Update lorawan-otaa.md (#285)

* Update lorawan-otaa.md

included the explanation at lorawan otaa about dev_eui

content/tutorials/lora/lorawan-otaa.md

@@ -6,12 +6,22 @@ aliases:
     - chapter/tutorials/lora/lorawan-otaa
 ---
 
-OTAA stands for Over The Air Authentication. With this method the LoPy sends a Join request to the LoRaWAN Gateway using the `APPEUI` and `APPKEY` provided. If the keys are correct the Gateway will reply to the LoPy with a join accept message and from that point on the LoPy is able to send and receive packets to/from the Gateway. If the keys are incorrect no response will be received and the `has_joined()` method will always return `False`.
+OTAA stands for Over The Air Authentication. With this method the LoPy sends a Join request to the LoRaWAN Gateway using the `app_eui` and `app_key` provided in your LoRaWAN application (Like TheThingsNetwork, Chirpstack etc.). If the keys are correct the Gateway will reply to the LoPy with a join accept message and from that point on the LoPy is able to send and receive packets to/from the Gateway. If the keys are incorrect no response will be received and the `has_joined()` method will always return `False`.
 
 The example below attempts to get any data received after sending the frame. Keep in mind that the Gateway might not be sending any data back, therefore we make the socket non-blocking before attempting to receive, in order to prevent getting stuck waiting for a packet that will never arrive.
 
-```python
+If everything works correctly, the device will print `Joined` to the terminal, and you should see a data packet arrive in your LoRaWAN application containing `0x01 0x02 0x03`
 
+
+Note: if the `dev_eui` it is not provided, the LoRa MAC of the device will be used in its place. You will need to change the `dev_eui` in your LoRaWAN application to the LoRa MAC address of the device. You can get the LoRa MAC using:
+```python
+from network import LoRa
+import binascii
+print(binascii.hexlify(LoRa().mac()).upper())
+```
+
+
+```python
 from network import LoRa
 import socket
 import time
@@ -25,18 +35,23 @@ import ubinascii
 # United States = LoRa.US915
 lora = LoRa(mode=LoRa.LORAWAN, region=LoRa.EU868)
 
-# create an OTAA authentication parameters
+# create an OTAA authentication parameters, change them to the provided credentials
 app_eui = ubinascii.unhexlify('ADA4DAE3AC12676B')
 app_key = ubinascii.unhexlify('11B0282A189B75B0B4D2D8C7FA38548B')
+#uncomment to use LoRaWAN application provided dev_eui
+#dev_eui = ubinascii.unhexlify('70B3D549938EA1EE')
 
 # join a network using OTAA (Over the Air Activation)
+#uncomment below to use LoRaWAN application provided dev_eui
 lora.join(activation=LoRa.OTAA, auth=(app_eui, app_key), timeout=0)
+#lora.join(activation=LoRa.OTAA, auth=(dev_eui, app_eui, app_key), timeout=0)
 
 # wait until the module has joined the network
 while not lora.has_joined():
     time.sleep(2.5)
     print('Not yet joined...')
 
+print('Joined')
 # create a LoRa socket
 s = socket.socket(socket.AF_LORA, socket.SOCK_RAW)
 
@@ -58,4 +73,3 @@ s.setblocking(False)
 data = s.recv(64)
 print(data)
 ```
-