MBED OS 5.9 LoRA set up in SF7

问题

Do you know how to set up the Spreading Factor to 12 in a Mbed-OS LoRaWAN protocol APIs to connect to a LoRaWAN network using OTAA?

I'm trying to make LoRA node to use Spreading Factor SF12, because the default one is SF7. I know that in the PHY layer we can change Radio configurations. There are several examples to change between the different sub-GHz frequency bands, however, I can't find one on how to change the LoRa modulation SF between 7 and 12 and with a bandwidth of 125 kHz. I'm using an SX1276 radio at EU 868 MHz config.

In the source code you can find the SF7-12 different configurations, but there is not a clear way to set it up. These configs are the definitions (#define) DR_0, DR_, etc ).

In the configuration file in the Phy part you find some example like this:

        "phy": {
        "help": "LoRa PHY region. 0 = EU868 (default), 1 = AS923,
         2 = AU915, 3 = CN470, 4 = CN779, 5 = EU433, 
         6 = IN865, 7 = KR920, 8 = US915, 9 = US915_HYBRID",
        "value": "0"
    },

But there is no examples or description for the Spreading Factor.

I would like to change it via source code, rather than the configuration file.

EDIT 1:

after Jon's answer, I add the following lines, but still not forcing the SF12 Joins.

retcode = lorawan.disable_adaptive_datarate ();
retcode = lorawan.set_datarate (0); //  DR_0

回答1:

Call:

lorawan.set_datarate(0); // SF12 125 KHz

Make sure to:

1. Disable ADR.
2. Either use ABP, or call the function above in the JOIN_SUCCESS event handler. This is because join procedure always starts at SF7, and then keeps the data rate on which the join succeeded.

来源：https://stackoverflow.com/questions/54882984/mbed-os-5-9-lora-set-up-in-sf7