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*
* Copyright (C) 2012 PX4 Development Team. All rights reserved.
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/**
* @file dsm.c
*
* Serial protocol decoder for the Spektrum DSM* family of protocols.
*
* Decodes into the global PPM buffer and updates accordingly.
*/
#include <nuttx/config.h>
#include <fcntl.h>
#include <unistd.h>
#include <termios.h>
#include <drivers/drv_hrt.h>
#define DEBUG
#include "px4io.h"
#define DSM_FRAME_SIZE 16
#define DSM_FRAME_CHANNELS 7
static int dsm_fd = -1;
static hrt_abstime last_rx_time;
static hrt_abstime last_frame_time;
static uint8_t frame[DSM_FRAME_SIZE];
static unsigned partial_frame_count;
static unsigned channel_shift;
unsigned dsm_frame_drops;
static bool dsm_decode_channel(uint16_t raw, unsigned shift, unsigned *channel, unsigned *value);
static void dsm_guess_format(bool reset);
static bool dsm_decode(hrt_abstime now, uint16_t *values, uint16_t *num_values);
int
dsm_init(const char *device)
{
if (dsm_fd < 0)
dsm_fd = open(device, O_RDONLY | O_NONBLOCK);
if (dsm_fd >= 0) {
struct termios t;
/* 115200bps, no parity, one stop bit */
tcgetattr(dsm_fd, &t);
cfsetspeed(&t, 115200);
t.c_cflag &= ~(CSTOPB | PARENB);
tcsetattr(dsm_fd, TCSANOW, &t);
/* initialise the decoder */
partial_frame_count = 0;
last_rx_time = hrt_absolute_time();
/* reset the format detector */
dsm_guess_format(true);
debug("DSM: ready");
} else {
debug("DSM: open failed");
}
return dsm_fd;
}
bool
dsm_input(uint16_t *values, uint16_t *num_values)
{
ssize_t ret;
hrt_abstime now;
/*
* The DSM* protocol doesn't provide any explicit framing,
* so we detect frame boundaries by the inter-frame delay.
*
* The minimum frame spacing is 11ms; with 16 bytes at 115200bps
* frame transmission time is ~1.4ms.
*
* We expect to only be called when bytes arrive for processing,
* and if an interval of more than 5ms 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) > 5000) {
if (partial_frame_count > 0) {
dsm_frame_drops++;
partial_frame_count = 0;
}
}
/*
* Fetch bytes, but no more than we would need to complete
* the current frame.
*/
ret = read(dsm_fd, &frame[partial_frame_count], DSM_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 < DSM_FRAME_SIZE)
return false;
/*
* Great, it looks like we might have a frame. Go ahead and
* decode it.
*/
partial_frame_count = 0;
return dsm_decode(now, values, num_values);
}
static bool
dsm_decode_channel(uint16_t raw, unsigned shift, unsigned *channel, unsigned *value)
{
if (raw == 0xffff)
return false;
*channel = (raw >> shift) & 0xf;
uint16_t data_mask = (1 << shift) - 1;
*value = raw & data_mask;
//debug("DSM: %d 0x%04x -> %d %d", shift, raw, *channel, *value);
return true;
}
static void
dsm_guess_format(bool reset)
{
static uint32_t cs10;
static uint32_t cs11;
static unsigned samples;
/* reset the 10/11 bit sniffed channel masks */
if (reset) {
cs10 = 0;
cs11 = 0;
samples = 0;
channel_shift = 0;
return;
}
/* scan the channels in the current frame in both 10- and 11-bit mode */
for (unsigned i = 0; i < DSM_FRAME_CHANNELS; i++) {
uint8_t *dp = &frame[2 + (2 * i)];
uint16_t raw = (dp[0] << 8) | dp[1];
unsigned channel, value;
/* if the channel decodes, remember the assigned number */
if (dsm_decode_channel(raw, 10, &channel, &value) && (channel < 31))
cs10 |= (1 << channel);
if (dsm_decode_channel(raw, 11, &channel, &value) && (channel < 31))
cs11 |= (1 << channel);
/* XXX if we cared, we could look for the phase bit here to decide 1 vs. 2-frame format */
}
/* wait until we have seen plenty of frames - 2 should normally be enough */
if (samples++ < 5)
return;
/*
* Iterate the set of sensible sniffed channel sets and see whether
* decoding in 10 or 11-bit mode has yielded anything we recognise.
