Building an HTTP Python Server with Mbed Client¶

This tutorial will guide you through creating an HTTP Python Server with Embedded Client using the Mbed Connect Cloud board. We first will cover creating the embedded client for the board.

Prerequisites¶

• An Mbed account - sign up here
• Install the Mbed serial driver (for Windows PCs)
• Python 2.7 - download here

Required Hardware¶

• Mbed Connect Cloud board
• An active WiFi network

Setup¶

1. Connect the board to your computer via the micro USB port.
2. The board mounts as a mass-storage device (like a USB drive). Verify that you can see it (the drive name will be DAPLINK).
3. Add the u-blox EVK-ODIN-W2 board to your online Mbed Compiler here

Making the Mbed application¶

Import the example program.

Printing to the screen¶

Open main.cpp

Instantiate a C12832 variable to write to the LCD screen on the board. Under 'GLOBAL VARIABLES HERE' add the following code:

C12832  lcd(PE_14, PE_12, PD_12, PD_11, PE_9);

Create a function to print to the LCD screen. Under 'FUNCTION DEFINITIONS HERE' add the following code:

void lcd_print(const char* message) {
    lcd.cls();
    lcd.locate(0, 3);
    lcd.printf(message);
}

Setting up WiFi¶

We will add some code to initialize the Mbed Connect Cloud board's WiFi module and connect to your local WPA/WPA2 WiFi network.

Open mbed_app.json

We need to tell Mbed the name of our WiFi network's SSID and password.

Under wifi-ssid change the "value" from SSID to your WiFi's name (for example: "\"iotlab\"") and do not remove the quotes.

Under wifi-password change the "value" from PASSWORD to your WiFi's password (for example: "\"abcd1234\"") and do not remove the quotes.

Open main.cpp

The Mbed Connect Cloud board uses the u-blox EVK-ODIN-W2 WiFi module. So to initialize our WiFi connection we need to instantiate a OdinWifiInterface variable. Under 'GLOBAL VARIABLES HERE' add the following code:

OdinWiFiInterface wifi;

Add some code to tell the WiFi module to connect to your wifi network. We will also print to the LCD the status of the WiFi connection. Under 'MAIN CODE HERE' add the following code:

lcd_print("Connecting...");
int ret = wifi.connect(MBED_CONF_APP_WIFI_SSID, MBED_CONF_APP_WIFI_PASSWORD, NSAPI_SECURITY_WPA_WPA2);
if (ret != 0) {
    lcd_print("Connection error.");
    return -1;
}
lcd_print("Successfully connected!");

The MBED_CONF_APP_WIFI_SSID and MBED_CONF_APP_WIFI_PASSWORD are variables defined by the mbed_app.json file we edited previously. NSAPI_SECURITY_WPA_WPA2 tells Mbed OS that our WiFi network has WPA or WPA2 security (so it requires an SSID and password).

Adding button interrupts¶

We will add some code to instantiate two InterruptIn variables so the SW1 and SW2 buttons on the board can interrupt the main program. We will also create two bool variables that change state (true/false) whenever the buttons are clicked.

Open main.cpp

Under 'GLOBAL VARIABLES HERE' add the following code:

InterruptIn post_button(PF_2);
InterruptIn get_put_button(PG_4);
volatile bool post_clicked = false;
volatile bool get_clicked = false;
volatile bool put_clicked = false;

Add two functions that will be called when either button interrupts the main function. When the post_button (SW1) is clicked, the post_clicked variable will be set to true. When the get_put_button (SW2) is clicked, the get_clicked variable will be set to true. Under 'FUNCTION DEFINITIONS HERE' add the following code:

void send_post() {
    post_clicked = true;
}

void send_get_put() {
    get_clicked = true;
}

Attach the send_post and send_get_put functions to the rising edge of their associated InterruptIn buttons. We will also add a while loop so the program will run indefinitely. Under 'MAIN CODE HERE' add the following code:

post_button.rise(&send_post);
get_put_button.rise(&send_get_put);
while (true) {
    // WHILE LOOP CODE HERE

}

Creating the HTTP Python Server¶

For this example, in order to send HTTP POST, GET, or PUT requests from our Mbed client code (main.cpp) we need to set up a local Python server to handle these requests. (If you are interested in using an already established database/API for your HTTP requests, check out the IFTTT tutorial here.)

Open server/server.py

This file contains a skeleton Python BaseHTTPServer. If we were to run this Python script through our command-line with python server.py, we would have a running local HTTP server but it would not be able to process HTTP POST, GET, or PUT requests. So, we need to add Python definitions in our MyHandler class to handle these requests. You can read more about Python's BaseHTTPServer here.

We will first create a definition for the HTTP POST request in the MyHandler class. Under 'HTTP REQUESTS HERE' add the following code:

    def do_POST(self):
        content = b"POST: Hello, Mbed!"
        self.send_response(200)
        self.send_header('Content-type', 'text/plain')
        self.send_header('Content-Length', len(content))
        self.end_headers()
        self.wfile.write(content)
        return

The definition do_POST allows the local server to process incoming POST requests, and send the response "POST: Hello, Mbed!". This response is always sent, regardless of the data in the incoming POST request.

Create similar definitions for the HTTP PUT and GET requests in the MyHandler class. Under 'HTTP REQUESTS HERE' add the following code:

    def do_GET(self):
        content = b"GET: Hello, Mbed!"
        self.send_response(200)
        self.send_header('Content-type', 'text/plain')
        self.send_header('Content-Length', len(content))
        self.end_headers()
        self.wfile.write(content)
        return

    def do_PUT(self):
        content = b"PUT: Hello, Mbed!"
        self.send_response(200)
        self.send_header('Content-type', 'text/plain')
        self.send_header('Content-Length', len(content))
        self.end_headers()
        self.wfile.write(content)
        return

Similarly to do_POST, the do_GET and do_PUT definitions allow the local server to process incoming GET and PUT requests, and send the responses "GET: Hello, Mbed!" or "PUT: Hello, Mbed!" respectively. These responses are always sent, regardless of the data in the incoming GET or PUT requests.

Sending HTTP requests¶

Earlier we set up our button interrupts for the Mbed client in main.cpp. Now we need to write code that sends HTTP POST, GET, or PUT requests to our local HTTP Python Server from the main function.

Compile and run¶

1. Select the u-blox EVK-ODIN-W2 target board in the upper right hand corner
2. Press "Compile"
3. Wait for a binary to be downloaded
4. Drag the binary to the DAPLINK disk
5. Press the board's reset button
6. The WiFi connection status should now be displaying on the board, once you see "Successfully connected!" you can press the button and see the number of button clicks on the LCD screen.