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Bluetooth HCI Transport Layer Functions
The following table shows the Bluetooth HCI transport layer functions
HCI_CloseConnection
This function closes an existing connection between the HCI layer in the stack and the Bluetooth hardware.
HCI_OpenConnection
This function opens a connection between the HCI layer in the stack and the Bluetooth hardware.
HCI_ReadHciParameters
This function obtains the hardware characteristics of the Bluetooth controller.
HCI_ReadPacket
This function reads the packet from the connected Bluetooth hardware.
HCI_SetCallback
This function obtains the HCI parameters.
HCI_StartHardware
This function initiates the Bluetooth hardware.
HCI_StopHardware
This function releases the Bluetooth hardware.
HCI_WritePacket
This function writes the packet to the connected Bluetooth hardware.
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This document describes the implementation of the standard Bluetooth™ HCI UART Transport Layer protocol on
a CSR Bluetooth (BlueCore™) chip. The HCI UART Transport Layer is a very simple protocol for carrying Host
Controller Interface (HCI) commands, events and data between a host and a Bluetooth module:
The Bluetooth specification [BT] defines three ways of making a physical connection between the host and a
Bluetooth module:
HCI USB Transport Layer [BT] section H2
HCI RS232 Transport Layer [BT] section H3
HCI UART Transport Layer [BT] section H4
Table 2.1: Options for Connecting Between Host and Bluetooth Module
CSR's BlueCore01b and BlueCore2-External (BlueCore) Bluetooth chips support the first and third of these
interfaces.
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You could imagine how if the HCI control is disabled, the TCU cannot the the BT module to open up a pairing connection or audio link.
Why the hell it is disabled after an E911 call is something I don't know.