In this chapter, we first explain how Bluetooth chipsets are connected physically and then provide information about popular Bluetooth chipset and their use with BTstack.

HCI Interface

The communication between a Host (a computer or an MCU) and a Host Controller (the actual Bluetooth chipset) follows the Host Controller Interface (HCI), see below. HCI defines how commands, events, asynchronous and synchronous data packets are exchanged. Asynchronous packets (ACL) are used for data transfer, while synchronous packets (SCO) are used for Voice with the Headset and the Hands-Free Profiles.

Host Controller to Host connection


On desktop-class computers incl. laptops, USB is mainly used as HCI transport layer. For USB Bluetooth chipsets, there is little variation: most USB dongles on the market currently contain a Broadcom BCM20702 or a CSR 851x chipset. It is also called H2.

On embedded systems, UART connections are used instead, although USB could be used as well.

For UART connections, different transport layer variants exist.


The most common one is the official "UART Transport", also called H4. It requires hardware flow control via the CTS/RTS lines and assumes no errors on the UART lines.


The "Three-Wire UART Transport", also called H5, makes use of the SLIP protocol to transmit a packet and can deal with packet loss and bit-errors by retransmission. While it is possible to use H5 really with "three wires" without hardware handshake, we recommend to use a full UART with hardware handshake. If your design lacks the hardware handshake, H5 is your only option.


The predecessor of H5. The main difference to H5 is that Even Parity is used for BCSP. To use BCSP with BTstack, you use the H5 transport and can call hci_transport_h5_enable_bcsp_mode


Finally, Texas Instruments extended H4 to create the "eHCILL transport" layer that allows both sides to enter sleep mode without loosing synchronisation. While it is easier to implement than H5, it it is only supported by TI chipsets and cannot handle packet loss or bit-errors.

H4 over SPI

Chipsets from Dialog Semiconductor and EM Marin allow to send H4 formatted HCI packets via SPI. SPI has the benefit of a simpler implementation for both Host Controller and Host as it does not require an exact clock. The SPI Master, here the Host, provides the SPI Clock and the SPI Slave (Host Controller) only has to read and update it's data lines when the clock line changes. The EM9304 supports an SPI clock of up to 8 Mhz. However, an additional protocol is needed to let the Host know when the Host Controller has HCI packet for it. Often, an additional GPIO is used to signal this.

HCI Shortcomings

Unfortunately, the HCI standard misses a few relevant details:

Documentation and Support

The level of developer documentation and support varies widely between the various Bluetooth chipset providers.

From our experience, only Texas Instruments and EM Microelectronics provide all relevant information directly on their website. Nordic Semiconductor does not officially have Bluetooth chipsets with HCI interface, but their documentation on the nRF5 series is complete and very informative. TI and Nordic also provide excellent support via their respective web forum.

Broadcom, whose Bluetooth + Wifi division has been acquired by the Cypress Semiconductor Corporation, provides developer documentation only to large customers as far as we know. It's possible to join their Community forum and download the WICED SDK. The WICED SDK is targeted at Wifi + Bluetooth Combo chipsets and contains the necessary chipset patch files.

CSR, which has been acquired by Qualcomm, provides all relevant information on their Support website after signing an NDA.

