Table of Contents

Overview

Serial Wi-Fi is a capability designed to provide easy, self-contained terminal access behavior over UART interface. This provides a “driver-less” solution and allows out-of-the box operation based on ASCII character set interpretation. This application runs over the UART interface and behaves as a command line interpreter. Leveraging the complete network stack integration, it allows secure, robust end-to-end communication.

Application details

The serial Wi-Fi application supports the following features:

1. Support to connect to an AP
2. Auto discovery of available peer using mDNS
   
3. All communication uses secure sockets
   
4. Once configured to a network, auto connect to the AP on every startup
5. Provision to override default mnemonic strings
6. Local/Remote Control modes, to issue commands to the locally/remotely connected Simplelink devices

The Serial Wi-Fi application operates in three modes:

1. Terminal/Interpreter mode: In this case the interpreter mode behaves as pure point-to-point cable replacement. This is the most common and obvious use in which a serial cable formerly used to carry information is replaced by SimpleLink devices on both end of the line.
   
2. Local control mode: In this mode it is possible to issue commands to the locally connected device. These commands encompass a close set of options to control the SimpleLink device while guaranteeing sandbox restrictions. The mode is entered using a predefined escape sequence //<
3. Remote control mode: This allows issuing commands to the remotely connected device. These commands encompass the same set of options to control the remote SimpleLink as the local device, while guaranteeing sandbox restrictions. The mode is entered using a predefined escape sequence //>

Warning: Current time must be set in the device. This time is used to validate the certificate. If the date is beyond the validity period of Certificate, sl_connect will return error. Update the below macros in serial_wifi.h.

    #define DATE                01    /* Current Date */
    #define MONTH               1     /* Month 1-12 */
    #define YEAR                2019  /* Current year */
    #define HOUR                12    /* Time - hours */
    #define MINUTE              00    /* Time - minutes */
    #define SECOND              00     /* Time - seconds */

Source Files briefly explained

• main.c - Creates to two tasks. Task 1: the interpreter task to operate the serial wifi functionality. Task 2: The UART task to receive data from the UART.
  
• serial_wifi.c - ALl the functionality of the serial wifi is implemented.
  
• uart_config.c - Configures the UART and the task to send and receive chars over the UART.
• conversions.c - All conversion utility API into different number formats.

Usage

1. Modify the date macros in serial_wifi.h as shown above. Rebuild the application and flash the binary to two LaunchPads using UniFlash.
2. Upon Reset, the device connects to the stored AP by the AUTO connect policy.If the device doesn’t connect to an AP in 6secs, the application prompts the user to connect to a known AP using the Local control mode. User can use the below command to connect to an AP in the local control mode:
   wlan_connect [ssid] [type] [Sec] [User] [key]
3. If this is the first boot after an Serial FLASH erase, one of the devices need to be configured for as mDNS server. This can be done by using the below command in local control mode.
   mDNS 0 <0-server, 1-client>
4. The client device automatically connects to the advertiser.

5. The communication is based on secure sockets. So, the server key, server certificate, client certificate need to be flashed in the SFLASH at file IDs 129,130,131 respectively. Please refer the Generating SSL certificates section for generating certificates using OpenSSL.
   For flashing the certificates using UniFlash, refer to the Uniflash documentation.

6. Click Add File
7. Rename the newly created file to “/cert/129.der”
   
8. In the Url field mention the local path to the Server key certificate.
   
9. Repeat the above steps for “/cert/130.der” and “/cert/131.der” for server key and client certificate respectively.
   
10. Check the Erase and Update check boxes and Program
11. Now the devices are connected in Interpreter mode.
    
12. \\< command is used to enter the local control mode, mode in which we control the local CC3200 device.
    

13. \\> command is used to enter the remote control mode, mode to control the remote CC3200 device.

Commands supported in the control modes are:

All communication is secure using secure sockets. Security method SSLV3 and Cipher RSA WITH RC4 128 SHA

Generating certificates using OpenSSL

The following are the steps for generating certificates using OpenSSL tool.

1. Generate CA file “ca.pem” and CA key “privkey.pem”
   • openssl req -out ca.pem -new -x509
2. Generate server certificate/key pair
   • openssl genrsa -out server.key 1024
   • openssl req -key server.key -new -out server.req
   • openssl x509 -req -in server.req -CA CA.pem -CAkey privkey.pem -CAserial file.srl -out server.pem
     • Contents of “file.srl” is a two digit number. eg. “00”
3. Convert the certificates and key generated to DER format.
   • For certificates: openssl x509 -in <input.crt> -inform PEM –out <output.crt> -outform DER
   • For key: rsa -in input.key -inform PEM -out output.key -outform DER
4. Flash the server key and the server certificate at /cert/129.der and /cert/130.der respectively.
   
5. Flash the CA certificate at /cert/131.der

Limitations/Known Issues