# rfListenBeforeTalk
SysConfig Notice
All examples will soon be supported by SysConfig, a tool that will help you graphically configure your software components. A preview is available today in the examples/syscfg_preview directory. Starting in 3Q 2019, with SDK version 3.30, only SysConfig-enabled versions of examples will be provided. For more information, click here.
Project Setup using the System Configuration Tool (SysConfig)
The purpose of SysConfig is to provide an easy to use interface for configuring drivers, RF stacks, and more. The .syscfg file provided with each example project has been configured and tested for that project. Changes to the .syscfg file may alter the behavior of the example away from default. Some parameters configured in SysConfig may require the use of specific APIs or additional modifications in the application source code. More information can be found in SysConfig by hovering over a configurable and clicking the question mark (?) next to it's name.
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 and CC1352P-4-LAUNCHXL boards).
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 done
The carrier sense command CMD_PROP_CS is prepended by a CMD_NOP in order to set an absolute trigger time TRIG_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 ms
If 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.