From: Martin Mareš <mj@ucw.cz>
Date: Fri, 12 Sep 2025 19:50:41 +0000 (+0200)
Subject: Neopixel square: Double-buffered DMA
X-Git-Url: http://mj.ucw.cz/gitweb/?a=commitdiff_plain;h=309cd67264553a1d451deda32fc6643425a99ae7;p=home-hw.git

Neopixel square: Double-buffered DMA
---

diff --git a/test-neopixel-square/main.c b/test-neopixel-square/main.c
index 7474061..9605a44 100644
--- a/test-neopixel-square/main.c
+++ b/test-neopixel-square/main.c
@@ -91,31 +91,34 @@ static void delay_ms(uint ms)
 /*** Neopixels ***/
 
 #define NPIX_PERIOD 90		// timer runs on 72 MHz, so 90 periods = 1250 ns
-#define NPIX_RESET 128		// the chip needs longer reset pulse than documented
 #define NPIX_NUM_LEDS 64
+#define NPIX_RESET 6		// length of reset pulse in LED slots
+				// The chip needs longer reset pulse than documented.
 #define B0 30
 #define B1 60
 
-byte neopixel_buf[NPIX_RESET + 24*NPIX_NUM_LEDS];
+static byte neopixel_leds[NPIX_NUM_LEDS][3];
+static byte neopixel_buf[2*24];
+static uint neopixel_index;
 
 static void neopixel_set(uint led, byte r, byte g, byte b)
 {
-	byte *buf = &neopixel_buf[NPIX_RESET + 24*led];
-
-	for (uint m=0x80; m; m >>= 1)
-		*buf++ = (g & m) ? B1 : B0;
-	for (uint m=0x80; m; m >>= 1)
-		*buf++ = (r & m) ? B1 : B0;
-	for (uint m=0x80; m; m >>= 1)
-		*buf++ = (b & m) ? B1 : B0;
+	cm_disable_interrupts();
+	neopixel_leds[led][0] = r;
+	neopixel_leds[led][1] = g;
+	neopixel_leds[led][2] = b;
+	cm_enable_interrupts();
 }
 
-
 static void neopixel_init(void)
 {
 	for (uint i=0; i < NPIX_NUM_LEDS; i++)
 		neopixel_set(i, 7, 7, 7);
 
+	// neopixel_buf is zero-initialized, which is a start of the reset sequence
+	memset(neopixel_buf, 0, sizeof(neopixel_buf));
+	neopixel_index = NPIX_NUM_LEDS;
+
 	// TIM4 update is connected to DMA1 channel 7
 
 	// FIXME: Strange programming sequence as specified in manual
@@ -136,6 +139,10 @@ static void neopixel_init(void)
 	dma_set_peripheral_size(DMA1, 7, DMA_CCR_PSIZE_16BIT);
 	dma_disable_peripheral_increment_mode(DMA1, 7);
 
+	dma_enable_half_transfer_interrupt(DMA1, 7);
+	dma_enable_transfer_complete_interrupt(DMA1, 7);
+	nvic_enable_irq(NVIC_DMA1_CHANNEL7_IRQ);
+
 	dma_enable_channel(DMA1, 7);
 
 	timer_set_prescaler(TIM4, 0);
@@ -154,6 +161,39 @@ static void neopixel_init(void)
 	timer_enable_counter(TIM4);
 }
 
+static inline void next_led(byte *buf)
+{
+	if (neopixel_index < NPIX_NUM_LEDS) {
+		byte *led = neopixel_leds[neopixel_index++];
+		byte r = led[0], g = led[1], b = led[2];
+		for (uint m=0x80; m; m >>= 1)
+			*buf++ = (g & m) ? B1 : B0;
+		for (uint m=0x80; m; m >>= 1)
+			*buf++ = (r & m) ? B1 : B0;
+		for (uint m=0x80; m; m >>= 1)
+			*buf++ = (b & m) ? B1 : B0;
+	} else {
+		for (uint i=0; i < 24; i++)
+			*buf++ = 0;
+		neopixel_index++;
+		if (neopixel_index == NPIX_NUM_LEDS + NPIX_RESET)
+			neopixel_index = 0;
+	}
+}
+
+void dma1_channel7_isr(void)
+{
+	if (DMA1_ISR & DMA_ISR_HTIF7) {
+		// Half transfer
+		DMA1_IFCR |= DMA_IFCR_CHTIF7;
+		next_led(neopixel_buf);
+	} else if (DMA1_ISR & DMA_ISR_TCIF7) {
+		// Transfer completed
+		DMA1_IFCR |= DMA_IFCR_CTCIF7;
+		next_led(neopixel_buf + 24);
+	}
+}
+
 /*** Game of Life ***/
 
 #define LN 8