Chipset Overview

Chipset Type HCI Transport BD_ADDR (1) SCO over HCI (2) LE DLE Multiple LE Roles (3) Classic SC (4) LE Addr Resolution BTstack folder Comment
Atmel ATWILC3000 LE H4 Yes n.a No No n.a. Don't know atwilc3000 BLE Firmware size: 60 kB
Broadcom UART Dual mode H4, H5 Rarely Partially (2) No Maybe (3) 43438: Yes bcm Max UART baudrate 2 mbps
Broadcom USB Dongles Dual mode USB Yes Yes No No BCM20702: No bcm
CSR UART Dual mode H4, H5, BCSP Rarely Partially (2) No No CSR8811: No csr
CSR USB Dongles Dual mode USB Mostly Yes No No CSR8510: No csr
Cypress CYW20704/7 Dual mode H4, H5, USB Don't know Partially (2) Yes Yes Yes Yes bcm
Cypress CYW20819 Dual mode H4, H5, USB Don't know Partially (2) Yes Yes Yes Don't know bcm Keep CTS high during power cycle
Cypress CYW43xxx Dual mode + Wifi H4, H5 Don't know Partially (2) Don't know On newer versions On wewer versions On newer versions bcm Bluetooth + Wifi Combo Controller
Cypress PSoC 4 LE H4 Don't know n.a. Yes Don't know n.a. Don't know HCI Firmware part of PSoC Creator kits examples
Dialog DA14531 LE H4 No n.a. Yes Yes n.a. Don't know da145xx Official HCI firmware included in BTstack
Dialog DA14581 LE H4, SPI No n.a. No No n.a. Don't know da145xx Official HCI firmware included in BTstack
Dialog DA14585 LE H4, SPI No n.a. Yes Yes n.a. Yes da145xx Official HCI firmware included in BTstack
Dialog DA1469x LE H4, SPI No n.a. Yes Yes n.a. Yes da145xx HCI Firmware part of DA1469x SDK
Espressif ESP32 Dual mode + Wifi VHCI, H4 Yes Yes Yes Yes Yes Don't know SoC with Bluetooth and Wifi
Espressif ESP32-S3,C3 LE + Wifi VHCI, H4 Yes No Yes Yes Yes Yes SoC with Bluetooth and Wifi
EM 9301 LE SPI, H4 No n.a. No No n.a. Don't know em9301 Custom HCI SPI implementation
EM 9304 LE SPI, H4 Yes n.a. Yes Yes n.a. Don't know em9301 Custom HCI SPI implementation
Intel Dual Wireless 3165, 8260, 8265 Dual mode USB Yes Probably Don't know Don't know Don't know Don't know intel Firmware size: 400 kB
Nordic nRF LE H4 Fixed Random n.a. Yes Yes n.a. Yes Requires HCI firmware
STM STLC2500D Classic H4 No Don't know n.a n.a. No n.a. stlc2500d Custom deep sleep management not supported
Renesas RX23W LE H4 No n.a. Yes Yes n.a . Don't know HCI Firmware part of BTTS
Realtek RTL8822CS Dual mode + Wifi H5 Yes Yes Don't know Don't know Don't know Don't know Requires initial firmware + config
Realtek USB Dongles Dual mode + Wifi USB Yes Yes Don't know Don't know Don't know Don't know realtek Requires initial firmware + config
Toshiba TC35661 Dual mode H4 No No No No No No tc3566 Only -007/009 models provide full HCI. See below
TI CC256x, WL183x Dual mode H4, H5, eHCILL Yes Yes No Yes for CC256XC No No cc256x Also WL185x, WL187x, and WL189x


  1. BD_ADDR: Indicates if Bluetooth chipset comes with its own valid MAC Address. Better Broadcom and CSR dongles usually come with a MAC address from the dongle manufacturer, but cheaper ones might come with identical addresses.
  2. SCO over HCI: All Bluetooth Classic chipsets support SCO over HCI in general. BTstack can receive SCO packets without problems. However, only TI CC256x has support for using SCO buffers in the Controller and a useful flow control. On CSR/Broadcom/Cypress Controllers, BTstack cannot queue multiple SCO packets in the Controller. Instead, the SCO packet must be sent periodically at the right time - without a clear indication about when this time is. The current implementation observes the timestamps of the received SCO packets to schedule sending packets. With full control over the system and no other Bluetooth data, this can be flawless, but it's rather fragile in general. For these, it's necessary to use the I2S/PCM interface for stable operation. , for those that are marked with No, we either didn't try or didn't found enough information to configure it correctly.
  3. Multiple LE Roles: Apple uses Broadcom Bluetooth+Wifi in their iOS devices and newer iOS versions support multiple concurrent LE roles, so at least some Broadcom models support multiple concurrent LE roles.


