Basic WebSocket Client Example

Use a websocket client to send sampled ADC and GPIO values to a remote server.

• SDK directory: apps/basic/websocket_client
• Zentri App Store: ZENTRI-BAWEBSOCCLNT

API Features Demonstrated

• WebSocket API
  • zn_websocket_disconnect
  • zn_websocket_connect
  • zn_websocket_register_client_event_handlers
  • zn_websocket_read
  • zn_websocket_write
• Event API
  • zn_event_register_timed
  • zn_event_register_periodic
  • zn_event_unregister
  • zn_event_issue
• Network API
  • zn_network_is_up
  • zn_network_close_all_streams
  • zn_network_register_event_handler
• Peripherals/GPIO API
  • zn_gpio_mask_get
• Periperhals/ADC API
  • zn_adc_sample
  • zn_adc_init
• Settings API
  • zn_load_app_settings

Platforms

This app works on:

• AMW004 - Zentri Mackerel evaluation board
• AMW106 - Zentri Moray evaluation board

Requirements and Prerequisites

This app requires a ZentriOS device with an ADC. For a device with no ADC, you can modify the code to substitute an appropriate data source.

Resources

The initialization file settings.ini is packaged into the .zap file downloaded to the device.

Description

This example app implements a websocket client that sends sampled ADC and GPIO values to the websocket.org echo server. The echoed text is received and displayed on the ZentriOS terminal.

Instructions for Use

Open a ZentriOS serial terminal to the device. See Getting Started, Opening a ZentriOS Terminal.

This app assumes your ZentriOS device is set up with the credentials to join your local network. If you have not already set up network credentials, on the ZentriOS terminal, run the commands:

network_up -s
save

See the Wi-Fi Command API documentation for network_up and save.

When the app runs, it displays output similar to the following:

> Attempting to connect to ws://echo.websocket.org
[Associating to Zentri_Guest]
> Obtaining IPv4 address via DHCP
IPv4 address: 192.168.6.113
[Associated]
> [Opening: echo.websocket.org:80]
> Resolving host: echo.websocket.org
Connecting (WS): 174.129.224.73:80
[2016-02-24 | 07:02:59: Opened: 0]
> Connected. Socket handle: 0
Network up
Echoed data: 000: gpios:35DA, adc: 1656mV
Echoed data: 001: gpios:F5DE, adc: 1656mV
Echoed data: 002: gpios:35DE, adc: 1655mV
Echoed data: 003: gpios:F5DE, adc: 1655mV
Echoed data: 004: gpios:35DE, adc: 1655mV
Echoed data: 005: gpios:F5DE, adc: 1655mV

Implementation

zn_app_init

• zn_network_register_event_handler sets up a handler that brings the network back up in the event of a network disconnection.
• zn_adc_init initializes the ADC
• zn_event_issue(websocket_attempt_connect_handler, NULL, 0) registers the attempt connection event, executes the associated handler, then unregisters it.

zn_app_deinit

• zn_network_close_all_streams cleans when the ZAP is shutting down

websocket_attempt_connect_handler

• Calls zn_websocket_connect to connect to the WebSocket echo server.
• If connecting to the WebSocket server fails, zn_event_register_timed sets up a timed event to try again after ATTEMPT_CONNECT_PERIOD.
• zn_websocket_register_client_event_handlers sets up:
  • websocket_disconnect_handler to run if the WebSocket disconnects
  • websocket_receive_handler to run when data is received on the WebSocket
• zn_event_register_periodic registers websocket_transmit_handler to run immediately, then every TRANSMIT_PERIOD milliseconds.

websocket_disconnect_handler

• Calls zn_event_unregister to turn off periodic transmission
• Calls websocket_receive_handler to read any remaining data
• Calls zn_event_issue to run websocket_attempt_connect_handler and reinstate the connection

websocket_receive_handler

• Calls zn_websocket_read to read the data from the WebSocket echo server.
• logs the echoed data to the ZentriOS terminal console.

websocket_transmit_handler

• Calls zn_gpio_mask_get(0xFFFF) to read all GPIO values
• Calls zn_adc_sample to read the ADC value in millivolts.
• Formats the data as a string
• Calls zn_websocket_write to send the data to the WebSocket echo server.

wlan_network_event_handler

• Calls zn_network_up to brings the network back up if it goes down

Source

See:

• websocket_client.c

See also:

• settings.ini