Why are data packets missing in Zigbee arduino setup? - api

Question

I made a setup which consists of 3 Zigbee's, 2 routers(Zigbee S2C's) and 1 coordinator(Zigbee S2). The routers are each connected to arduino nano which collects data from 2 FSR's and an IMU(frame type: zigbee transmit request and packet size 46 bytes) and sends it to the Coordinator attached to an arduino UNO. All the Xbees are in API mode 2 and working at a baud rate of 115200. I am using a library called "Simple Zigbee Library" to send all the collected data to the Coordinator. The collection and sending of data works fine except that there are packets lost in the way. The nano's sample data at a frequency of around 25Hz independently. The coordinator tries to read the data send from the zigbees(using the library of course) in every loop, but unfortunately, it seems to receive only around 40-45 samples.(Should have been 25*2=50 samples total from the 2 xbees). Can anybody suggest why this is happening. I need as less data loss as possible for my setup to achieve its motive. Any kind of help is appreciated.
P.S: It may be important to mention that the coordinator is reading the data only from one xbee in each loop.
As can be seen under the "Source" heading of this image of data received by the coordinator, "19" and "106" are the addresses of the routers and there are data packets dropped intermittently
Thank you.
void setup()
{
// Start the serial ports ...
Serial.begin( 115200 );
while( !Serial ){;} // Wait for serial port (for Leonardo only).
xbeeSerial.begin( 115200 );
// ... and set the serial port for the XBee radio.
xbee.setSerial( xbeeSerial );
// Set a non-zero frame id to receive Status packets.
xbee.setAcknowledgement(true);
}
void loop()
{
// While data is waiting in the XBee serial port ...
while( xbee.available() )
{
// ... read the data.
xbee.read();
// If a complete message is available, display the contents
if( xbee.isComplete() ){
Serial.print("\nIncoming Message: ");
printPacket( xbee.getIncomingPacketObject() );
}
}
delay(10); // Small delay for stability
// That's it! The coordinator is ready to go.
}
// Function for printing the complete contents of a packet //
void printPacket(SimpleZigBeePacket & p)
{
//Serial.print( START, HEX );
//Serial.print(' ');
//Serial.print( p.getLengthMSB(), HEX );
//Serial.print(' ');
//Serial.print( p.getLengthLSB(), HEX );
//Serial.print(' ');
// Frame Type and Frame ID are stored in Frame Data
uint8_t checksum = 0;
for( int i=10; i<p.getFrameLength(); i++){
Serial.print( p.getFrameData(i), HEX );
Serial.print(' ');
checksum += p.getFrameData(i);
}
// Calculate checksum based on summation of frame bytes
checksum = 0xff - checksum;
Serial.print(checksum, HEX );
Serial.println();
}

Answer 1

Although you claim to be using 115,200bps, posted code shows you opening the serial ports at 9600 baud, definitely not fast enough for 2500 bytes/second (50 packets/second * 45 bytes/packet * 110% for overhead) received from XBee and dumped by printPacket()). Remember that 802.15.4 is always 250kbps over the air, and the XBee module's serial port configuration is just for local communications with the host.
Make sure your routers are sending unicast (and not broadcast) packets to keep the radio traffic down.
You should verify that sending is working before troubleshooting code on the coordinator. Update the code on your routers to see if you get a successful Transmit Status packet for every packet sent. Aiming for 50Hz seems like a bit much -- you're trying to send 45 bytes (is that the full size of the API frame?) every 20ms.
Are you using a hardware serial port on the Arduino for both the XBee module and Serial.print()? How much time does each call to printPacket() take? If you reduce the code in printPacket() to a bare minimum (last byte of sender's address and the 1-byte frame ID), do you see all packets come through (an indication that you're spending too much time dumping the packets).

Answer 2

I'm concerned with the code you're using in loop. I don't know the deep internals of how the Arduino works, but does that 10ms delay block other code from processing data? What if you simplify it:
void loop()
{
xbee.read();
// Process any complete frames.
while (xbee.isComplete()){
Serial.print("\nIncoming Message: ");
printPacket( xbee.getIncomingPacketObject() );
}
}
But before going too far, you should isolate the problem would by connecting the coordinator to a terminal emulator on a PC to monitor the frame rate. If all frames arrive then there's an issue on the coordinator. If they don't, work on your router code first.