The ATILC3000 Bluetooth/Wifi combo controller has been used with Linux on embedded devices by Atmel/Microchip. Drivers and documentation are available from a GitHub repository. The ATWILC3000 has a basic HCI implementation stored in ROM and requires a firmware image to be uploaded before it can be used. The BLE Controller is qualified as QDID 99659. Please note: the BLE firmware is around 60 kB. It might need a separate Wifi firmware as well.

BD Addr can be set with vendor-specific command although all chipsets have an official address stored. The BD_ADDR lookup results in "Newport Media Inc." which was acquired by Atmel in 2014.

Baud rate can be set with a custom command.

BTstack integration: btstack_chipset_atwilc3000.c contains the code to download the Bluetooth firmware image into the RAM of the ATWILC3000. After that, it can be normally used by BTstack.

Broadcom/Cypress Semiconductor

Before the Broadcom Wifi+Bluetooth division was taken over by Cypress Semiconductor, it was not possible to buy Broadcom chipset in low quantities. Nevertheless, module manufacturers like Ampak created modules that contained Broadcom BCM chipsets (Bluetooth as well as Bluetooth+Wifi combos) that might already have been pre-tested for FCC and similar certifications.

A popular example is the Ampak AP6212A module that contains an BCM 43438A1 and is used on the Raspberry Pi 3, the RedBear Duo, and the RedBear IoT pHAT for older Raspberry Pi models.

The CYW20704 A2 controller supports both DLE as well as multiple LE roles and is available e.g. from LairdTech as UART module (BT860), USB module (BT850), and USB dongle.

Interestingly, the CYW20704 exhibits the same UART flow control bug as the CC2564. You can add ENABLE_CYPRESS_BAUDRATE_CHANGE_FLOWCONTROL_BUG_WORKAROUND to activate a workaround and/or read the bug & workardound description in the TI section below.

The best source for documentation on vendor specific commands so far has been the source code for blueZ and the Bluedroid Bluetooth stack from Android, but with the takeover by Cypress, documentation is directly available.

Broadcom USB dongles do not require special configuration, however SCO data is not routed over USB by default.

The PSoC 4 SoCs can be programmed with the "BLE DTM" HCI Firmware from the PSoC Creator Kit Examples. The UART baudrate is set to 115200. For higher baud rates, the clocks probably need to be configured differently, as the IDE gives a warning about this.

The CYW20819 can be used as a SoC with Cypress' Bluetooth stack. To use it as a regular Bluetooth Controller over HCI H4, CTS must be asserted during Power-Up / Reset.

The CYW43xxx series contains a Wifi and Bluetooth Controller. The Bluetooth Controller can be used independent from the Wifi part.

Init scripts: For UART connected chipsets, an init script has to be uploaded after power on. For Bluetooth chipsets that are used in Broadcom Wifi+Bluetooth combos, this file often can be found as a binary file in Linux distributions with the ending '.hcd' or as part of the WICED SDK as C source file that contains the init script as a data array for use without a file system.

To find the correct file, Broadcom chipsets return their model number when asked for their local name.

BTstack supports uploading of the init script in two variants: using .hcd files looked up by name in the posix-h4 port and by linking against the init script in the WICED port. While the init script is processed, the chipsets RTS line goes high, but only 2 ms after the command complete event for the last command from the init script was sent. BTstack waits for 10 ms after receiving the command complete event for the last command to avoid sending before RTS goes high and the command fails.

BD Addr can be set with a custom command. A fixed address is provided on some modules, e.g. the AP6212A, but not on others.

