/****************************************************************************
*
* 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,
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* BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS
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* ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
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*
****************************************************************************/
/**
* @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 18
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 (chancount > 17) {
/* 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;
}
