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-rw-r--r--examples/test-usart-dma.cpp127
1 files changed, 78 insertions, 49 deletions
diff --git a/examples/test-usart-dma.cpp b/examples/test-usart-dma.cpp
index 10ebe8b..7dafc3a 100644
--- a/examples/test-usart-dma.cpp
+++ b/examples/test-usart-dma.cpp
@@ -12,9 +12,9 @@
*
* This example isn't very robust; don't use it in production. In
* particular, since the buffer keeps filling (DMA_CIRC_MODE is set),
- * if you keep typing after filling the buffer, you'll overwrite
- * earlier bytes; this may happen before those earlier bytes are done
- * printing.
+ * if you keep sending characters after filling the buffer, you'll
+ * overwrite earlier bytes; this may happen before those earlier bytes
+ * are done printing.
*
* This code is released into the public domain.
*/
@@ -25,29 +25,65 @@
#include <wirish/wirish.h>
-#define BAUD 9600
+/*
+ * Configuration and state
+ */
+// USART and DMA configuration
#define USART USART2
#define USART_HWSER Serial2
#define USART_DMA_DEV DMA1
#define USART_RX_DMA_CHANNEL DMA_CH6
-#define USART_TX BOARD_USART2_TX_PIN
-#define USART_RX BOARD_USART2_RX_PIN
-
-#define BUF_SIZE 8
-uint8 rx_buf[BUF_SIZE];
+#define BAUD 9600
-dma_irq_cause irq_cause;
+// This will store received USART characters.
+#define BUF_SIZE 20
+char rx_buf[BUF_SIZE];
+// The interrupt handler, rx_dma_irq(), sets this to 1.
volatile uint32 irq_fired = 0;
+// Used to store the ISR bits inside rx_dma_irq(). This helps explain
+// what's going on inside loop(); see comments below.
+volatile uint32 isr = 0;
+
+/*
+ * Helper functions
+ */
+
+// This is our DMA interrupt handler.
+void rx_dma_irq(void) {
+ irq_fired = 1;
+ isr = dma_get_isr_bits(USART_DMA_DEV, USART_RX_DMA_CHANNEL);
+}
+
+// Configure the USART receiver for use with DMA:
+// 1. Turn it on.
+// 2. Set the "DMA request on RX" bit in USART_CR3 (USART_CR3_DMAR).
+void init_usart(void) {
+ USART_HWSER.begin(BAUD);
+ USART->regs->CR3 = USART_CR3_DMAR;
+}
+
+// Configure the DMA controller to serve DMA requests from the USART.
+void init_dma_xfer(void) {
+ dma_init(USART_DMA_DEV);
+ dma_setup_transfer(USART_DMA_DEV, USART_RX_DMA_CHANNEL,
+ &USART->regs->DR, DMA_SIZE_8BITS,
+ rx_buf, DMA_SIZE_8BITS,
+ (DMA_MINC_MODE | DMA_CIRC_MODE | DMA_TRNS_CMPLT));
+ dma_set_num_transfers(USART_DMA_DEV, USART_RX_DMA_CHANNEL,
+ BUF_SIZE - 1);
+ dma_attach_interrupt(USART_DMA_DEV, USART_RX_DMA_CHANNEL, rx_dma_irq);
+ dma_enable(USART_DMA_DEV, USART_RX_DMA_CHANNEL);
+}
-void init_usart(void);
-void init_dma_xfer(void);
-void rx_dma_irq(void);
+/*
+ * setup() and loop()
+ */
void setup(void) {
pinMode(BOARD_LED_PIN, OUTPUT);
-
+ rx_buf[BUF_SIZE - 1] = '\0'; // null-terminate rx_buf so we can print it
init_dma_xfer();
init_usart();
}
@@ -56,56 +92,49 @@ void loop(void) {
toggleLED();
delay(100);
- dma_channel_reg_map *ch_regs = dma_channel_regs(USART_DMA_DEV,
- USART_RX_DMA_CHANNEL);
+ // See if the interrupt handler got called since the last time we
+ // checked.
if (irq_fired) {
USART_HWSER.println("** IRQ **");
+ // Notice how the ISR bits show transfer complete _and_
+ // half-complete here, but the ISR bits we print next will be
+ // zero. That's because the variable "isr" gets set _inside_
+ // rx_dma_irq(). After it exits, libmaple cleans up by
+ // clearing the ISR bits. (If it didn't, and we forgot to, the
+ // interrupt would repeatedly fire forever.)