SCO data can be configured with a custom command found in the bluez sources. It works with USB chipsets. The chipsets don't implement the SCO Flow Control that is used by BTstack for UART connected devices. A forum suggests to send SCO packets as fast as they are received since both directions have the same constant speed.

Baud rate can be set with custom command. The baud rate resets during the warm start after uploading the init script. So, the overall scheme is this: start at default baud rate, get local version info, send custom Broadcom baud rate change command, wait for response, set local UART to high baud rate, and then send init script. After sending the last command from the init script, reset the local UART. Finally, send custom baud rate change command, wait for response, and set local UART to high baud rate.

BTstack integration: The common code for all Broadcom chipsets is provided by btstack_chipset_bcm.c. During the setup, btstack_chipset_bcm_instance function is used to get a btstack_chipset_t instance and passed to hci_init function.

SCO Data can be routed over HCI for both USB dongles and UART connections, however BTstack does not support flow control for UART connections. HSP and HFP Narrow Band Speech is supported via I2C/PCM pins. Newer Controllers provide an mSBC codec that allows to use HSP/HFP incl. WBS over PCM/I2S with ENABLE_BCM_PCM_WBS.

CSR / Qualcomm Incorporated

CSR plc has been acquired by Qualcomm Incorporated in August 2015.

Similar to Broadcom, the best source for documentation is the source code for blueZ.

CSR USB dongles do not require special configuration and SCO data is routed over USB by default.

CSR chipset do not require an actual init script in general, but they allow to configure the chipset via so-called PSKEYs. After setting one or more PSKEYs, a warm reset activates the new setting.

BD Addr can be set via PSKEY. A fixed address can be provided if the chipset has some kind of persistent memory to store it. Most USB Bluetooth dongles have a fixed BD ADDR.

SCO data can be configured via a set of PSKEYs. We haven't been able to route SCO data over HCI for UART connections yet.

Baud rate can be set as part of the initial configuration and gets actived by the warm reset.

BTstack integration: The common code for all Broadcom chipsets is provided by btstack_chipset_csr.c. During the setup, btstack_chipset_csr_instance function is used to get a btstack_chipset_t instance and passed to hci_init function. The baud rate is set during the general configuration.

SCO Data is routed over HCI for USB dongles, but not for UART connections. HSP and HFP Narrow Band Speech is supported via I2C/PCM pins.

Dialog Semiconductor / Renesas

Daialo Semiconductor has been aquired by Renesas in February 2021.

They offers the DA145xx and DA1469xx series of LE-only SoCs that can be programmed with an HCI firmware. The HCI firmware can be uploaded on boot into SRAM or stored in the OTP (One-time programmable) memory, or in an external SPI.

The 581 does not implement the Data Length Extension or supports multiple concurrent roles, while later versions support it.

The mechanism to boot a firmware via UART/SPI has mostly stayed the same, while the set of supported interfaces and baudrates have slightly changed.

The DA1469x uses an external flash. The DA 1469x SDK contains a HCI firmware that can be compiled and downloaded into flash using the SmartSnippets Studio.

Unexpected issues: - DA14585 cannot scan for other devices if advertising is enabled alhtough it supports multiple Peripheral/Central roles (last check:

BD Addr fixed to 80:EA:CA:00:00:01. No command in HCI firmware to set it differently. Random addresses could be used instead.

Baud rate: The baud rate is fixed at 115200 with the provided firmware. A higher baud rate could be achieved by re-compiling the HCI firmware.

BTstack integration: btstack_chipset_da145xx.c contains the code to download the provided HCI firmware into the SRAM of the DA145xx. After that, it can be used as any other HCI chipset. No special support needed for DA1469x after compiling and flashing the HCI firmware.

Espressif ESP32

The ESP32 is a SoC with a built-in Dual mode Bluetooth and Wifi radio. The HCI Controller is implemented in software and accessed via a so called Virtual HCI (VHCI) interface. It supports both LE Data Length Extensions (DLE) as well as multiple LE roles. Since ESP-IDF v4.3, SCO-over-HCI is usable for HSP/HFP.

