Why does it take so long for my VSP to establish a connection with a mobile device?

Answer

When making a connection to a mobile device using Lyra VSP service there is a 15 second delay between the "Ring" indication and the "Connect" indication. This issue is not actually a firmware issue and more of a use case issue. In a scenario whereby two LYRAs are connecting the central LYRA will kickstart the process and once a BT connection is established a series of commands will be sent to the peripheral LYRA to ensure the VSP is ready. These things include writing the Modem in & out descriptors to ensure that the devices are in a good state to send data (set to '1') and also to enable notifications on the Modem out and TX characteristics so that the central can view the incoming data. 

In the case where a different non-LYRA device is acting as a central for a VSP connection these extra commands are not automatically sent therefore it is the central devices responsibility to send these commands. 

Writing to the Modem In characteristic is the write that specifically breaks the peripheral LYRA out of the delay state and to output the connect message. This does not necessarily mean that the VSP system is ready the other commands must also be sent for the system to work properly.

If the 15 second delay is not tolerable for an application, then simply writing any value to the MODEM_IN Characteristic will allow the connection handshake to proceed. For example, using nRF Connect to connect to the Lyra DVK running VSP and connected to a UwTerminalX terminal, a simple write of 0xFF results will then show connect.