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Operation
CCA operations
CCA is a method of collision avoidance that is implemented by detecting the energy level on the
transmission channel before starting the transmission. The CCA threshold (defined by the CA
parameter) defines the energy level that it takes to block a transmission attempt. For example, if CCA
is set to the default value of 0x41 (which is interpreted as -65 dBm) then energy detected above the -
65 dBm level (for example -60 dBm) temporarily blocks a transmission attempt. But if the energy level
is less than that (for example -70 dBm), the transmission is not blocked. The intent of this feature is to
prevent simultaneous transmissions on the same channel.
You can disable CCA by setting CA to 0. Disabling CCA can improve latency in noisy environments, but it
can also interfere with other devices that are operating on the same channel.
In the event that the energy level exceeds the threshold, the transmission is blocked for a random
number of backoff periods. The number of backoff periods is defined by the following formula: random
(2^n - 1), where n is defined by the RN parameter and increments after each CCA failure. When RN is
set to its default value of 0, then 2^n -1 is 0, preventing any delay before the first energy detection on
a new frame. However, n increments after each CCA failure, giving a greater range for the number of
backoff periods between each energy detection cycle.
In the event that five energy detection cycles occur and each one detects too much energy, the
application tries again 1 to 48 ms later. After the application retries are exhausted, then the
transmission fails with a CCA error.
Whenever the MAC code reports a CCA failure, meaning that it performed five energy detection cycles
with exponential random back-offs, and each one failed, the EC parameter is incremented. The EC
parameter can be read at any time to find out how noisy the operating channel is. It continues to
increment until it reaches its maximum value of 0xFFFF. To get new statistics, you can set EC back to
0.
Serial interface
The XBee3 802.15.4 RF Module interfaces to a host device through a serial port. The device can
communicate through its serial port with:
Through logic and voltage compatible universal asynchronous receiver/transmitter (UART).
n
Through a level translator to any serial device, for example, through an RS-232 or USB
n
interface board.
Serial receive buffer
When serial data enters the device through the DIN pin, it stores the data in the serial receive buffer
until the device can process it. Under certain conditions, the device may not be able to process data in
the serial receive buffer immediately. If large amounts of serial data are sent to the device such that
the serial receive buffer would overflow, then it discards new data. If the UART is in use, you can avoid
this by the host side by honoring CTS flow control.
Serial transmit buffer
When the device receives RF data, it moves the data into the serial transmit buffer and sends it out
the UART. If the serial transmit buffer becomes full and the system buffers are also full, then it drops
the entire RF data packet. Whenever the device receives data faster than it can process and transmit
the data out the serial port, there is a potential of dropping data.
XBee3® 802.15.4 RF Module User Guide
Serial interface
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