I sat down and read the Semtech SX1276 datasheet paying close attention to any references to CRCs and headers. Then to test some ideas I modified my Receive Basic test harness to see if I could reliably reproduce the problem with my stress test harness.
public sealed class StartupTask : IBackgroundTask
{
private const int ChipSelectLine = 25;
private const int ResetLine = 17;
private Rfm9XDevice rfm9XDevice = new Rfm9XDevice(ChipSelectLine, ResetLine);
public void Run(IBackgroundTaskInstance taskInstance)
{
// Put device into LoRa + Sleep mode
rfm9XDevice.RegisterWriteByte(0x01, 0b10000000); // RegOpMode
// Set the frequency to 915MHz
byte[] frequencyWriteBytes = { 0xE4, 0xC0, 0x00 }; // RegFrMsb, RegFrMid, RegFrLsb
rfm9XDevice.RegisterWrite(0x06, frequencyWriteBytes);
rfm9XDevice.RegisterWriteByte(0x0F, 0x0); // RegFifoRxBaseAddress
rfm9XDevice.RegisterWriteByte(0x01, 0b10000101); // RegOpMode set LoRa & RxContinuous
while (true)
{
// Wait until a packet is received, no timeouts in PoC
Debug.WriteLine("Receive-Wait");
byte IrqFlags = rfm9XDevice.RegisterReadByte(0x12); // RegIrqFlags
while ((IrqFlags & 0b01000000) == 0) // wait until RxDone cleared
{
Task.Delay(20).Wait();
IrqFlags = rfm9XDevice.RegisterReadByte(0x12); // RegIrqFlags
Debug.Write(".");
}
Debug.WriteLine("");
if ((IrqFlags & 0b00100000) == 0b00100000)
{
Debug.WriteLine("Payload CRC error");
}
byte regHopChannel = rfm9XDevice.RegisterReadByte(0x1C);
Debug.WriteLine(string.Format("regHopChannel {0}", Convert.ToString((byte)regHopChannel, 2).PadLeft(8, '0')));
byte currentFifoAddress = rfm9XDevice.RegisterReadByte(0x10); // RegFifiRxCurrent
rfm9XDevice.RegisterWriteByte(0x0d, currentFifoAddress); // RegFifoAddrPtr*
byte numberOfBytes = rfm9XDevice.RegisterReadByte(0x13); // RegRxNbBytes
// Allocate buffer for message
byte[] messageBytes = new byte[numberOfBytes];
for (int i = 0; i < numberOfBytes; i++)
{
messageBytes[i] = rfm9XDevice.RegisterReadByte(0x00); // RegFifo
}
rfm9XDevice.RegisterWriteByte(0x12, 0xff); // RegIrqFlags clear all the bits
string messageText = UTF8Encoding.UTF8.GetString(messageBytes);
Debug.WriteLine("Received {0} byte message {1}", messageBytes.Length, messageText);
Debug.WriteLine(string.Format("RegIrqFlags {0}", Convert.ToString((byte)IrqFlags, 2).PadLeft(8, '0')));
Debug.WriteLine("Receive-Done");
}
}
}
The RegHopChannel register has a flag indicating whether there was a CRC extracted from the packet header.
regHopChannel 00000000
Received 23 byte message 1 Hello Arduino LoRa! 1
RegIrqFlags 01010000
Receive-Done
Receive-Wait
…………………………..
regHopChannel 00000000
Received 23 byte message 1 Hello Arduino LoRa! 2
RegIrqFlags 01010000
Receive-Done
Receive-Wait
……………………………
regHopChannel 00000000
Received 23 byte message 1 Hello Arduino LoRa! 3
RegIrqFlags 01010000
Receive-Done
Receive-Wait
I then modified my Arduino-LoRa library based client to include a CRC
void setup() {
Serial.begin(9600); // initialize serial
while (!Serial);
Serial.println("LoRa Duplex - Set sync word");
// override the default CS, reset, and IRQ pins (optional)
LoRa.setPins(csPin, resetPin, irqPin);// set CS, reset, IRQ pin
if (!LoRa.begin(915E6)) { // initialize ratio at 915 MHz
Serial.println("LoRa init failed. Check your connections.");
while (true); // if failed, do nothing
}
LoRa.enableCrc(); // BHL This was my change
LoRa.setSyncWord(0x12); // ranges from 0-0xFF, default 0x34, see API docs
LoRa.dumpRegisters(Serial);
Serial.println("LoRa init succeeded.");
}
void loop() {
if (millis() - lastSendTime > interval) {
String message = "5 Hello Arduino LoRa! "; // send a message
message += msgCount;
sendMessage(message);
Serial.println("Sending " + message);
lastSendTime = millis(); // timestamp the message
//interval = random(2000) + 1000; // 2-3 seconds
interval = 1000;
}
// parse for a packet, and call onReceive with the result:
onReceive(LoRa.parsePacket());
}
void sendMessage(String outgoing) {
LoRa.beginPacket(); // start packet
LoRa.print(outgoing); // add payload
LoRa.endPacket(); // finish packet and send it
msgCount++; // increment message ID
}
void onReceive(int packetSize) {
if (packetSize == 0) return; // if there's no packet, return
// read packet header bytes:
String incoming = "";
while (LoRa.available()) {
incoming += (char)LoRa.read();
}
Serial.println("Message: " + incoming);
Serial.println("RSSI: " + String(LoRa.packetRssi()));
Serial.println("Snr: " + String(LoRa.packetSnr()));
Serial.println();
}
When I powered up a single client and the payload had a CRC
............................... regHopChannel 01000000 Received 23 byte message 6 Hello Arduino LoRa! 6 RegIrqFlags 01010000 Receive-Done Receive-Wait ................................. regHopChannel 01000000 Received 23 byte message 6 Hello Arduino LoRa! 7 RegIrqFlags 01010000 Receive-Done Receive-Wait ................................. regHopChannel 01000000 Received 23 byte message 6 Hello Arduino LoRa! 8 RegIrqFlags 01010000 Receive-Done Receive-Wait ...............................
Then when I increased the number of clients I started getting corrupted messages with CRC errors.
Received 24 byte message 6 Hello Arduino LoRa! 32 RegIrqFlags 01010000 Receive-Done Receive-Wait ............... regHopChannel 01000001 Received 25 byte message 8 Hello Arduino LoRa! 114 RegIrqFlags 01010000 Receive-Done Receive-Wait Payload CRC error regHopChannel 01000000 Received 24 byte message s��=��(��p�^j�\ʏ����� RegIrqFlags 01100000 Receive-Done Receive-Wait ............. regHopChannel 01000000 Received 24 byte message 6 Hello Arduino LoRa! 33 RegIrqFlags 01010000 Receive-Done Receive-Wait ............... regHopChannel 01000001 Received 25 byte message 8 Hello Arduino LoRa! 115 RegIrqFlags 01010000 Receive-Done Receive-Wait
I need to do some more testing but now I think the problem was the RegIrqFlags PayloadCRCError flag was never going to get set because there was no CRC on the payload.
devMobile's blog 