The newer ESP32-S3 and ESP32-C3 SoCs have a newer LE Controller that also supports 2M-PHY, but does support Classic (BR/EDR) anymore.

ALl can either be used as an SoC with the application running on the ESP32 itself or can be configured as a regular Bluetooth HCI Controller. BTstack can work either on the SoC itself or on another MCU with the ESP32 connected via 4-wire UART.

See Espressif's ESP-Hosted firmware for use as Bluetooth/Wifi Ccontroller.

EM Microelectronic Marin

For a long time, the EM9301 has been the only Bluetooth Single-Mode LE chipset with an HCI interface. The EM9301 can be connected via SPI or UART. The UART interface does not support hardware flow control and is not recommended for use with BTstack. The SPI mode uses a proprietary but documented extension to implement flow control and signal if the EM9301 has data to send.

In December 2016, EM released the new EM9304 that also features an HCI mode and adds support for optional Bluetooth 4.2. features. It supports the Data Length Extension and up to 8 LE roles. The EM9304 is a larger MCU that allows to run custom code on it. For this, an advanced mechanism to upload configuration and firmware to RAM or into an One-Time-Programmable area of 128 kB is supported. It supports a superset of the vendor specific commands of the EM9301.

EM9304 is used by the 'stm32-l073rz-em9304' port in BTstack. The port.c file also contains an IRQ+DMA-driven implementation of the SPI H4 protocol specified in the datasheet.

BD Addr must be set during startup for EM9301 since it does not have a stored fix address. The EM9304 comes with an valid address stored in OTP.

SCO data is not supported since it is LE only.

Baud rate can be set for UART mode. For SPI, the master controls the speed via the SPI Clock line. With 3.3V, 16 Mhz is supported.

Init scripts are not required although it is possible to upload small firmware patches to RAM or the OTP memory (EM9304 only).

BTstack integration: The common code for the EM9304 is provided by btstack_chipset_em9301.c. During the setup, btstack_chipset_em9301_instance function is used to get a btstack_chipset_t instance and passed to hci_init function. It enables to set the BD Addr during start.

Intel Dual Wireless 8260, 8265

Wifi/Bluetooth combo cards mainly used in mobile computers. The Bluetooth part requires the upload of a firmware file and a configuration file. SCO, DLE, Multiple roles not tested.

Nordic nRF5 series

The Single-Mode LE chipsets from the Nordic nRF5 series chipsets usually do not have an HCI interface. Instead, they provide an LE Bluetooth Stack as a binary library, the so-called SoftDevices. Developer can write their Bluetooth application on top of this library. Since the chipset can be programmed, it can also be loaded with a firmware that provides a regular HCI H4 interface for a Host.

An interesting feature of the nRF5 chipsets is that they can support multiple LE roles at the same time, e.g. being Central in one connection and a Peripheral in another connection. Also, the nRF52 SoftDevice implementation supports the Bluetooth 4.2 Data Length Extension.

Both nRF5 series, the nRF51 and the nRF52, can be used with an HCI firmware. The nRF51 does not support encrypted connections at the moment (November 18th, 2016) although this might become supported as well.

BD ADDR is not set automatically. However, during production, a 64-bit random number is stored in the each chip. Nordic uses this random number as a random static address in their SoftDevice implementation.

SCO data is not supported since it is LE only.

Baud rate is fixed to 115200 by the patch although the firmware could be extended to support a baud rate change.

Init script is not required.

BTstack integration: Support for a nRF5 chipset with the Zephyr Controller is provided by btstack_chipset_zephyr.c. It queries the static random address during init.

