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In CSR’s HCI-stack design, the BlueCore chip handles Bluetooth controller functions through the Host Controller Interface (HCI); the separate host processor runs the Bluetooth protocols and application above HCI. CSR also documented BlueCore2 options that moved more of the stack onto the chip, including an RFCOMM variant and a virtual-machine (VM) variant.
What runs on a CSR chip versus the Bluetooth host?
For the HCI-based BlueCore4-External architecture, the chip runs the radio, baseband and link-controller functions, link manager, and controller-side HCI firmware. The host computer or embedded host runs the upper Bluetooth layers and the application. CSR’s 2005 BlueCore4-External datasheet describes the split directly: “In the implementation shown in Figure 9.1 the internal processor runs the Bluetooth stack up to the Host Controller Interface (HCI). The Host processor must provide all upper layers including the application.”
In a typical host implementation, those upper layers can include L2CAP, RFCOMM, SDP, GAP, and Bluetooth profiles. The precise set depends on the host operating system and the application’s needs; HCI itself does not supply those host-side protocols or the application.
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Where the boundary sits
- On the controller: radio and lower-level Bluetooth controller functions, including the link manager and HCI firmware.
- Between controller and host: HCI command, event, and data packets cross a transport such as USB or UART.
- On the host: the Bluetooth host stack implements upper protocols and profiles, and the application uses those services.
The Bluetooth SIG’s Core Specification 5.4 describes the Host Controller Transport Layer as the physical-bus driver that lets the two HCI sides exchange information. HCI is therefore the standard boundary between host-side Bluetooth software and controller-side functions; it is not itself a complete Bluetooth stack.
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What does CSR’s HCI stack provide?
“HCI stack” in this context refers to the controller implementation up to the HCI boundary, not to all the software a Bluetooth application needs. The CSR controller accepts host HCI commands and returns HCI events and data over its transport. The host stack uses that interface to direct controller operations while handling the higher-level protocols and profiles.
This division lets the host use a standard controller interface instead of implementing radio and link-control behavior itself. It does not remove the need for a host Bluetooth stack in the HCI-only arrangement: the host still needs the upper layers required by its use case, plus its application.
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Do you need a host Bluetooth stack above HCI?
Yes, if the selected CSR architecture ends at HCI. In that configuration, the host must provide the upper Bluetooth layers and application. For BlueCore4-External, the 2005 datasheet explicitly assigns all upper layers, including the application, to the host processor.
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How do the HCI, RFCOMM, and VM options differ?
CSR documented multiple software boundaries for BlueCore2. The comparison below describes those historical architecture options, not a claim about every CSR product or a current controller.
| Architecture | What runs on the BlueCore chip | What remains on the host | Main trade-off |
|---|---|---|---|
| HCI | Bluetooth controller stack through HCI. | Upper Bluetooth layers and application. | More host software and processing, with greater flexibility to manage upper layers on the host. |
| RFCOMM variant | Upper layers through RFCOMM. | Software above the on-chip RFCOMM boundary, including any host-side application components the design requires; the datasheet does not specify a universal host stack for every application. | Moves more Bluetooth functionality onto the chip and reduces host-side software needs compared with HCI-only. |
| VM variant | All software layers and the application on the internal RISC processor. | No host-side Bluetooth stack or application is required for the documented on-chip application arrangement; the datasheet does not establish that every system needs no host for any purpose. | Can reduce or eliminate host-side software and processing, while placing the application in the chip’s VM environment. |
CSR summarized the choice in its BlueCore2-External datasheet: “Running the upper stack on BlueCore2-External reduces (or eliminates, in the case of a virtual machine (VM) application) the need for host-side software and processing time. Running the upper layers on the host processor allows greater flexibility.” In practical terms, HCI favors a conventional host-managed stack; RFCOMM and VM shift increasing responsibility to the chip.
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How do HCI, H4, BCSP, and BCCMD relate?
HCI is the host-controller interface: the commands, events, and data exchanged between host and controller. USB and UART are transports used to carry HCI traffic. For UART, H4 is a standard HCI transport protocol. BlueCore2 documentation also describes BCSP, a CSR-proprietary reliable alternative to standard H4 UART transport. BCSP is not a higher Bluetooth protocol such as RFCOMM, nor is it a general replacement name for HCI.
CSR also documented approximately 50 manufacturer-specific BCCMD HCI extension commands for device configuration and control on BlueCore2. These are vendor-specific commands, distinct from portable Bluetooth HCI commands; software that depends on them should not assume another manufacturer’s controller supports them.
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Which historical BlueCore specifications do these details describe?
These architecture descriptions come from legacy CSR product documents. The BlueCore2 source describes Bluetooth specification v1.1-era firmware; the BlueCore4-External source describes Bluetooth v2.0 + EDR firmware. Their feature figures should not be generalized to current Bluetooth controllers.
- BlueCore2 and BlueCore4 datasheets describe support for up to seven active slaves in those historical product generations; this is not a general guarantee for current Bluetooth devices.
- The BlueCore4-External datasheet (2005) lists enhanced data rates of 2 and 3 Mbps for that product generation; those figures are not a statement of current controller performance.
For a current design, use the datasheet and software documentation for the exact controller and host platform. The legacy CSR documents establish how those BlueCore architectures divided responsibilities, not what an unspecified present-day device implements.
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