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rc/dsm: remove system field check, add new validity checks
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 - unfortunately we can't depend on the system field due to potential
external binding and non-genuine Spektrum equipment
 - reject any DSM frame with duplicate channels
 - add 16 channel mask
 - tighten valid PWM range 990-2010us (±100% travel is 1102-1898µs)
 - update RCTest rejected frame count
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@dagar @LorenzMeier
dagar authored and LorenzMeier committed Jan 1, 2021
1 parent 4b0038b commit a6274bc
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Showing 2 changed files with 132 additions and 116 deletions.
228 changes: 121 additions & 107 deletions src/lib/rc/dsm.cpp
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Original file line number Diff line number Diff line change
@@ -1,6 +1,6 @@
/****************************************************************************
*
* Copyright (c) 2012-2015 PX4 Development Team. All rights reserved.
* Copyright (c) 2012-2020 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
Expand Down Expand Up @@ -44,6 +44,7 @@
#include <px4_platform_common/defines.h>

#include <fcntl.h>
#include <math.h>
#include <unistd.h>
#include <termios.h>
#include <string.h>
Expand All @@ -53,6 +54,8 @@
#include "common_rc.h"
#include <drivers/drv_hrt.h>

#include <include/containers/Bitset.hpp>

#if defined(__PX4_NUTTX)
#include <nuttx/arch.h>
#define dsm_udelay(arg) up_udelay(arg)
Expand Down Expand Up @@ -106,7 +109,7 @@ static uint16_t dsm_chan_count = 0; /**< DSM channel count */
*/
static bool dsm_decode_channel(uint16_t raw, unsigned shift, uint8_t &channel, uint16_t &value)
{
if (raw == 0xffff) {
if (raw == 0 || raw == 0xffff) {
return false;
}

Expand All @@ -118,41 +121,81 @@ static bool dsm_decode_channel(uint16_t raw, unsigned shift, uint8_t &channel, u
static constexpr uint16_t MASK_1024_SXPOS = 0x03FF;

channel = (raw & MASK_1024_CHANID) >> 10; // 6 bits
uint16_t servo_position = (raw & MASK_1024_SXPOS); // 10 bits
value = servo_position * 2; // scale to be equivalent to 2048 mode

const uint16_t servo_position = (raw & MASK_1024_SXPOS); // 10 bits

if (channel > DSM_MAX_CHANNEL_COUNT) {
PX4_DEBUG("invalid channel: %d\n", channel);
return false;
}

// PWM_OUT = (ServoPosition x 1.166μs) + Offset
static constexpr uint16_t offset = 903; // microseconds
value = roundf(servo_position * 1.166f) + offset;

// Spektrum range is 903μs to 2097μs (Specification for Spektrum Remote Receiver Interfacing Rev G 9.1)
// ±100% travel is 1102µs to 1898 µs
if (value < 990 || value > 2010) {
// if the value is unrealistic, fail the parsing entirely
PX4_DEBUG("channel %d invalid range %d", channel, value);
return false;
}

return true;

} else if (shift == 11) {
// 2048 Mode
// Bits 15 Servo Phase
// Bits 14-11 Channel ID
// Bits 10-0 Servo Position
static constexpr uint16_t MASK_2048_CHANID = 0x7800;
static constexpr uint16_t MASK_2048_SXPOS = 0x07FF;

// from Spektrum Remote Receiver Interfacing Rev G Page 6
uint8_t chan = (raw & MASK_2048_CHANID) >> 11;
uint16_t servo_position = 0;

// from Spektrum Remote Receiver Interfacing Rev G Page 6
const bool phase = raw & (2 >> 15); // the phase is part of the X-Plus address (bit 15)
uint8_t chan = (raw >> 11) & 0x0F;

if (chan < 12) {
// Normal channels
static constexpr uint16_t MASK_2048_SXPOS = 0x07FF;
servo_position = (raw & MASK_2048_SXPOS);

} else if (chan == 12) {
// XPlus channels
chan += ((raw >> 9) & 0x03);

const bool phase = raw & (2 >> 15); // the phase is part of the X-Plus address (bit 15)

if (phase) {
chan += 4;
}

if (chan > DSM_MAX_CHANNEL_COUNT) {
PX4_DEBUG("invalid channel: %d\n", chan);
return false;
}

servo_position = (raw & 0x01FF) << 2;

channel = chan;

