hardfault fixed

.gitignore

@@ -1 +1,2 @@
 *.pdf
+dma_example/

dma_gpio2/src/bin/main.rs

@@ -29,9 +29,10 @@ unsafe fn start_dma<'a>(
 }
 
 pub const TIM6_UP_REQ: Request = 4;
+const DMA_BUF_WORDS: usize = 256;
 
-static BUF_A: StaticCell<[u32; 8]> = StaticCell::new();
-static BUF_B: StaticCell<[u32; 8]> = StaticCell::new();
+static BUF_A: StaticCell<[u32; DMA_BUF_WORDS]> = StaticCell::new();
+static BUF_B: StaticCell<[u32; DMA_BUF_WORDS]> = StaticCell::new();
 
 #[embassy_executor::main]
 async fn main(_spawner: Spawner) {
@@ -41,8 +42,16 @@ async fn main(_spawner: Spawner) {
     init_tim6_for_uart(p.TIM6, BAUD, TX_OVERSAMPLE);
     dump_tim6_regs();
 
-    let buf_a = BUF_A.init([0u32; 8]);
-    let buf_b = BUF_B.init([0u32; 8]);
+    let buf_a = BUF_A.init([0u32; DMA_BUF_WORDS]);
+    let buf_b = BUF_B.init([0u32; DMA_BUF_WORDS]);
+    // Pre-fill with idle high level (stop bit level)
+    let idle = 1u32 << TX_PIN_BIT;
+    for w in buf_a.iter_mut() {
+        *w = idle;
+    }
+    for w in buf_b.iter_mut() {
+        *w = idle;
+    }
 
     let mut opts = TransferOptions::default();
     opts.priority = Priority::VeryHigh;
@@ -58,10 +67,25 @@ async fn main(_spawner: Spawner) {
             (&*buf_b, &mut *buf_a)
         };
 
+        // Start from idle pattern
+        let idle = 1u32 << TX_PIN_BIT;
+        for w in cpu_buf.iter_mut() {
+            *w = idle;
+        }
+
         let used = encode_uart_frames(TX_PIN_BIT, b"Hello marshmallow\r\n", cpu_buf).await;
 
+        let len = if used == 0 { 1 } else { used };
+        // At least one word so DMA is always valid.
+
         let transfer = unsafe {
-            start_dma(ch0.reborrow(), TIM6_UP_REQ, odr_ptr, &dma_buf[..used], opts)
+            start_dma(
+                ch0.reborrow(),
+                TIM6_UP_REQ,
+                odr_ptr,
+                &dma_buf[..len],
+                opts,
+            )
         };
         transfer.await;
         using_a = !using_a;