# Direct Test Mode (DTM)

DTM is a standard mechanism defined in the Bluetooth Specification for testing the radio performance and interoperability of Bluetooth Low Energy devices. Through DTM, an external Bluetooth test instrument can use a UART interface to issue standardized HCI (host control interface) commands. DTM is required for Bluetooth and some regulatory approval processes. Therefore, if your product design needs DTM, you must expose a UART interface. Refer to the hci sample application for details on how to use DTM.

Another option for configuring RSL15 in DTM mode is to develop the GAP API DTM command in embedded firmware, and then the application can send DTM GAP commands to the Bluetooth Low Energy stack whenever DTM is required. For example, DTM can be requested by an external device through any interface such as UART, SPI, or I2C with a custom protocol, or it can be commanded by a Bluetooth Low Energy device over a Bluetooth Low Energy link. DTM is useful for final product testing in manufacturing or the production line.

IMPORTANT: All unrelated DTM activities must first be deleted for direct test mode to work properly.