} else {
PX4_DEBUG("invalid channel: %d\n", chan);
return false;
}

channel = chan;
value = servo_position;

// PWM_OUT = (ServoPosition x 0.583μs) + Offset
static constexpr uint16_t offset = 903; // microseconds
value = roundf(servo_position * 0.583f) + offset;

// Spektrum range is 903μs to 2097μs (Specification for Spektrum Remote Receiver Interfacing Rev G 9.1)
// ±100% travel is 1102µs to 1898 µs
if (value < 990 || value > 2010) {
// if the value is unrealistic, fail the parsing entirely
PX4_DEBUG("channel %d invalid range %d", channel, value);
return false;
}

return true;
}

Expand Down Expand Up @@ -180,6 +223,12 @@ dsm_guess_format(bool reset)
return false;
}

px4::Bitset<DSM_MAX_CHANNEL_COUNT> channels_found_10;
px4::Bitset<DSM_MAX_CHANNEL_COUNT> channels_found_11;

bool cs10_frame_valid = true;
bool cs11_frame_valid = true;

/* scan the channels in the current dsm_frame in both 10- and 11-bit mode */
for (unsigned i = 0; i < DSM_FRAME_CHANNELS; i++) {

Expand All @@ -190,15 +239,43 @@ dsm_guess_format(bool reset)
uint16_t value = 0;

/* 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, 10, channel, value)) {
// invalidate entire frame (for 1024) if channel already found, no duplicate channels per DSM frame
if (channels_found_10[channel]) {
cs10_frame_valid = false;

} else {
channels_found_10.set(channel);
}
}

if (dsm_decode_channel(raw, 11, channel, value)) {
// invalidate entire frame (for 2048) if channel already found, no duplicate channels per DSM frame
if (channels_found_11[channel]) {
cs11_frame_valid = false;

} else {
channels_found_11.set(channel);
}
}
}

if (dsm_decode_channel(raw, 11, channel, value) && (channel < 31)) {
cs11 |= (1 << channel);
// add valid cs10 channels
if (cs10_frame_valid) {
for (unsigned channel = 0; channel < DSM_FRAME_CHANNELS; channel++) {
if (channels_found_10[channel]) {
cs10 |= 1 << channel;
}
}
}

/* XXX if we cared, we could look for the phase bit here to decide 1 vs. 2-dsm_frame format */
// add valid cs11 channels
if (cs11_frame_valid) {
for (unsigned channel = 0; channel < DSM_FRAME_CHANNELS; channel++) {
if (channels_found_11[channel]) {
cs11 |= 1 << channel;
}
}
}

samples++;
Expand Down Expand Up @@ -229,8 +306,10 @@ dsm_guess_format(bool reset)
0x1ff, /* 9 channels (DX9, etc.) */
0x3ff, /* 10 channels (DX10) */
0x1fff, /* 13 channels (DX10t) */
0x3fff /* 18 channels (DX10) */
0xffff, /* 16 channels */
0x3ffff,/* 18 channels (DX10) */
};

unsigned votes10 = 0;
unsigned votes11 = 0;

Expand Down Expand Up @@ -470,91 +549,6 @@ dsm_decode(hrt_abstime frame_time, uint16_t *values, uint16_t *num_values, bool
}
}

/*
* The second byte indicates the protocol and frame rate. We have a
* guessing state machine, so we don't need to use this. At any rate,
* these are the allowable values:
*
* 0x01 22MS 1024 DSM2
* 0x12 11MS 2048 DSM2
* 0xa2 22MS 2048 DSMX
* 0xb2 11MS 2048 DSMX
*/
const uint8_t system = dsm_frame[1];

switch (system) {
case 0x00: // SURFACE DSM1
// unsupported
PX4_DEBUG("ERROR system: SURFACE DSM1 (%X) unsupported\n", system);
return false;

case 0x01: // DSM2 1024 22ms
if (dsm_channel_shift != 10) {
dsm_guess_format(true);
return false;
}

break;

