The nitty-gritty of how the Arduino WiFi shield goes wrong any time a network connection is dropped

1. The Arduino WiFi
Shield
Katrina Ellison Geltman
March 27, 2014

2. Arduino UNO R3 Arduino WiFi Shield

3. Arduino UNO R3 Arduino WiFi Shield

4. +
vs.

5. +
vs.

6. How it Works

7. Getting set up

8. In the beginning, you have:
WiFi ShieldComputer WiFi Router
Arduino

9. You set up the basics.
WiFi ShieldComputer WiFi Router
Arduino
WiFi
driver
Server
object

10. You request to connect to the network.
WiFi ShieldComputer WiFi Router
Arduino
WiFi
driver
Request to connect
Server
object

11. If you’re lucky, you connect successfully.
WiFi ShieldComputer WiFi Router
Arduino
WiFi
driverConnection successful
Request to connect
Server
object

12. You start your server.
WiFi ShieldComputer WiFi Router
Arduino
Server
object
Start TCP server
listening on
socket/port

13. Now it’s listening on a particular port.
WiFi ShieldComputer WiFi Router
Arduino
Server
object

14. You’re ready to go!

15. Receiving requests

16. After setup, everything happens in an event loop.
WiFi ShieldComputer WiFi Router
Arduino
Server
object
Your
code
loop

17. At the beginning of the loop, a client object is created
using the server’s port & socket.
WiFi ShieldComputer WiFi Router
Arduino
Server
object
Your
code
loop
Client
object

18. The client asks the WiFi shield whether it’s active or not.
WiFi ShieldComputer WiFi Router
Arduino
Server
object
Your
code
loop
Client
object
What's my state?

19. If there haven’t been any requests, the client stays inactive.
WiFi ShieldComputer WiFi Router
Arduino
Server
object
Your
code
loop
Client
object
Inactive
The event loop ends and re-starts.

20. If there haven’t been any requests, the client stays inactive.
!
!
The event loop ends and re-starts.
!
I think the client is deleted, but I’m not sure how.
WiFi ShieldComputer WiFi Router
Arduino
Server
object
Your
code
loop

21. If a request has been made, the client activates.
WiFi ShieldComputer WiFi Router
Arduino
Server
object
Your
code
loop
Client
object
Client
object
Connected!
Request!

22. Your code asks the client object for whatever data it’s receiving
from your computer.
WiFi ShieldComputer WiFi Router
Arduino
Server
object
Your
code
loop
Client
object
Client
object
Connected!
Request!
Data?

23. If there is data, it reads it and processes it.
WiFi ShieldComputer WiFi Router
Arduino
Server
object
Your
code
loop
Client
object
Client
object
Read data
Request!
Read data

24. (This is when your Arduino actually does something)
WiFi ShieldComputer WiFi Router
Arduino
Server
object
Your
code
loop
Client
object
Client
object
Read data
Request!
Read data

25. Then the event loop ends and the client is deleted.
WiFi ShieldComputer WiFi Router
Arduino
Server
object
Your
code
loop

26. !
Then the event loop ends and the client is deleted.
(I think)
WiFi ShieldComputer WiFi Router
Arduino
Server
object
Your
code
loop
Client
object
Inactive

27. That’s all good

28. How it goes wrong

29. Bad things happen if you disconnect from the network.
WiFi ShieldComputer WiFi Router
Arduino
Server
object
Your
code
loop

30. The server doesn’t reconnect properly when the network reconnects.
WiFi ShieldComputer WiFi Router
Arduino
Server
object
Your
code
loop

31. We know this from our debugging output.
Socket Port
Server
Status
Client
Status
-1 0 0 0

32. We know this from our debugging output.
Socket Port
Server
Status
Client
Status
-1 0 0 0
0 80 0 0

33. We know this from our debugging output.
Socket Port
Server
Status
Client
Status
-1 0 0 0
0 80 0 0
Still active!

34. We know this from our debugging output.
Socket Port
Server
Status
Client
Status
-1 0 0 0
0 80 0 0
No server!

35. How to fix it

36. Manually release
port & socket

37. void WiFiServer::disconnect() {
WiFiClass::_state[_sock] = -1;
WiFiClass::_server_port[_sock] = 0;
}

38. void WiFiServer::disconnect() {
WiFiClass::_state[_sock] = -1;
WiFiClass::_server_port[_sock] = 0;
}

39. void WiFiServer::disconnect() {
WiFiClass::_state[_sock] = -1;
WiFiClass::_server_port[_sock] = 0;
}

40. void WiFiServer::disconnect() {
WiFiClass::_state[_sock] = -1;
WiFiClass::_server_port[_sock] = 0;
}

41. Socket Port
Server
Status
Client
Status
-1 0 0 0

42. Socket Port
Server
Status
Client
Status
-1 0 0 0
0 80 1 0

43. It works!

44. It works!
… trivially

45. It doesn’t work
if a client object has ever
been created

46. It doesn’t work
if a client object has ever
been created
This makes it useless!

47. It doesn’t work
if a client object has ever
been created
usually
This makes it useless!

48. Memory
management?

49. But available memory
does not change from
loop to loop.

50. I don’t really
know C++

51. Or network
programming

52. So I would love to
hear your ideas

53. But whether or not you
can help

54. Your main takeaway
should be

55. Arduino UNO R3 Arduino WiFi Shield