*
* XXX Note that due to what seem to be bugs in the DSM2 high-resolution
* stream, we may want to sniff for longer in some cases when we think we
* are talking to a DSM2 receiver in high-resolution mode (so that we can
* reject it, ideally).
* See e.g. http://git.openpilot.org/cru/OPReview-116 for a discussion
* of this issue.
*/
static uint32_t masks[] = {
0x3f, /* 6 channels (DX6) */
0x7f, /* 7 channels (DX7) */
0xff, /* 8 channels (DX8) */
0x1ff, /* 9 channels (DX9, etc.) */
0x3ff, /* 10 channels (DX10) */
0x3fff /* 18 channels (DX10) */
};
unsigned votes10 = 0;
unsigned votes11 = 0;
for (unsigned i = 0; i < (sizeof(masks) / sizeof(masks[0])); i++) {
if (cs10 == masks[i])
votes10++;
if (cs11 == masks[i])
votes11++;
}
if ((votes11 == 1) && (votes10 == 0)) {
channel_shift = 11;
debug("DSM: 11-bit format");
return;
}
if ((votes10 == 1) && (votes11 == 0)) {
channel_shift = 10;
debug("DSM: 10-bit format");
return;
}
/* call ourselves to reset our state ... we have to try again */
debug("DSM: format detect fail, 10: 0x%08x %d 11: 0x%08x %d", cs10, votes10, cs11, votes11);
dsm_guess_format(true);
}
static bool
dsm_decode(hrt_abstime frame_time, uint16_t *values, uint16_t *num_values)
{
/*
debug("DSM frame %02x%02x %02x%02x %02x%02x %02x%02x %02x%02x %02x%02x %02x%02x %02x%02x",
frame[0], frame[1], frame[2], frame[3], frame[4], frame[5], frame[6], frame[7],
frame[8], frame[9], frame[10], frame[11], frame[12], frame[13], frame[14], frame[15]);
*/
/*
* If we have lost signal for at least a second, reset the
* format guessing heuristic.
*/
if (((frame_time - last_frame_time) > 1000000) && (channel_shift != 0))
dsm_guess_format(true);
/* we have received something we think is a frame */
last_frame_time = frame_time;
/* if we don't know the frame format, update the guessing state machine */
if (channel_shift == 0) {
dsm_guess_format(false);
return false;
}
/*
* The encoding of the first two bytes is uncertain, so we're
* going to ignore them for now.
*
* Each channel is a 16-bit unsigned value containing either a 10-
* or 11-bit channel value and a 4-bit channel number, shifted
* either 10 or 11 bits. The MSB may also be set to indicate the
* second frame in variants of the protocol where more than
* seven channels are being transmitted.
*/
for (unsigned i = 0; i < DSM_FRAME_CHANNELS; i++) {
uint8_t *dp = &frame[2 + (2 * i)];
uint16_t raw = (dp[0] << 8) | dp[1];
unsigned channel, value;
if (!dsm_decode_channel(raw, channel_shift, &channel, &value))
continue;
/* ignore channels out of range */
if (channel >= PX4IO_INPUT_CHANNELS)
continue;
/* update the decoded channel count */
if (channel >= *num_values)
*num_values = channel + 1;
/* convert 0-1024 / 0-2048 values to 1000-2000 ppm encoding in a very sloppy fashion */
if (channel_shift == 11)
value /= 2;
value += 998;
/*
* Store the decoded channel into the R/C input buffer, taking into
* account the different ideas about channel assignement that we have.
*
* Specifically, the first four channels in rc_channel_data are roll, pitch, thrust, yaw,
* but the first four channels from the DSM receiver are thrust, roll, pitch, yaw.
*/
switch (channel) {
case 0:
channel = 2;
break;
case 1:
channel = 0;
break;
case 2:
channel = 1;
default:
break;
}
values[channel] = value;
}
/*
* XXX Note that we may be in failsafe here; we need to work out how to detect that.
*/
return true;
}