To use these chipsets with BTstack, you need to install an arm-none-eabi gcc toolchain and the nRF5x Command Line Tools incl. the J-Link drivers, checkout the Zephyr project, apply a minimal patch to help with using a random static address, and flash it onto the chipset:

  $ make BOARD=nrf52_pca10040


Realtek provides Dual-Mode Bluetooth Controllers with USB and UART (H4/H5) interfaces as well as combined Bluetooth/WiFi Controllers, which are also available as M.2 modules for laptops. They commonly require to download a patch and a configuration file. Patch and configuration file can be found as part of their Linux drivers.

BD ADDR is stored in Controller.

SCO data can either be routed over HCI with working flow control or over I2S/PCM. The 8822CS supports mSBC codec internally.

Baud rate is set by the config file.

Init script is required.

BTstack integration: H4/H5 Controller require firmware upload. 'rtk_attach' can be used for this. For USB Controllers, btstack_chipset_realtek.c implements the patch and config upload mechanism. See port/libusb for details on how to use it.

Renesas Electronics

Please see Dialog Semiconducator for DA14xxx Bluetooth SoCs above.

Renesas currently has 3 LE-only SoCs: the older 16-bit RL78 and the newer RX23W and the RA4W1. For the newer SoCs, Renesas provides a pre-compiled HCI firmware as well as an HCI project for their e2 Studio IDE. The HCI firmware needs to be programmed into the SoC Flash.

Both newer SoC provide the newer Bluetooth 5.0 features like DLE, 2M-PHY, Long Range, and Multiple Roles.

To install the HCI Firmware on the Target Board for RX23W:

BD Addr fixed to 74:90:50:FF:FF:FF. A Windows tool in the BTTS suite allows to set a public Bluetooth Address.

Baud rate: The baud rate is fixed at 115200 resp. 2000000 with the provided firmware images. With 2 mbps, there's no need to update the baudrate at run-time.

BTstack integration: No special support needed.


STMicroelectronics has several different Bluetooth series.


It offers the Bluetooth V2.1 + EDR chipset STLC2500D that supports SPI and UART H4 connection.

BD Addr can be set with custom command although all chipsets have an official address stored.

SCO data might work. We didn't try.

Baud rate can be set with custom command. The baud rate change of the chipset happens within 0.5 seconds. At least on BTstack, knowning exactly when the command was fully sent over the UART is non-trivial, so BTstack switches to the new baud rate after 100 ms to expect the command response on the new speed.

Init scripts are not required although it is possible to upload firmware patches.

BTstack integration: Support for the STLC2500C is provided by btstack_chipset_stlc.c. During the setup, btstack_chipset_stlc2500d_instance function is used to get a btstack_chipset_t instance and passed to hci_init function. It enables higher UART baud rate and to set the BD Addr during startup.


The BlueNRG series is an LE-only SoC which can be used with an HCI Firmware over a custom SPI interface.


The new STM32-WB5x series microcontroller is an SoC with a multi-protocol 2.4 Ghz radio co-processor. It provides a virtual HCI interface.

Texas Instruments CC256x series

The Texas Instruments CC256x series is currently in its fourth iteration and provides a Classic-only (CC2560), a Dual-mode (CC2564), and a Classic + ANT (CC2567) model. A variant of the Dual-mode chipset is also integrated into TI's WiLink 8 Wifi+Bluetooth combo modules of the WL183x, WL185x, WL187x, and WL189x series. Some of the latter support ANT as well.

The CC256x chipset is connected via an UART connection and supports the H4, H5 (since third iteration), and eHCILL.

The latest generation CC256xC chipsets support multiple LE roles in parallel.

TI provides an alternative firmware that integrates an SBC Codec in the Bluetooth Controller itself for Assisted A2DP (A3DP) and Assisted HFP (Wide-band speech support). While this can save computation and code size on the main host, it cannot be used together with BLE, making it useless in most projects.

The different CC256x chipset can be identified by the LMP Subversion returned by the hci_read_local_version_information command. TI also uses a numeric way (AKA) to identify their chipsets. The table shows the LMP Subversion and AKA number for the CC256x and the WL18xx series.

