Lorawan Class C no data received

[HelgeFH]

Update: looks like Class C reception generate a CRC error every time something is received.
Class A reception works without problem.

Another strange thing is the interrupt latency. IRQ goes high but it takes up 30ms or more before SPI CS goes low.

Hi,
I have a board using an ESP32 and a Ra01H (SX1276) module. Radiolib 7.4.0.0 using PlatformIO and Arduino framework.
I have Class A up and running with TTN and Loriot EU868. Works like a charm.
I have now added the lines from the example to change to Class C. And I have connected to the gateway Loriot.
I can see the "C" is received in the Loriot server. The device is set to Class C in the Loriot dashboard.
My problem is that nothing is received when I send a downlink in the Loriot dashboard.
I am also a bit curious, The code for the poll of class C states that no SPI communication is done just a check of a boolean. How does it then know if something is received in the SX1276?:
"// check if a Class C downlink is ready for processing
// tip: internally, this just checks a boolean;
// it does not poll the radio over SPI.
// tip: you are not required to continuously call
// this function; you can do other stuff in between.
// however, a downlink may be overwritten if you
// don't call this function in time for the previous one."

Here is a print of my log:

[LoRaWAN] ActivateOTAA response: -1118
[LoRaWan] Save LoraWan LW Data to FRAM
[LoRaWAN] DevAddr: 0x3190D66C
[LoRaWAN] Initial Datarate: 3
[25.584794] RLB_PRO: [MAC] 0x03
[25.584910] RLB_PRO: 00000000: 3f                                               ?                 
[25.595263] RLB_PRO: LinkAdrReq: dataRate = 3, txSteps = 15, nbTrans = 0
[25.605577] RLB_PRO: LinkAdrAns: 07
[LoRaWAN] ADR mode: 1
[LoRaWAN] Switch to Class C
[LoRaWAN] Sending a confirmed uplink to confirm Class C
[25.618078] RLB_PRO: Uplink (FCntUp = 0) encoded:
[25.618307] RLB_PRO: 00000000: 5b 32 35 2e 35 39 35 32 36 33 5d 20 52 4c 42 5f  [25.595263] RLB_
[25.629822] RLB_PRO: 00000010: 80 6c d6 90 31 80 00 00 01 f0 64 72 52 65        .l..1.....drRe    

[25.645450] RLB_PRO: Frequency = 868.300 MHz, TX = 16 dBm
[25.649522] RLB_PRO: [LoRa] SF = 9, BW = 125.0 kHz, CR = 4/5, IQ: U
[25.821730] RLB_PRO: Uplink sent (ToA = 165 ms)

[25.823958] RLB_PRO: Frequency = 869.525 MHz, TX = 16 dBm
[25.827897] RLB_PRO: [LoRa] SF = 12, BW = 125.0 kHz, CR = 4/5, IQ: D
[25.831870] RLB_PRO: Opened RxC window
[26.785667] RLB_PRO: Closed RxC window

[26.787362] RLB_PRO: Frequency = 868.300 MHz, TX = 16 dBm
[26.791107] RLB_PRO: [LoRa] SF = 9, BW = 125.0 kHz, CR = 4/5, IQ: D
[26.817938] RLB_PRO: Rx1 window open (114 + 10 ms)
[26.932647] RLB_PRO: Rx1 window closing
[26.968594] RLB_PRO: Downlink (AFCntDown = 0) decoded:
[26.969912] RLB_PRO: 00000000: 49 00 00 00 00 01 6c d6 90 31 00 00 00 00 00 08  I.....l..1......
[26.980348] RLB_PRO: 00000010: 60 6c d6 90 31 a0 00 00 c1 37 54 b4              `l..1....7T.      

[26.993723] RLB_PRO: Frequency = 869.525 MHz, TX = 16 dBm
[26.996272] RLB_PRO: [LoRa] SF = 12, BW = 125.0 kHz, CR = 4/5, IQ: D
[26.999801] RLB_PRO: Opened RxC window
[LoRaWAN] Confirmed uplink sent, device should now be in Class C
[LoRaWAN] Lorawan Ready!
[LoRaWAN] Lora connected!
[LoRaWAN] Class C polled 11 times!
[LoRaWAN] Class C polled 11 times!
[LoRaWAN] Class C polled 11 times!

And the code:

if (LoraSettings.lora_version == LORA_VERSION_1_1C || LoraSettings.lora_version == LORA_VERSION_1_04C)
    {
        uint8_t downlinkPayload[MAX_MESSAGE_LORA];
        LoRaWANEvent_t downlinkDetails = {};
        size_t downlinkSize = 0;
        static uint8_t cnt = 0;

        cnt++;
        if (cnt > 10)
        {
            if (LoraSettings.debug_level > 0)
                Log.printf("[LoRaWAN] Class C polled %u times!\n", cnt);
            // short delay before first uplinks
            cnt = 0;
        }

        // check if a Class C downlink is ready for processing
        // tip: internally, this just checks a boolean;
        //      it does not poll the radio over SPI.
        // tip: you are not required to continuously call
        //      this function; you can do other stuff in between.
        //      however, a downlink may be overwritten if you
        //      don't call this function in time for the previous one.
        state = node.getDownlinkClassC(downlinkPayload, &downlinkSize, &downlinkDetails);

        if (state > RADIOLIB_ERR_NONE)
        {
            message_from_lora_t LoraMessage = {};

            if (LoraSettings.debug_level > 0)
                Log.printf("[LoRaWAN] Received a Class C downlink!\n");

            HandleDownlink(downlinkPayload, downlinkSize, &downlinkDetails, &LoraMessage, state);

            // print extra information about the event
            Log.printf("[LoRaWAN] Event information:\n");
            Log.printf("   Datarate:        %u", downlinkDetails.datarate);
            Log.printf("   Frequency:       %.3f MHz", downlinkDetails.freq);
            Log.printf("   Frequency error: %.1f Hz", radio.getFrequencyError());
            Log.printf("   Frame count:     %u", downlinkDetails.fCnt);
            Log.printf("   Port:            %u", downlinkDetails.fPort);
            Log.printf("   Confirming:      %u", downlinkDetails.confirming);
            Log.printf("   RSSI:            %.1f dBm", radio.getRSSI());
            Log.printf("   SNR:             %.1f dB", radio.getSNR());
            Log.printf("   Cast:            ");
            Log.println(downlinkDetails.multicast ? "Multi" : "Uni");
        }
    }

Thanks for any input.
Helge

[StevenCellist]

Hi @HelgeFH, sorry for replying a bit slow.
Your code is quite customized, and it even has some things that have left me wondering how it works, such as the variable cnt which appears to be declared locally but also somewhere 'higher'.
Have you tried the Class C example? It's simplistic, but it gets the job done and helps confirming whether there is something happening in your own code, or something with the setup.

Regarding the boolean check: RadioLib configures an interrupt that fires when the DIO pin triggers (upon reception of a packet). This interrupt sets the boolean which is subsequently checked in the Class C function. This function also checks whether the downlink that was received is actually meant for this device based on DevAddr etc.
As this is just a boolean check, there is no harm in checking it infinitely; actively polling the radio over SPI wouldn't be ideal. Hence the comment :)

[StevenCellist]

Right, I didn't see your edit when replying. Goes to show that it is much more useful to add that as a comment than editing into your post :)

The SX1278 has some CRC check problems, but these are in principle tackled by RadioLib:

RadioLib/src/protocols/LoRaWAN/LoRaWAN.cpp

Lines 1749 to 1753 in 90511e8

	// downlink frames are sent without CRC, which will raise error on SX127x
	// we can ignore that error
	if(state == RADIOLIB_ERR_LORA_HEADER_DAMAGED) {
	state = RADIOLIB_ERR_NONE;
	}

What happens when you remove these lines?

RadioLib/src/protocols/LoRaWAN/LoRaWAN.cpp

Lines 1755 to 1760 in 90511e8

	if(state != RADIOLIB_ERR_NONE) {
	#if !RADIOLIB_STATIC_ONLY
	delete[] downlinkMsg;
	#endif
	return(state);
	}

That is kinda wonk, but it will be useful to see if it resolves your problems.

[HelgeFH]

Thanks for the input.

