1 files changed, 255 insertions, 0 deletions
diff --git a/src/modules/px4iofirmware/sbus.c b/src/modules/px4iofirmware/sbus.c
new file mode 100644
index 000000000..073ddeaba
--- /dev/null
+++ b/src/modules/px4iofirmware/sbus.c
@@ -0,0 +1,255 @@
+/****************************************************************************
+ *
+ *   Copyright (C) 2012 PX4 Development Team. All rights reserved.
+ *
+ * Redistribution and use in source and binary forms, with or without
+ * modification, are permitted provided that the following conditions
+ * are met:
+ *
+ * 1. Redistributions of source code must retain the above copyright
+ *    notice, this list of conditions and the following disclaimer.
+ * 2. Redistributions in binary form must reproduce the above copyright
+ *    notice, this list of conditions and the following disclaimer in
+ *    the documentation and/or other materials provided with the
+ *    distribution.
+ * 3. Neither the name PX4 nor the names of its contributors may be
+ *    used to endorse or promote products derived from this software
+ *    without specific prior written permission.
+ *
+ * THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
+ * "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+ * LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
+ * FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
+ * COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
+ * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
+ * BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
+ * OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED
+ * AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
+ * LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
+ * ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
+ * POSSIBILITY OF SUCH DAMAGE.
+ *
+ ****************************************************************************/
+
+/**
+ * @file sbus.c
+ *
+ * Serial protocol decoder for the Futaba S.bus protocol.
+ */
+
+#include <nuttx/config.h>
+
+#include <fcntl.h>
+#include <unistd.h>
+#include <termios.h>
+
+#include <systemlib/ppm_decode.h>
+
+#include <drivers/drv_hrt.h>
+
+#define DEBUG
+#include "px4io.h"
+#include "protocol.h"
+#include "debug.h"
+
+#define SBUS_FRAME_SIZE		25
+#define SBUS_INPUT_CHANNELS	16
+
+static int sbus_fd = -1;
+
+static hrt_abstime last_rx_time;
+static hrt_abstime last_frame_time;
+
+static uint8_t	frame[SBUS_FRAME_SIZE];
+
+static unsigned partial_frame_count;
+
+unsigned sbus_frame_drops;
+
+static bool sbus_decode(hrt_abstime frame_time, uint16_t *values, uint16_t *num_values);
+
+int
+sbus_init(const char *device)
+{
+	if (sbus_fd < 0)
+		sbus_fd = open(device, O_RDONLY | O_NONBLOCK);
+
+	if (sbus_fd >= 0) {
+		struct termios t;
+
+		/* 100000bps, even parity, two stop bits */
+		tcgetattr(sbus_fd, &t);
+		cfsetspeed(&t, 100000);
+		t.c_cflag |= (CSTOPB | PARENB);
+		tcsetattr(sbus_fd, TCSANOW, &t);
+
+		/* initialise the decoder */
+		partial_frame_count = 0;
+		last_rx_time = hrt_absolute_time();
+
+		debug("S.Bus: ready");
+
+	} else {
+		debug("S.Bus: open failed");
+	}
+
+	return sbus_fd;
+}
+
+bool
+sbus_input(uint16_t *values, uint16_t *num_values)
+{
+	ssize_t		ret;
+	hrt_abstime	now;
+
+	/*
+	 * The S.bus protocol doesn't provide reliable framing,
+	 * so we detect frame boundaries by the inter-frame delay.
+	 *
+	 * The minimum frame spacing is 7ms; with 25 bytes at 100000bps
+	 * frame transmission time is ~2ms.
+	 *
+	 * We expect to only be called when bytes arrive for processing,
+	 * and if an interval of more than 3ms passes between calls,
+	 * the first byte we read will be the first byte of a frame.
+	 *
+	 * In the case where byte(s) are dropped from a frame, this also
+	 * provides a degree of protection. Of course, it would be better
+	 * if we didn't drop bytes...
+	 */
+	now = hrt_absolute_time();
+
+	if ((now - last_rx_time) > 3000) {
+		if (partial_frame_count > 0) {
+			sbus_frame_drops++;
+			partial_frame_count = 0;
+		}
+	}
+
+	/*
+	 * Fetch bytes, but no more than we would need to complete
+	 * the current frame.
+	 */
+	ret = read(sbus_fd, &frame[partial_frame_count], SBUS_FRAME_SIZE - partial_frame_count);
+
+	/* if the read failed for any reason, just give up here */
+	if (ret < 1)
+		return false;
+
+	last_rx_time = now;
+
+	/*
+	 * Add bytes to the current frame
+	 */
+	partial_frame_count += ret;
+
+	/*
+	 * If we don't have a full frame, return
+	 */
+	if (partial_frame_count < SBUS_FRAME_SIZE)
+		return false;
+
+	/*
+	 * Great, it looks like we might have a frame.  Go ahead and
+	 * decode it.
+	 */
+	partial_frame_count = 0;
+	return sbus_decode(now, values, num_values);
+}
+
+/*
+ * S.bus decoder matrix.
