Does Bluetooth Low Energy need advertising channels for reconnecting?
Under standard BLE protocol, if the 3 advertising channels are jammed (by someone other than you, since you would be violating FCC regulations if you were the culprit), it is not possible for the devices to establish, or re-establish, a connection. Your idea of hunting for open non-advertising channels could be implemented by gaining low-level access to the radios, but it wouldn't meet the BLE standard.
With BLE, the peripheral device advertises in quick bursts, sequentially on the 3 reserved advertising channels, and listens briefly after each advertisement for a request from a central device on that same channel. That might be a request for more information, or a request to connect.
Once connected, the devices negotiate a &quot;connection interval&quot; that determines when, and on which channel, they plan to exchange data again, using one of the 37 non-advertising channels. When it's time for their &quot;date&quot; the central will transmit to the peripheral, and the peripheral can respond.
Because the device clocks aren't perfect, the devices have to start listening early, and keep listening later, than the exact planned time. That's why the BLE protocol specifies a maximum connection interval. If it's too long, the devices might miss each other because of drifts in their clocks.
The peripheral, if it has no new information, can skip the planned connection a few times, determined by the &quot;maximum slave latency&quot;. This can save energy for the peripheral. But again, you're not going to get to a once-a-day schedule using this technique.
It isn't impossible to do what you want, you will just have to devise your own protocol layered on top of BLE. Use low-power, crystal-controlled clocks that are as precise as you can reasonably get. Then wake both devices once a day; the peripheral should advertise and the central should scan. The duration of this will be determined by how accurate your device clocks are. Once the connection is made, the devices go back to sleep with their clocks running. You'll also have to decide when to give up if the connection is missed, and what to do about it (cache data for later, etc).