case 0x02: // DSM2 1024 (MC24)
// unsupported
PX4_DEBUG("ERROR system: DSM2 1024 (MC24) (%X) unsupported\n", system);
return false;

case 0x12: // DSM2 2048 11ms
if (dsm_channel_shift != 11) {
dsm_guess_format(true);
return false;
}

break;

case 0x23: // SURFACE DSM2 16.5ms
// unsupported
PX4_DEBUG("ERROR system: DSM2 16.5ms (%X) unsupported\n", system);
return false;

case 0x50: // DSM MARINE
// unsupported
PX4_DEBUG("ERROR system: DSM MARINE (%X) unsupported\n", system);
return false;

case 0x92: // DSMJ
// unsupported
PX4_DEBUG("ERROR system: DSMJ (%X) unsupported\n", system);
return false;

case 0xA2: // DSMX 22ms OR DSMR 11ms or DSMR 22ms
if (dsm_channel_shift != 11) {
dsm_guess_format(true);
return false;
}

break;

case 0xA4: // DSMR 5.5ms
// unsupported
PX4_DEBUG("ERROR system: DSMR 5.5ms (%X) unsupported\n", system);
return false;

case 0xAE: // NOT_BOUND
PX4_DEBUG("ERROR system: NOT_BOUND (%X) unsupported\n", system);
return false;

case 0xB2: // DSMX 11ms
if (dsm_channel_shift != 11) {
dsm_guess_format(true);
return false;
}

break;

default:
// ERROR
PX4_DEBUG("ERROR system: %X unsupported\n", system);
return false;
}

/*
* Each channel is a 16-bit unsigned value containing either a 10-
* or 11-bit channel value and a 4-bit channel number, shifted
Expand All @@ -563,18 +557,42 @@ dsm_decode(hrt_abstime frame_time, uint16_t *values, uint16_t *num_values, bool
* seven channels are being transmitted.
*/

px4::Bitset<DSM_MAX_CHANNEL_COUNT> channels_found;

unsigned channel_decode_failures = 0;

for (unsigned i = 0; i < DSM_FRAME_CHANNELS; i++) {

uint8_t *dp = &dsm_frame[2 + (2 * i)];
uint16_t raw = (dp[0] << 8) | dp[1];

// ignore
if (raw == 0 || raw == 0xffff) {
continue;
}

uint8_t channel = 0;
uint16_t value = 0;

if (!dsm_decode_channel(raw, dsm_channel_shift, channel, value)) {
channel_decode_failures++;
continue;
}

// discard entire frame if at least half of it (4 channels) failed to decode
if (channel_decode_failures >= 4) {
return false;
}

// abort if channel already found, no duplicate channels per DSM frame
if (channels_found[channel]) {
PX4_DEBUG("duplicate channel %d\n\n", channel);
return false;

} else {
channels_found.set(channel);
}

/* reset bit guessing state machine if the channel index is out of bounds */
if (channel > DSM_MAX_CHANNEL_COUNT) {
dsm_guess_format(true);
Expand Down Expand Up @@ -614,11 +632,7 @@ dsm_decode(hrt_abstime frame_time, uint16_t *values, uint16_t *num_values, bool
break;
}

// Scaling
// See Specification for Spektrum Remote Receiver Interfacing Rev G 2019 January 22
// 2048 Mode Scaling: PWM_OUT = (ServoPosition x 0.583μs) + Offset (903μs)
// scaled integer for decent accuracy while staying efficient (0.583us ~= 1194/2048)
values[channel] = (value * 1194) / 2048 + 903;
values[channel] = value;
}

/*
Expand Down
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4 comments on commit a6274bc

@mengchaoheng
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Why can't the v1.11.x version be used for SITL debugging with vscode, but the current master branch version can. I want to use a tag on the stable branch for secondary development instead of master, because I found that the current master branch upload to CUAV nano is abnormal, and the initial ringtone is incomplete. So, I need the stability of the stable branch, and I need to perform single-step debugging in vscode. What should I do?

@mcsauder
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This isn the conundrum of continuous development.

@mengchaoheng
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This isn the conundrum of continuous development.

What do you mean？This isn't the conundrum of continuous development？

@mcsauder
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Fix it and your will be the answer.

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