This gave some more info. My log looks like below.
I write a message for every 50 polls of the "getDownlinkClassC(...)"
I can see that it actually receives something from my Loriot gateway that is like 6 meters away.
First one has a partly correct device address. Second one is from someone else that is way off.

Looks like I am back to the issue "Too Early". It looks like the gateway is sending the info anyway. Can the "Too Early" issue influence the data sent by the gateway?

I run a Dragino LPS8 without the GPS but with working NTP. Semtech Packet Forwarder udp4.2

Device log:
[LoRaWAN] Class C polled 50 times!
[LoRaWAN] Class C polled 50 times!
[97.692717] RLB_PRO: Device address mismatch, expected 0x310AE318, got 0x0E0AE318
[LoRaWAN] Receive Class C failed - RADIOLIB_ERR_DOWNLINK_MALFORMED (-1102)
[LoRaWAN] Event information:
Frequency error: -3886.8 Hz
RSSI: -61.0 dBm
SNR: 11.5 dB
Cast: Uni
[LoRaWAN] Class C polled 50 times!
[LoRaWAN] Class C polled 50 times!
[LoRaWAN] Class C polled 50 times!
[LoRaWAN] Class C polled 50 times!
[145.760736] RLB_PRO: Device address mismatch, expected 0x310AE318, got 0x9064C32A
[LoRaWAN] Receive Class C failed - RADIOLIB_ERR_DOWNLINK_MALFORMED (-1102)
[LoRaWAN] Event information:
Frequency error: -2477.0 Hz
RSSI: -115.5 dBm
SNR: -12.5 dB
Cast: Uni
[LoRaWAN] Class C polled 50 times!
[LoRaWAN] Class C polled 50 times!

//-----------------------

Gateway log:
Wed Dec 10 13:43:17 2025 daemon.info fwd[926]:
Wed Dec 10 13:43:17 2025 daemon.info fwd[926]: [PKTS~][server-DOWN] {"txpk":{"imme":true,"freq":869.525,"rfch":0,"powe":27,"modu":"LORA","datr":"SF12BW125","codr":"4/5","ipol":true,"size":15,"data":"YBjjCjGAAwA4PnfzQUT2"}}
Wed Dec 10 13:43:17 2025 daemon.info fwd[926]: [INFO~][PKTS][server-DOWN] a packet will be sent in "immediate" mode
Wed Dec 10 13:43:17 2025 daemon.info fwd[926]: [DEBUG~][PKTS][server-DOWN] A packet enqueue, us=259416905, cur_us=258416905
Wed Dec 10 13:43:17 2025 daemon.info fwd[926]: [MACINFO~][UNCONF_DOWN]:{"ADDR":"310AE318", "Size":15, "InvertPol":true, "FCtrl":{"ADR":1,"ACK":0, "FPending":0, "FOptsLen":0}, "FCnt":3, "FPort":56, "MIC":"F64441F3"}
Wed Dec 10 13:43:18 2025 daemon.info fwd[926]: INFO: tx_start_delay=1495 (1495.500000) - (1497, bw_delay=1.500000, notch_delay=0.000000)
Wed Dec 10 13:43:18 2025 daemon.info fwd[926]: [INFO~][JIT] send done on rf_chain 0 in count_us=259416905 with freq=869525000, SF64

And Loriot log:

2025-12-10 14:44:36.265 | DATA_DOWN | Sending downlink failed: TOO_EARLY \| "instance":"eu2-lns-core-1","gweui":"A84041FFFF1DA263","fcnt":4,"deveui":"F2F5BDFFFE73C974","appid":BE0108F3,"freq":"868.3000","datr":"SF9BW125","cr":"4/5","len":21,"confirmed":false,"FPending":false,"userid":1826,"organizationId":1790 -- | -- | --

[StevenCellist]

Once again sorry for the slow reply.
Your problem sounds suspiciously similar to #1638. Can you try the fix that is proposed there? I think it will resolve your problems.

[HelgeFH]

Thanks. That looks like it will work. I now receive messages from Loriot in Class C.
I will have to run some tests especially around sending the "C" with confirm.
Looks like it returns with 0 = RADIOLIB_ERR_NONE so Not Confirmed.

Thanks for your help.