+ USART_HWSER.print("ISR bits: 0x");
+ USART_HWSER.println(isr, HEX);
irq_fired = 0;
}
+
+ // Print the ISR (interrupt status register) bits.
+ //
+ // Notice that the "transfer half-complete" ISR flag gets set when
+ // we reach the rx_buf half-way point. This is true even though we
+ // don't tell the DMA controller to interrupt us on a
+ // half-complete transfer. That is, the ISR bits get set at the
+ // right times no matter what; we just don't get interrupted
+ // unless we asked. (If an error or other problem occurs, the
+ // relevant ISR bits will get set in the same way).
USART_HWSER.print("[");
USART_HWSER.print(millis());
USART_HWSER.print("]\tISR bits: 0x");
uint8 isr_bits = dma_get_isr_bits(USART_DMA_DEV, USART_RX_DMA_CHANNEL);
USART_HWSER.print(isr_bits, HEX);
- USART_HWSER.print("\tCCR: 0x");
- USART_HWSER.print(ch_regs->CCR, HEX);
- USART_HWSER.print("\tCNDTR: 0x");
- USART_HWSER.print(ch_regs->CNDTR, HEX);
- USART_HWSER.print("\tBuffer contents: ");
- for (int i = 0; i < BUF_SIZE; i++) {
- USART_HWSER.print('\'');
- USART_HWSER.print(rx_buf[i]);
- USART_HWSER.print('\'');
- if (i < BUF_SIZE - 1) USART_HWSER.print(", ");
- }
- USART_HWSER.println();
+
+ // Print the contents of rx_buf. If you keep typing after it fills
+ // up, the new characters will overwrite the old ones, thanks to
+ // DMA_CIRC_MODE.
+ USART_HWSER.print("\tCharacter buffer contents: '");
+ USART_HWSER.print(rx_buf);
+ USART_HWSER.println("'");
if (isr_bits == 0x7) {
USART_HWSER.println("** Clearing ISR bits.");
dma_clear_isr_bits(USART_DMA_DEV, USART_RX_DMA_CHANNEL);
}
}
-/* Configure USART receiver for use with DMA */
-void init_usart(void) {
- USART_HWSER.begin(BAUD);
- USART->regs->CR3 = USART_CR3_DMAR;
-}
-
-/* Configure DMA transmission */
-void init_dma_xfer(void) {
- dma_init(USART_DMA_DEV);
- dma_setup_transfer(USART_DMA_DEV, USART_RX_DMA_CHANNEL,
- &USART->regs->DR, DMA_SIZE_8BITS,
- rx_buf, DMA_SIZE_8BITS,
- (DMA_MINC_MODE | DMA_CIRC_MODE | DMA_TRNS_CMPLT));
- dma_set_num_transfers(USART_DMA_DEV, USART_RX_DMA_CHANNEL, BUF_SIZE);
- dma_attach_interrupt(USART_DMA_DEV, USART_RX_DMA_CHANNEL, rx_dma_irq);
- dma_enable(USART_DMA_DEV, USART_RX_DMA_CHANNEL);
-}
-
-void rx_dma_irq(void) {
- irq_fired = true;
-}
+// ------- init() and main() --------------------------------------------------
// Force init to be called *first*, i.e. before static object allocation.
// Otherwise, statically allocated objects that need libmaple may fail.