dzikuvx / quadmeup_crossbow

Cheap DIY RC link based on LoRa 868MHz modules

QuadMeUp Crossbow

QuadMeUp Crossbow is a DIY project that gives 5km (at least) of RC link for UAV (airplanes and drones) for a price below $40. I uses SX1278 (LoRa 868MHz/915MHz) compatible (like HopeRF RFM95W) radio modules connected to Arduino compatible boards. It can be regular Arduino connected via SPI to SX1278 or dedicated board like Adafruit Feather 32u4 RFM LoRa or LoRa32u4 II

Future versions might be ported to ESP32 LoRa32 modules.

Current state

Works:

• Getting data from OpenTX radio using SBUS protocol
• Transmitting 10 channels to RX modules
• Frequency hopping
• Getting basic telemetry from RX module
• Sending 10 channels using SBUS to flight controller
• Binding
• Allowing to use OLED on TX to get basic data

Needs implementation:

• TX module configuration
• RX configuration from TX module
• Sending telemetry from TX to OpenTX radio

Dependencies

To compile, following libraries have to be installed:

• U8g2 for OLED support in TX module
• FlashStorage for EEPROM-Emulation if using a SAMD-Board (M0 etc.)

Protocol

Byte	Description	Notes
1	Frame type & used radio channel	bits 7-5 defines frame, bits 4-0 current radio channel
2 - 33	Payload	lenghth defined by frame type
payload length + 2	CRC	using crc8_dvb_s2 method

CRC

CRC is computed using crc8_dvb_s2 method. Initial CRC value for each frame CRC is equal to CRC of 4 bind bytes (unique for transmitter module).

Frame types

Value	Value hex	Description	Direction	Payload length
0000	0x0	RC channels data RC_DATA	TX -> RX	9
0001	0x1	Receiver health and basic telemetry RX_HEALTH	RX -> TX	6
0010	0x2	Request receiver configuration	TX -> RX	no used
0011	0x3	Receiver configuration	RX -> TX	no used
0100	0x4	Set receiver configuration	TX -> RX	no used
0101	0x5	PING frame, uses 9 byte payload	TX -> RX	4
0110	0x6	PONG frame, the same payload as PING	RX -> TX	4
0111	0x7	BIND frame, transmitted by TX only during binding	TX -> RX	4

RC_DATA frame format

Protocol allows to send 10 RC channels in total encoded as following

• channels 1 to 4 encoded using 10 bits each (5 bytes)
• channels 5 to 6 encoded using 8 bits each (2 bytes)
• channels 7 to 10 encoded using 4 bits per channel (2 bytes)

Total length of RC_DATA payload is 9 bytes

RX_HEALTH frame format

Byte	Description
1	RX RSSI
2	RX SNR
3	RX supply volatage, sent in 0,1V
4	RX analog input 1 sent in 0,1V
5	RX analog input 2 sent in 0,1V
6	Flags

Flags

Bit	Meaning
00000001	Device in Failsafe mode

PING and PONG frames

PING and PONG frames are to determine packet roundrip between TX and RX module. TX sends PING frame with curent micros. If RX receives PING frame, it respons its payload as PONG frame.

BIND frame format

Byte	Description
1	Bind key byte 0
2	Bind key byte 1
3	Bind key byte 2
4	Bind key byte 3

RSSI

1. Receiver RSSI for the last received packet is injected as channel 11
2. RSSI is scaled in dB, NOT percent
3. RSSI 0 in flight controller means 40dB RSSI in RF chip on last packet received
4. RSSI 100 in flight controller means 140dB RSSI in RF chip on last packet received
5. Rule of thumb: to increase distance twice you need 6dB assuming nothing else will interfere
6. If at 100m RSSI is 40, at 200m should be around 34
7. I had successful flight with RSSI below 15
8. With correct antennas, at 1km RSSI should be around 20

OpenTX setup

OpenTX 2.2.1 or newer is required with enabled External RF SBUS.

• Mode: SBUS
• Channel Range: 1-16
• Refresh rate: 12.0ms or 14.0ms normal

Tested with:

• FrSky X9D Plus

Flight controller setup

RX module outputs RC data using SBUS protocol. The trick is that DIY RX module does not have inverters, so SBUS signal is also not inverted (TTL standard), while standard SBUS protocol used inverted serial signal.

That mean the following:

• RX module TX line can be connected to any free UART RX pin
• On F3 or F7 boards flight controller has to be configured not to use inverted SBUS (refer to flight controller docs)
• On F4 flight controllers inverios has to be configured only when using dedicated SBUS serial port

Manual

Binding

After flashing TX and RX, binding is required.

1. Power up TX module
2. Navigate using button #1 to "Bind" option
3. Long press button #2 to enter Bind Mode
4. Power up RX
5. RX LED flashes quickly when in bind mode
6. After RX receives bind packet, LED goes to constanly ON state
7. When RX LED is solid ON, leave bind mode by long pressing button #2

TX module connection diagram

RX module connection diagram

quadmeup_crossbow's Issues

Lost model alarm on RX module

Remove PacketID from QSP frame

It has no sense to use it. It only takes one byte without giving anything valuable

Watchdog to go back to IDLE if frame did not completed in reasonable time

Add CRSF support on the TX module

PPM is crappy. Use CRSF protocol to get data from Taranis

RX_HEALTH encoding and decoding

S.Bus input for TX module

opentx/opentx#5225 implemented S.Bus output on PPM pin. Add S.Bus instead of PPM for better performance

Drop frame channelID field in favor of CRC with salt

ChannelID (1st byte) of QSP packet is not needed. Drop it and instead use 32bit salt key for authorization:

Instead of initializing CRC with 0 at the beginning of frame encoding and decoding, initialize it with 32 bit secret key used to bind both TX and RX. This way, 1 byte will be saved and packets will be protected by crc only

