L2CAP
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SWRU271H–October 2010–Revised April 2019
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The Bluetooth Low Energy Protocol Stack
5.6 L2CAP
The L2CAP layer sits on top of the HCI layer and transfers data between the upper layers of the host
(GAP, GATT, application, and so forth) and the lower-layer protocol stack. This layer multiplexes higher-
level protocol and reassembles data exchanged between the host and the protocol stack. L2CAP permits
higher-level protocols and applications to transmit and receive upper-layer data packets (L2CAP Service
Data Units, [SDUs]) up to 64KB long. The amount of memory available on the specific device
implementation limits the size. The CC254x and the 1.4.1 stack support an effective MTU size of 23, set
by L2CAP_MTU_SIZE in l2cap.h. Changing the L2CAP_MTU_SIZE results in incorrect stack behavior.
5.7 HCI
The HCI layer is a thin layer that transports commands and events between the host and controller. In a
network processor application, the HCI layer is implemented by a transport protocol such as SPI or UART.
In embedded SoC projects, the HCI layer is implemented through function calls and callbacks. The
commands and events discussed previously in this guide pass from the given layer through the HCI layer
to the controller and through the controller to the HCI layer.
5.7.1 HCI Extension Vendor-Specific Commands
Some HCI extension vendor-specific commands extend the functionality of the controller for the
application and host. For a description of HCI extension commands and examples for an SoC project, see
Appendix G.
5.7.2 Receiving HCI Extension Events in the Application
Like the GAP and ATT layers, the HCI extension commands result in HCI extension events being passed
passing from the controller to the host. Additional steps are required to receive these HCI extension
events at the application.
The GAPRole task registers receive HCI Extension events in gapRole_init() by default:
The GAPRole task receives events related to HCI extension commands, even if they are called from the
application. Implement a callback function to pass these events from the GAPRole task back to the
application if needed in the application.
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