Chipset LMP Subversion AKA
CC2560 0x191f 6.2.31
CC2560A, CC2564, CC2567 0x1B0F 6.6.15
CC256xB 0x1B90 6.7.16
CC256xC 0x9a1a 6.12.26
WL18xx 0xac20 11.8.32

SCO data: Routing of SCO data can be configured with the HCI_VS_Write_SCO_Configuration command.

Baud rate can be set with HCI_VS_Update_UART_HCI_Baudrate. The chipset confirms the change with a command complete event after which the local UART is set to the new speed. Oddly enough, the CC256x chipsets ignore the incoming CTS line during this particular command complete response.

If you've implemented the hal_uart_dma.h without an additional ring buffer (as recommended!) and you have a bit of delay, e.g. because of thread switching on a RTOS, this could cause a UART overrun. If this happens, BTstack provides a workaround in the HCI H4 transport implementation by adding ENABLE_CC256X_BAUDRATE_CHANGE_FLOWCONTROL_BUG_WORKAROUND. If this is enabled, the H4 transport layer will resort to "deep packet inspection" to first check if its a TI controller and then wait for the HCI_VS_Update_UART_HCI_Baudrate. When detected, it will tweak the next UART read to expect the HCI Command Complete event.

BD Addr can be set with HCI_VS_Write_BD_Addr although all chipsets have an official address stored.

Init Scripts. In order to use the CC256x chipset an initialization script must be obtained and converted into a C file for use with BTstack. For newer revisions, TI provides a main.bts and a ble_add_on.bts that need to be combined.

The Makefile at chipset/cc256x/ is able to automatically download and convert the requested file. It does this by:

./ main.bts [ble_add_on.bts] output_file.c

BTstack integration: - The common code for all CC256x chipsets is provided by btstack_chipset_cc256x.c. During the setup, btstack_chipset_cc256x_instance function is used to get a btstack_chipset_t instance and passed to hci_init function. btstack_chipset_cc256x_lmp_subversion provides the LMP Subversion for the selected init script. - SCO Data is be routed over HCI with ENABLE_SCO_OVER_HCI or to PCM/I2S with ENABLE_SCO_OVER_PCM. Wide-band speech is supported in both cases. For SCO-over-HCI, BTstack implements the mSBC Codec. For SCO-over-I2S, Assisted HFP can be used. - Assisted HFP: BTstack provides support for Assisted HFP mode if enabled with ENABLE_CC256X_ASSISTED_HFP and SCO is routed over PCM/I2S with ENABLE_SCO_OVER_PCM. During startup, the PCM/I2S is configured and the HFP implementation will enable/disable the mSBC Codec for Wide-band-speech when needed.

Known issues: - The CC2564C v1.5 may loose the connection in Peripheral role with a Slave Latency > 0 when the Central updates Connection Parameters. See


The Toshiba TC35661 Dual-Mode chipset is available in three variants: standalone incl. binary Bluetooth stack, as a module with embedded stack or with a regular HCI interface. The HCI variant has the model number TC35661–007 resp TC35561-009 for the newer silicon.

We first tried their USB Evaluation Stick that contains an USB-to-UART adapter and the PAN1026 module that contains the TC35661 -501. While it does support the HCI interface and Bluetooth Classic operations worked as expected, LE HCI Commands are not supported. With the -007 and the -009 models, everything works as expected.

SCO data does not seem to be supported.

Baud rate can be set with custom command.

BD Addr must be set with custom command. It does not have a stored valid public BD Addr.

Init Script is not required. A patch file might be uploaded.

BTstack integration: Support for the TC35661 series is provided by btstack_chipset_tc3566x.c. During the setup, btstack_chipset_tc3566x_instance function is used to get a btstack_chipset_t instance and passed to hci_init function. It enables higher UART baud rate and sets the BD Addr during startup.