Latest master PPM reading not stable

Reading packet from radio takes too much time and interferes with PPM processing

Inject RSSI as one of output PPM channels

LoRa transmitting should not be blocking

Do not block loop during LoRa transmitting.
Add a periodic check if radio is done with TX and then switch radio to receive

Improve RC_DATA frame performance

Improve scaling, get rid of map function and find out faster way to scale input into output

OLED display for TX module to display basic statistics

sbus decoding should timeout

SBUS incoming frame whould be rejected if:

• decoding is taking more than n ms. 5ms should be enough. Check just to be sure
• we should ignore everything after successful frame since frames are send every 6-7ms

Telemetry feedback from RX to TX

TX should be transmitting only when receiving input from radio

This is the only way to ensure failsafe can be triggered in RX when radio dies

Verify PPM input on TX module

Send RX device state in RX_HEALTH frame to indicate RX in failsafe mode

increase power RF

hi developer
can you increase power RF 500mw or 1w ? 100mw is small for longrange
thanks

Use slots for transmitting and receiving

TX determines transmission slots and transmits with constant frequency. RX can answer only after TX originated frame was received

SBUS output from RX module

Enable OLED only on button

There is no need to enable OLED processing all the time

Get rid of Serial mode, it does not makes sense

PCB

Can we work together to produce a set of hardware to sell?

Implement SBUS on RX side and deprecate PPM output

Verify PPM output on RX module

Frame length should be hardcoded

Frame length should be hardcoded to frame type.
Do not send frame length, get it from frame type.
Available 4 bits in header can be used to send current channel

ability for RX and TX to change frequency

Adafruit OLED library is a resource hugger, get rid of it

PING/PONG should be only part of debug process

Which libraries do you use?

I'm trying to locate correct sources and versions of all libraries but I'm not sure I got it correct. It would be great if you could help me out, maybe update the readme?

My PPMReader (https://github.com/Nikkilae/PPM-reader) does not match the function prototype
"
crossbow:38: error: no matching function for call to 'PPMReader::PPMReader(int, int, bool)'
PPMReader ppmReader(PPM_INPUT_PIN, PPM_INPUT_INTERRUPT, true);
"

Then I need to fix the InterruptHandler to support the M0 chip but thats another story :)

Improve radio sleep/wake/receive/transmit cycle

• QSP Success:
  • sleep
  • transmit
  • receive after TX is done
• on QSP failure
  • sleep
  • receive again

No RSSI output on RX module

Issues on M0

Hi there,

i tried your code on adafruit m0 and it needs adjustmends, because the M0 does not have an EEPROM.

In platform_node.h:

#ifdef ARDUINO_AVR_FEATHER32U4 #include <EEPROM.h> #endif

In platform_node.cpp:
`
void PlatformNode::seed(void) {
uint8_t val;
#ifdef ARDUINO_AVR_FEATHER32U4
val = EEPROM.read(EEPROM_ADDRESS_BIND_KEY_SEEDED);

if (val != 0xf1) {
    EEPROM.write(EEPROM_ADDRESS_BIND_0, random(1, 255)); //Yes, from 1 to 254
    EEPROM.write(EEPROM_ADDRESS_BIND_1, random(1, 255)); //Yes, from 1 to 254
    EEPROM.write(EEPROM_ADDRESS_BIND_2, random(1, 255)); //Yes, from 1 to 254
    EEPROM.write(EEPROM_ADDRESS_BIND_3, random(1, 255)); //Yes, from 1 to 254
    EEPROM.write(EEPROM_ADDRESS_BIND_KEY_SEEDED, 0xf1);
} 

#endif
}

void PlatformNode::loadBindKey(uint8_t key[]) {
#ifdef ARDUINO_AVR_FEATHER32U4
key[0] = EEPROM.read(EEPROM_ADDRESS_BIND_0);
key[1] = EEPROM.read(EEPROM_ADDRESS_BIND_1);
key[2] = EEPROM.read(EEPROM_ADDRESS_BIND_2);
key[3] = EEPROM.read(EEPROM_ADDRESS_BIND_3);
#endif
}

void PlatformNode::saveBindKey(uint8_t key[]) {
#ifdef ARDUINO_AVR_FEATHER32U4
EEPROM.write(EEPROM_ADDRESS_BIND_0, key[0]);
EEPROM.write(EEPROM_ADDRESS_BIND_1, key[1]);
EEPROM.write(EEPROM_ADDRESS_BIND_2, key[2]);
EEPROM.write(EEPROM_ADDRESS_BIND_3, key[3]);
EEPROM.write(EEPROM_ADDRESS_BIND_KEY_SEEDED, 0xf1);
#endif
}
`

Best regards,
David

PING PONG frame to determine time in the air and other statistics

Change RSSI scaling

Change rssi scaling to correspond to dB scale. Since radio usually looses signal at 40dB, scale like so:

• constrain reported RSSI to 40-140
• substract 40
• multiply by 10
• add 1000

RF settings description

Can you write something about RF settings? It's very important for reliable, delay and radius.. Thanks

uint32_t frequency = 867000000;
uint32_t loraBandwidth = 250000;
uint8_t loraSpreadingFactor = 7;
uint8_t loraCodingRate = 6;

Why you don't change TxPower?

LoRa.setTxPower(17); //2-17

Thanks!

Some kind of alarm when RSSI goes LOW

CRSF should use 400kbps not 420kbps

serial passtrough (?)

Detect Failsafe state on TX side

Improve TX module indication

onboard led should be blinking if radio is working normally

Can sx1278 433m works

I bought an sx1278 works on 433m, but I don't know whether it can be supported by software.

Some ideas? I hate to waste my money:-(