The Bluetooth specification includes the definition of a standardised interface (the Host Controller Interface) to the hardware of a Bluetooth communications module. It defines the interface between the HCI Driver software (part of the Host, i.e. the user of the Bluetooth communications module) and the Host Controller firmware (part of the Bluetooth communications module itself).
- HCI Command Packets: commands are issued by the HCI Driver to the Host Controller:
- Packet indicator (for UART interfaces) of 1.
- Op-code (16 bits): identifies the command:
OGF (Op-code Group Field, most significant 6 bits);
OCF (Op-code Command Field, least significant 10 bits).- Parameter length (8-bit): total length of all parameters in bytes.
- Command parameters: the number of parameters and their length is command specific.
- HCI Data Packets for ACL data:
- Packet indicator (for UART interfaces) of 2.
- Control information (16 bits):
Broadcast flag (most significant 2 bits):00 = point-to-point packet (no broadcast);Packet boundary flag (2 bits):
01 = Active Slave Broadcast;
10 = Parked Slave Broadcast.01 = continuing packet of a higher level message;Connection handle (least significant 12 bits).
10 = first packet of a higher level message.- Data length (16 bits): total length of data in bytes.
- Data.
- HCI Data Packets for SCO data:
- Packet indicator (for UART interfaces) of 3.
- Connection handle (12 bits).
- Unused (4 bits).
- Data length (8 bits): total length of data in bytes.
- Data.
- HCI Event Packets: the Host Controller notifies the HCI Driver of events:
- Packet indicator (for UART interfaces) of 4.
- Event code (8 bits): identifies the event.
- Parameter length (8-bit): total length of all parameters in bytes.
- Event parameters: the number of parameters and their length is event specific.
Commands are processed asynchronously, so the completion of a command (and its return data) is reported by a Command Complete event. Commands may be processed in parallel, so a later command may complete before an earlier command.
The Bluetooth specification also defines standard mechanisms for transmitting HCI packets between an HCI Driver and Host Controller, via a Host Controller Transport Layer such as: