rfListenBeforeTalk Example
Listen Before Talk TX Example
Example Summary
The Listen Before Talk (LBT) TX example illustrates how to implement a simple, proprietary LBT algorithm using command chaining.
When sending a packet, the radio first enters RX mode using CMD_PROP_CS. If the channel is IDLE (the RSSI is below RSSI_THRESHOLD) for IDLE_TIME_US, the the radio enters TX and transmits a packet. If the channel is BUSY (RSSI above RSSI_THRESHOLD), the radio enters RX again to check the channel once more. This is repeated max CS_RETRIES_WHEN_BUSY number of times. The command chain will either finish with a packet being sent (if the channel is IDLE), or after checking the channel CS_RETRIES_WHEN_BUSY times.
Default Settings:
- CS_RETRIES_WHEN_BUSY: 10
- RSSI_THRESHOLD: -90 dBm
- IDLE_TIME_US: 5000 us
Peripherals Exercised
Board_PIN_LED1- Toggled for every packet sent.
Resources & Jumper Settings
If you’re using an IDE (such as CCS or IAR), please refer to Board.html in your project directory for resources used and board-specific jumper settings. Otherwise, you can find Board.html in the directory <SDK_INSTALL_DIR>/source/ti/boards/<BOARD>.
Example Usage
The example code can be tested with SmartRF Studio or the rfPacketRX examples as the receiver, and a jammer at 868.0 MHz (433.92 MHz for the CC1350-LAUNCHXL-433 board).
Application Design Details
The command chain contains the following commands:
- CMD_NOP
- CMD_PROP_CS
- CMD_COUNT_BRANCH
- CMD_PROP_TX
The following image shows the control flow:
-------------
| |
| CMD_NOP |
| |
-------------
|
|
|
-----------------
PROP_DONE_IDLE | |
---------<-------------------| CMD_PROP_CS |----<---
| | | |
| ----------------- |
| | |
| PROP_DONE_BUSY | RF_cmdCountBranch.counter-- > 0
| | |
| ---------------------- |
| | | |
| | CMD_COUNT_BRANCH |-->--
| | | |
| ---------------------- |
| . |
| . RF_cmdCountBranch.counter-- = 0
| . |
| ----------------- |
| | | |
--------->-------------------| CMD_PROP_TX | |
| | |
----------------- |
| |
|--------<--------
|
Last command in chain doneThe carrier sense command CMD_PROP_CS is prepended by a CMD_NOP in order to set an absolute trigger time TIRG_ABSTIME for the whole chain. All other commands trigger immediately after each other. If CMD_PROP_CS indicates a busy channel it is restarted by CMD_COUNT_BRANCH for maximum RF_cmdCountBranch.counter times. Only if CMD_PROP_CS observes an IDLE channel, CMD_PROP_TX is executed.
On a successful transmission, Board_PIN_LED1 toggles and the sequence number in the packet is incremented. If CMD_PROP_TX can not be executed due to an always BUSY channel, the command chain is restarted again after PACKET_INTERVAL_MS which is 200 ms by default.
Once triggered, all commands runs entirely on the RF core and do not involve the main CPU. The CC13xx enters Standby mode in-between packet transmissions. The following image shows the characteristic current consumption profile when channel is IDLE (the radio is in RX for 5 ms):
TX TX
---- ----
| | | |
| | | |
| | | |
| | | |
RX | | RX | |
---- | ---- |
| | | |
| | Standby | |
------ ---------------------------------------- ----------------
<--------------------------------------------->
200 msIf the channel is BUSY, the CMD_PROP_CS command is called 10 times. The synthesizer is kept on in between the commands.
With the default settings it takes ~250 us to determine if the channel is BUSY or not. Considering a worst case of 10 retries, the execution of the whole chain takes around 5.6 ms, including CMD_NOP and CMD_COUNT_BRANCH.
Note for IAR users: When using the CC1310DK, the TI XDS110v3 USB Emulator must be selected. For the CC1310_LAUNCHXL, select TI XDS110 Emulator. In both cases, select the cJTAG interface.