### Bluetooth: Mesh OnOff Model


#### Overview

This is a simple application demonstrating a Bluetooth mesh multi-element node.
Each element has a mesh onoff client and server
model which controls one of the 4 sets of buttons and LEDs .

Prior to provisioning, an unprovisioned beacon is broadcast that contains
a UUID. Each button controls the state of its
corresponding LED and does not initiate any mesh activity.

The models for button 1 and LED 1 are in the node's root element.
The 3 remaining button/LED pairs are in elements 1 through 3.
Assuming the provisioner assigns 0x100 to the root element,
the secondary elements will appear at 0x101, 0x102 and 0x103.

After provisioning, the button clients must
be configured to publish and the LED servers to subscribe.

If a LED server is provided with a publish address, it will
also publish its status on an onoff state change.

If a button is pressed only once within a 1 second interval, it sends an
"on" message. If it is pressed more than once, it
sends an "off" message. The buttons are quite noisy and are debounced in
the button_pressed() interrupt handler. An interrupt within 250ms of the
previous is discarded. This might seem a little clumsy, but the alternative of
using one button for "on" and another for "off" would reduce the number
of onoff clients from 4 to 2.

#### Requirements
************

This sample has been tested on the Nordic nRF52840-PDK board, but would
likely also run on the nrf52_pca10040 board.

#### Building and Running
********************

Prior to provisioning, each button controls its corresponding LED as one
would expect with an actual switch.

Provisioning is done using the BlueZ meshctl utility. Below is an example that
binds button 2 and LED 1 to application key 1. It then configures button 2
to publish to group 0xc000 and LED 1 to subscribe to that group.

```
discover-unprovisioned on
provision <discovered UUID>
menu config
target 0100
appkey-add 1
bind 0 1 1000                # bind appkey 1 to LED server on element 0 (unicast 0100)
sub-add 0100 c000 1000       # add subscription to group address c000 to the LED server
bind 1 1 1001                # bind appkey 1 to button 2 on element 1 (unicast 0101)
pub-set 0101 c000 1 0 0 1001 # publish button 2 to group address c000
```

The meshctl utility maintains a persistent JSON database containing
the mesh configuration. As additional nodes (boards) are provisioned, it
assigns sequential unicast addresses based on the number of elements
supported by the node. This example supports 4 elements per node.

The first or root element of the node contains models for configuration,
health, and onoff. The secondary elements only
have models for onoff. The meshctl target for configuration must be the
root element's unicast address as it is the only one that has a
configuration server model.

If meshctl is gracefully exited, it can be restarted and reconnected to
network 0x0.

The meshctl utility also supports a onoff model client that can be used to
change the state of any LED that is bound to application key 0x1.
This is done by setting the target to the unicast address of the element
that has that LED's model and issuing the onoff command.
Group addresses are not supported.