SX126x LoRa driver

Currently experimental. Changes can happen in the near future

Short introduction SX126x

The SX126x is a LoRa chip by Semtech. Sometimes also found in chips with a different name but the same or similar sillicon underneath. The older variant is the SX127x, even with its higher number. The SX126x series promises to improve the link budget. They also added extra spreading factors.

Userspace API

This driver is controlled as a character device and using IOCTL commands. At the time of writing this 24 February, 2025, there is no common API yet for LoRa devices and other RF devices under /lpwan. Currently this driver is controlled using older and driver specific IOCTL commands.

Basic RF IOCTL

See nuttx/wireless/ioctl.h : WLIOC_x for setting the basic radio parameters such as frequency and power. Please note that frequency and power may be limited by the lower driver part at board logic.

Driver dedicated IOCTL

See nuttx/wireless/lpwan/sx126x.h : SX126XIOC_x for the commands.

• The packet type is set using SX126XIOC_PACKETTYPESET which takes sx126x_packet_type_e enums. Currently only LoRa packet type is supported.

• LoRa modulation and LoRa packets are configured using SX126XIOC_LORACONFIGSET which takes a pointer to a sx126x_lora_config_s struct. This struct contains two seperate structures for modulation and packet config. It is recommended to follow the SX126x datasheet at https://www.semtech.com/products/wireless-rf/lora-connect/sx1262 that gives more details about each parameter.

Reading / Writing

Reading and writing is done with the read and write functions.

• write directly transmits the given bytes using the IOCTL configured parameters. Does not support polling or timeout yet.

• read takes a pointer to sx126x_read_header_s, which contains information about the received packet. RSSI and SNR aren’t implemented yet. Note that packets are always received, even when the CRC (checked using ``.crc_error``) failed.

Board implementation

The driver can be registered in board logic using sx126x_register, which takes an SPI bus, lower driver and the path. More information about struct sx126x_lower_s parameters can be found in detail under nuttx/wireless/lpwan/sx126x.h : struct sx126x_lower_s.

This driver supports multiple instances using dev_number found in sx126x_lower_s. This is done so multiple channels of LoRa can work at the same time.

Do not assign random numbers in get_pa_values. This can damage the device depending on model. Refer to the manual

Testing

A simple ping-pong test is made to test the device. This code can be used as an example. It is found here https://pastebin.com/71CdKZvm It takes the following parameters:

• -f sets frequency in hz. The range is board depended.

• -b sets the bandwidth in kHz and automatically selects the closest sx126x supported bandwidth.

• -s sets the spreading factor between 5 and 12.

• -p sets the power in dBm. The range is board depended. Also PA setting depended.

• -t perform transmit. Transmit the following bytes.

• -r perform receive. Receives and print.

• -x perform transmit and receive. Transmits following bytes and attempts to receives immidiately. To be used with -e echo

• -e perform echo. Listen and repeat.