		 * issue.
+		 */
+		reset_delay_cycles = cfg->bus.config.frequency * cfg->reset_delay / USEC_PER_SEC;
+
+		/*
+		 * Define dummy buffers that will send zeroes on the MOSI line for the duration of
+		 * the reset delay.
+		 */
+		buffers[0].buf = NULL;
+		buffers[0].len = reset_delay_cycles / 8;
+		buf = &buffers[1];
+		buffers[2].buf = NULL;
+		buffers[2].len = reset_delay_cycles / 8;
+	}
+
+	buf->buf = cfg->px_buf;
+	buf->len = buf_len;
+
 	/*
 	 * Convert pixel data into an SPI bitstream. The bitstream contains
 	 * pixel data in color mapping on-wire format (e.g. GRB, GRBW, RGB,
@@ -182,7 +204,9 @@ static int ws2812_strip_update_rgb(const struct device *dev,
 	 * Display the pixel data.
 	 */
 	rc = spi_write_dt(&cfg->bus, &tx);
-	ws2812_reset_delay(cfg->reset_delay);
+	if (!IS_ENABLED(CONFIG_WS2812_STRIP_SPI_RESET_DELAY_SEND_NOP)) {
+		ws2812_reset_delay(cfg->reset_delay);
+	}
 
 	return rc;
 }