LoRa module, SX1278 (Ra-02)

The Ai-Thinker Ra-02: a Semtech SX1278 LoRa radio, 433 MHz, on SPI, with a spring antenna. LoRa.h (Sandeep Mistry's library) or RadioLib.h drives it.

Aliases: LoRa, Ra-02, SX1278, SX1276, RFM95W, RFM96W, LoRa radio. The SX1276 (the 868 and 915 MHz RFM95W) is the same chip with the same registers, so this part is that one too.

Every LoRa module on the canvas shares the air

Put two modules down, wire each to its own board, and they hear each other, with no wire between them. As for the nRF24L01, the air is the extra pin ANT: Mokxi joins the ANT of every LoRa module on the canvas, and that line is the room. Two modules that send at the same time spoil each other's packets.

A LoRa module and an nRF24L01 do not hear each other: they have airs of their own.

Pins

Pin	What it does
GND	Ground.
3.3V	Supply, 1.8 to 3.7 V. Use the board's 3.3 V pin.
RST	Reset, active low.
DIO0	Packet received, packet sent or channel scan done, as the sketch maps it.
DIO1	Receive timeout. DIO2 to DIO5 stay low.
NSS	SPI chip select.
SCK, MOSI, MISO	SPI.
ANT	The air every LoRa module on the canvas shares. Leave it unwired.

LoRa.h's default Uno wiring is NSS to 10, RST to 9 and DIO0 to 2, with SCK, MOSI and MISO on 13, 11 and 12. LoRa.setPins(ss, reset, dio0) changes it.

What is modeled

The chip is a register file behind SPI, as Semtech's datasheet has it, and RegVersion reads 0x12, so LoRa.begin() and RadioLib's begin() find it. In LoRa mode the modes are the datasheet's: sleep, standby, transmit, receive continuous, receive single (which gives up after SymbTimeout symbols) and channel activity detection, with the FIFO, its base addresses and pointer, the IRQ flags and mask, and DIO0 and DIO1.

Packets take as long as real ones. The bit rate is LoRa's, set by the spreading factor and the bandwidth, and the preamble goes first: a 20 byte packet takes about 60 ms at SF7 and 125 kHz, and well over a second at SF12. A sketch that sends faster than that finds the radio still busy.

A receiver takes a packet only when its frequency, sync word, spreading factor and bandwidth match the sender's, as a real one does. The CRC goes with the packet when it is on, and a spoiled packet raises PayloadCrcError.

What is not modeled

The radio itself: distance, walls, fading and output power. packetRssi() and getRSSI() always report -45 dBm and packetSnr() 9.5 dB, a strong close signal, because there is no distance in the simulator. The coding rate is stored but does not change the time on air, and modules with different coding rates still hear each other. Invert IQ and implicit header mode are stored and not modeled. The FSK modem is not modeled: in FSK mode nothing is sent or heard. There is no LoRaWAN gateway or network server, so LMIC and The Things Network sketches have nobody to join.

Common mistakes

5 V on 3.3V. The SX1278 is a 3.3 V chip.

Different settings on the two sides. LoRa.begin(433E6) on one board and LoRa.begin(915E6) on the other, or a setSyncWord() on only one, and nothing arrives.

Sending in a tight loop. At SF12 one packet is over a second on the air.