SPRacingF3Mini - First cut of transponder system.

Implementation is DMA buffer based, just like the WS2811 LED Strip code,
so that there is minimal impact on flight performance.

Currently the code is mutually exclusive with the LED_STRIP code and
there is no means to set the transponder code.

src/main/drivers/transponder_ir.c

@@ -0,0 +1,115 @@
+/*
+ * This file is part of Cleanflight.
+ *
+ * Cleanflight is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * Cleanflight is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with Cleanflight.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include <stdbool.h>
+#include <stdint.h>
+#include <string.h>
+
+#include "platform.h"
+
+#include "build_config.h"
+
+#include "drivers/transponder_ir.h"
+
+/*
+ * Implementation note:
+ * Using around over 700 bytes for a transponder DMA buffer is a little excessive, likely an alternative implementation that uses a fast
+ * ISR to generate the output signal dynamically based on state would be more memory efficient and would likely be more appropriate for
+ * other targets.  However this approach requires very little CPU time and is just fire-and-forget.
+ *
+ * On an STM32F303CC 720 bytes is currently fine and that is the target for which this code was designed for.
+ */
+uint8_t transponderIrDMABuffer[TRANSPONDER_DMA_BUFFER_SIZE];
+
+volatile uint8_t transponderIrDataTransferInProgress = 0;
+
+void transponderIrInit(void)
+{
+    memset(&transponderIrDMABuffer, 0, TRANSPONDER_DMA_BUFFER_SIZE);
+    transponderIrHardwareInit();
+}
+
+bool isTransponderIrReady(void)
+{
+    return !transponderIrDataTransferInProgress;
+}
+
+static uint16_t dmaBufferOffset;
+
+void updateTransponderDMABuffer(const uint8_t* transponderData)
+{
+    uint8_t byteIndex;
+    uint8_t bitIndex;
+    uint8_t toggleIndex;
+
+    for (byteIndex = 0; byteIndex < TRANSPONDER_DATA_LENGTH; byteIndex++) {
+
+        uint8_t byteToSend = *transponderData;
+        transponderData++;
+        for (bitIndex = 0; bitIndex < TRANSPONDER_BITS_PER_BYTE; bitIndex++)
+        {
+            bool doToggles = false;
+            if (bitIndex == 0) {
+                doToggles = true;
+            } else if (bitIndex == TRANSPONDER_BITS_PER_BYTE - 1) {
+                doToggles = false;
+            } else {
+                doToggles = byteToSend & (1 << (bitIndex - 1));
+            }
+
+            for (toggleIndex = 0; toggleIndex < TRANSPONDER_TOGGLES_PER_BIT; toggleIndex++)
+            {
+                if (doToggles) {
+                    transponderIrDMABuffer[dmaBufferOffset] = BIT_TOGGLE_1;
+                } else {
+                    transponderIrDMABuffer[dmaBufferOffset] = BIT_TOGGLE_0;
+                }
+                dmaBufferOffset++;
+            }
+            transponderIrDMABuffer[dmaBufferOffset] = BIT_TOGGLE_0;
+            dmaBufferOffset++;
+        }
+    }
+}
+
+void transponderIrWaitForTransmitComplete(void)
+{
+    static uint32_t waitCounter = 0;
+
+    while(transponderIrDataTransferInProgress) {
+        waitCounter++;
+    }
+}
+
+void transponderIrUpdateData(const uint8_t* transponderData)
+{
+    transponderIrWaitForTransmitComplete();
+
+    updateTransponderDMABuffer(transponderData);
+}
+
+
+void transponderIrTransmit(void)
+{
+    transponderIrWaitForTransmitComplete();
+
+    dmaBufferOffset = 0;
+
+    transponderIrDataTransferInProgress = 1;
+    transponderIrDMAEnable();
+}
+