Receiving Meshtastic Packets

[AkaBkn]

I'm using SX1262 in LilyGO T-Deck Plus.

For a test of reception, I try to receive packets from meshtastic.

so i set EU_868 LongFast params like that

#define SX126X_LORA_SF11 0x0b
uint8_t spreadingFactor = SX126X_LORA_SF11;
uint8_t bandwidth = SX126X_LORA_BW_250_0;
uint8_t codingRate = SX126X_LORA_CR_4_5;
uint16_t preambleLength = 16;
uint8_t payloadLen = 0;
int16_t syncword = 0x2b;
frequencyInHz = 869525000;

after LoRaConfig()

SetSyncWord(syncword);

there is no error in the serial log, however, i never receive any packets. if I trace the irqRegs in LoRaReceive i see that i get SX126X_IRQ_PREAMBLE_DETECTED but never SX126X_IRQ_RX_DONE.

have you an idea what i do wrong? do i need to configure DIO1 and handle interrupts like that?

[nopnop2002]

There is no information about LilyGO T-Deck Plus.

You must review this product's schematic to properly configure these options.

• Using TCXO(Temperature-Compensated Crystal Oscillator)
  SX1262/1268 can use the TCXO.
  If the TCXO is used, the XTB pin is not connected.
  However, the 6th pin (DIO3) of the SX1262/1268 can be used to power the TCXO.
  Explanation for TXCO and antenna control is here.
  Ra-01S / Ra-01SH does not use TCXO.

• Power supply modes
  SX1262/1268 has two power supply modes.
  One is that only LDO used in all modes.
  Another is that DC_DC+LDO used for STBY_XOSC,FS, RX and TX modes.
  Explanation for LDO and DCDC selection is here.
  Ra-01S / Ra-01SH use only LDO in all modes.

• RF-Switching
  In general, use DIO2 to switch the RF-Switch.
  However, some tranceiver use an external gpio to switch the RF-Switch.
  Ra-01S / Ra-01SH use the SC70-6 integrated load switch to switch between RFO and RFI.
  Ra-01S / Ra-01SH use DIO2 to control this.
  DIO2 = 1, CTRL = 0, RFC for SC70-6 goes to RF1 --> Switch to Tx Mode.
  DIO2 = 0, CTRL = 1, RFC for SC70-6 gose to RF2 --> Switch to Rx Mode.

---

Is this correct?

uint8_t payloadLen = 0;

[AkaBkn]

Meshtastic (https://meshtastic.org/) is a LoRa mesh networking system.

TCXO / LDO: pretty sure i have those correct, also played with all the combinations.

RF switching: on the schematics, (https://github.com/Xinyuan-LilyGO/T-Deck/blob/master/schematic/schematic.pdf) i only see DIO1 connected.

payloadLen = 0; meens no fixed lenght if I'm not mistaken. Should be correct.

Why would it receive the preamble but not the packet?

[nopnop2002]

The LoRa modem employs two types of packet formats: explicit and implicit.

Do the packet formats of both match?

[AkaBkn]

Yes, payloadLen = 0 should mean explicit, which should match.

[nopnop2002]

Can transmission and reception work properly using two LilyGO T-Deck Plus?

[AkaBkn]

Unfortunatly I have only one. I send meshtastic messages from a LilyGO T-Echo, it has the same SX1262 LoRa chip in there.

[nopnop2002]

It seems that the LoRa packet formats do not match.

Are the packet header lengths the same on both sides?

[AkaBkn]

some other guy also has the same parameters (https://www.reddit.com/r/meshtastic/comments/1ec1us8/meshtastic_packets_not_received_by_a_standard/)

how would you change the packet header length?
is SX126X_REG_PACKECT_LENGTH also important?

[AkaBkn]

got it! it was the wrong sync word.
meshtastic is using radiolib, which sets the syncword like that:

in src/modules/SX126x/SX126x.h

   786      \param controlBits Undocumented control bits, required for compatibility purposes.
...
   789    int16_t setSyncWord(uint8_t syncWord, uint8_t controlBits = 0x44);

in src/modules/SX126x/SX126x.cpp

   815    const uint8_t data[2] = {(uint8_t)((syncWord & 0xF0) | ((controlBits & 0xF0) >> 4)), (uint8_t)(((syncWord & 0x0F) << 4) | (controlBits & 0x0F))};
   816    return(writeRegister(RADIOLIB_SX126X_REG_LORA_SYNC_WORD_MSB, data, 2));

so when using 0x2b as sync word, actually 0x24b4 gets written to the register.

maybe you want to also change SetSyncWord or add a note ?