+ *
+ * Each channel value can come from up to 3 input bytes. Each row in the
+ * matrix describes up to three bytes, and each entry gives:
+ *
+ * - byte offset in the data portion of the frame
+ * - right shift applied to the data byte
+ * - mask for the data byte
+ * - left shift applied to the result into the channel value
+ */
+struct sbus_bit_pick {
+	uint8_t byte;
+	uint8_t rshift;
+	uint8_t mask;
+	uint8_t lshift;
+};
+static const struct sbus_bit_pick sbus_decoder[SBUS_INPUT_CHANNELS][3] = {
+	/*  0 */ { { 0, 0, 0xff, 0}, { 1, 0, 0x07, 8}, { 0, 0, 0x00,  0} },
+	/*  1 */ { { 1, 3, 0x1f, 0}, { 2, 0, 0x3f, 5}, { 0, 0, 0x00,  0} },
+	/*  2 */ { { 2, 6, 0x03, 0}, { 3, 0, 0xff, 2}, { 4, 0, 0x01, 10} },
+	/*  3 */ { { 4, 1, 0x7f, 0}, { 5, 0, 0x0f, 7}, { 0, 0, 0x00,  0} },
+	/*  4 */ { { 5, 4, 0x0f, 0}, { 6, 0, 0x7f, 4}, { 0, 0, 0x00,  0} },
+	/*  5 */ { { 6, 7, 0x01, 0}, { 7, 0, 0xff, 1}, { 8, 0, 0x03,  9} },
+	/*  6 */ { { 8, 2, 0x3f, 0}, { 9, 0, 0x1f, 6}, { 0, 0, 0x00,  0} },
+	/*  7 */ { { 9, 5, 0x07, 0}, {10, 0, 0xff, 3}, { 0, 0, 0x00,  0} },
+	/*  8 */ { {11, 0, 0xff, 0}, {12, 0, 0x07, 8}, { 0, 0, 0x00,  0} },
+	/*  9 */ { {12, 3, 0x1f, 0}, {13, 0, 0x3f, 5}, { 0, 0, 0x00,  0} },
+	/* 10 */ { {13, 6, 0x03, 0}, {14, 0, 0xff, 2}, {15, 0, 0x01, 10} },
+	/* 11 */ { {15, 1, 0x7f, 0}, {16, 0, 0x0f, 7}, { 0, 0, 0x00,  0} },
+	/* 12 */ { {16, 4, 0x0f, 0}, {17, 0, 0x7f, 4}, { 0, 0, 0x00,  0} },
+	/* 13 */ { {17, 7, 0x01, 0}, {18, 0, 0xff, 1}, {19, 0, 0x03,  9} },
+	/* 14 */ { {19, 2, 0x3f, 0}, {20, 0, 0x1f, 6}, { 0, 0, 0x00,  0} },
+	/* 15 */ { {20, 5, 0x07, 0}, {21, 0, 0xff, 3}, { 0, 0, 0x00,  0} }
+};
+
+static bool
+sbus_decode(hrt_abstime frame_time, uint16_t *values, uint16_t *num_values)
+{
+	/* check frame boundary markers to avoid out-of-sync cases */
+	if ((frame[0] != 0x0f) || (frame[24] != 0x00)) {
+		sbus_frame_drops++;
+		return false;
+	}
+
+	/* if the failsafe or connection lost bit is set, we consider the frame invalid */
+	if ((frame[23] & (1 << 2)) && /* signal lost */
+	    (frame[23] & (1 << 3))) { /* failsafe */
+
+		/* actively announce signal loss */
+		*values = 0;
+		return false;
+	}
+
+	/* we have received something we think is a frame */
+	last_frame_time = frame_time;
+
+	unsigned chancount = (PX4IO_INPUT_CHANNELS > SBUS_INPUT_CHANNELS) ?
+			     SBUS_INPUT_CHANNELS : PX4IO_INPUT_CHANNELS;
+
+	/* use the decoder matrix to extract channel data */
+	for (unsigned channel = 0; channel < chancount; channel++) {
+		unsigned value = 0;
+
+		for (unsigned pick = 0; pick < 3; pick++) {
+			const struct sbus_bit_pick *decode = &sbus_decoder[channel][pick];
+
+			if (decode->mask != 0) {
+				unsigned piece = frame[1 + decode->byte];
+				piece >>= decode->rshift;
+				piece &= decode->mask;
+				piece <<= decode->lshift;
+
+				value |= piece;
+			}
+		}
+
+		/* convert 0-2048 values to 1000-2000 ppm encoding in a very sloppy fashion */
+		values[channel] = (value / 2) + 998;
+	}
+
+	/* decode switch channels if data fields are wide enough */
+	if (PX4IO_INPUT_CHANNELS > 17 && chancount > 15) {
+		chancount = 18;
+
+		/* channel 17 (index 16) */
+		values[16] = (frame[23] & (1 << 0)) * 1000 + 998;
+		/* channel 18 (index 17) */
+		values[17] = (frame[23] & (1 << 1)) * 1000 + 998;
+	}
+
+	/* note the number of channels decoded */
+	*num_values = chancount;
+
+	return true;
+}