Host Controller Interface (HCI)

For more details : Download the HCI Specification from the SIG website, or visit the Documents Page.

Table Of Contents

4.1  HCI Functional Entities

    The HCI is functionally broken up into 3 separate parts :

*Diagram Source: Courtesy of Bluetooth SIG,HCI Specs, Fig 1.2 , p 544

4.1.1  HCI Firmware    (location: Host Controller)

   HCI Firmware , is located on the Host Controller , (e.g. the actual Bluetooth hardware device). The HCI firmware implements the HCI Commands for the Bluetooth hardware by accessing baseband commands, link manager commands, hardware status registers, control registers, and event registers. The term Host Controller means the HCI-enabled Bluetooth device

4.1.2  HCI Driver         (location: Host)

    HCI Driver , which is located on the Host (e.g. software entity). The Host will receive asynchronous notifications of HCI events, HCI events are used for notifying the Host when something occurs. When the Host discovers that an event has occurred it will then parse the received event packet to determine which event occurred. The term Host means the HCI-enabled Software Unit.

4.1.3  Host Controller Transport Layer   (location: Intermediate Layers)

    The HCI Driver and Firmware communicate via the Host Controller Transport Layer , i.e. a definition of the several layers that may exist between the HCI driver on the host system and the HCI firmware in the Bluetooth hardware. These intermediate layers, the Host Controller Transport Layer, should provide the ability to transfer data without intimate knowledge of the data being transferred. Several different Host Controller Layers can be used, of which 3 have been defined initially for Bluetooth : USB , UART and RS232. The Host should receive asynchronous notifications of HCI events independent of which Host Controller Transport Layer is used.

4.2  HCI Commands

   The HCI provides a uniform command method of accessing the Bluetooth hardware capabilities. The HCI Link commands provide the Host with the ability to control the link layer connections to other Bluetooth devices. These commands typically involve the Link Manager (LM) to exchange LMP commands with remote Bluetooth devices. The HCI Policy commands are used to affect the behaviour of the local and remote LM. These Policy commands provide the Host with methods of influencing how the LM manages the piconet. The Host Controller and Baseband commands, Informational commands , and Status commands provide the Host access to various registers in the Host Controller.

4.2.1  HCI-Specific Information Exchange

   The Host Controller Transport Layer provides transparent exchange of HCI-specific information. These transporting mechanisms provide the ability for the Host to send HCI commands, ACL data, and SCO data to the Host Controller. These transport mechanisms also provide the ability for the Host to receive HCI events, ACL data, and SCO data from the Host Controller. Since the Host Controller Transport Layer provides transparent exchange of HCI-specific information, the HCI specification specifies the format of the commands, events, and data exchange between the Host and the Host Controller.

4.2.2  Link Control Commands

    The Link Control commands allow the Host Controller to control connections to other Bluetooth devices. When the Link Control commands are used, the Link Manager (LM) controls how the Bluetooth piconets and scatternets are established and maintained. These commands instruct the LM to create and modify link layer connections with Bluetooth remote devices, perform Inquiries of other Bluetooth devices in range, and other LMP commands.

4.2.3  Link Policy Commands

   The Link Policy Commands provide methods for the Host to affect how the Link Manager manages the piconet. When Link Policy Commands are used, the LM still controls how Bluetooth piconets and scatternets are established and maintained, depending on adjustable policy parameters. These policy commands modify the Link Manager behaviour that can result in changes to the link layer connections with Bluetooth remote devices.

4.2.4  Host Controller & Baseband Commands

   The Host Controller & Baseband Commands provide access and control to various capabilities of the Bluetooth hardware. These parameters provide control of Bluetooth devices and of the capabilities of the Host Controller, Link Manager, and Baseband. The host device can use these commands to modify the behaviour of the local device.

4.2.5  Informational Parameters

    The Informational Parameters are fixed by the manufacturer of the Bluetooth hardware. These parameters provide information about the Bluetooth device and the capabilities of the Host Controller, Link Manager, and Baseband. The host device cannot modify any of these parameters.

4.2.6  Status Parameters

   The Host Controller modifies all status parameters. These parameters provide information about the current state of the Host Controller, Link Manager, and Baseband. The host device cannot modify any of these parameters other than to reset certain specific parameters.

4.2.7  Testing Commands

   The Testing commands are used to provide the ability to test various functionality's of the Bluetooth hardware. These commands provide the ability to arrange various conditions for testing.

4.3  HCI Events/ Error Codes/ Flow Control

4.3.1   Flow Control

    Flow control is used in the direction from the Host to the Host Controller to avoid filling up the Host Controller data buffers with ACL data destined for a remote device (connection handle) that is not responding. It is the Host that manages the data buffers of the Host Controller.

4.3.2  HCI Events

    A number of different events are defined for the HCI layer. The events provide a method to return parameters and data associated for each event. 32 HCI different events have been implemented so far, they range from Inquiry Complete Event to Page Scan Repetition Mode Change Event. See the main HCI specs for mode details.

4.3.3  HCI Error Codes

    A large number of error codes have been defined for the HCI layer. When a command fails, Error codes are returned to indicate the reason for the error. 35 HCI error codes have so far been defined, from Unknown HCI Command to LMP PDU Not Allowed.See the main HCI specs for mode details.

4.4  Bluetooth-defined Host Controller Transport Layers

4.4.1  UART Transport Layer

    The objective of the HCI UART Transport Layer is to make it possible to use the Bluetooth HCI over a serial interface between two UARTs on the same PCB. The HCI UART Transport Layer assumes that the UART communication is free from line errors. Event and data packets flow through this layer, but the layer does not decode them.

4.4.2  RS232 Transport Layer

   The objective of the HCI RS232 Transport Layer is to make it possible to use the Bluetooth HCI over one physical RS232 interface between the Bluetooth Host and the Bluetooth Host Controller. Event and data packets flow through this layer, but the layer does not decode them.

4.4.3  USB Transport Layer

   The objective of the Universal Serial Bus (USB) Transport Layer is to the use a USB hardware interface for Bluetooth hardware (which can be embodied in one of two ways: as a USB dongle, or integrated onto the motherboard of a notebook PC). A class code will be used that is specific to all USB Bluetooth devices. This will allow the proper driver stack to load, regardless of which vendor built the device. It also allows HCI commands to be differentiated from USB commands across the control endpoint.

Note , the above text contains excerpts from the Bluetooth SIG's Specification, as well as various interpretations of the Specs. For complete details of the various sections, consult the actual Bluetooth Specification.