CC112X/CC1175
SWRU295C
Page 39 of 108
RX can be terminated and resumed later.
RX can be restarted to receive the new packet. For this to be successful, the new packet must
have signal energy that is sufficiently higher than the current packet to allow correct demodulation.
This scenario is mainly for high throughput protocols where nodes communicate with several
nodes at various distances.
CC112X
has several signals that can be used to indicate a collision. These are
, and
described in Table 10.
Section 5.9.1 and Section 5.9.2 give two examples on how collision detection can be implemented.
5.9.1
RSSI Based Detection
During packet reception a collision can be detected as a step in RSSI. When the first packet is
detected, the carrier sense level can be changed to a value higher than the RSSI for the current
packet. If a new packet with higher signal power appears on the air, a carrier sense interrupt will tell
the MCU to restart RX. The SRX strobe can be used to immediately restart the demodulator to catch
the incoming packet. Since an SFRX strobe cannot be issued from RX state one should read the
NUM_RXBYTES
register to find out how many bytes belong to the first packet.
5.9.2
Preamble Based Detection
A
more
robust
indicator
is
to
use
the
preamble
detector
(enabled
by
setting
). If a new message is incoming it will start with a preamble. A PQT
interrupt can then be used to indicate a collision. If the data of the current message can contain a
010101 preamble sequence, a check for a step in RSSI should also be done before running the SRX
strobe command. Also in this case one should read the
over again.
5.10
Clear Channel Assessment (CCA)
The Clear Channel Assessment (CCA) is used to indicate if the current channel is free or busy. The
current CCA state is viewable on GPIO1 or GPIO3 by setting
(15). There are also two other flags related to the CCA feature available. These are
and are available on GPIO2 and GPIO0 respectively by
using the same
configuration as for
a decision has been made as to whether the channel is busy or not and
if the radio went to TX or not after
PKT_CFG2.CCA_MODE
selects the mode to use when determining CCA.
CC112X
is in the RX state, the TX or
FSTXON state is only entered if the clear channel requirements are fulfilled (
asserted). Otherwise, the chip will remain in RX. If the channel then becomes available, the radio will
not enter TX or FSTXON state before a new strobe command is sent on the SPI interface
11
. This
feature is called TX on CCA/LBT. Five CCA requirements can be programmed:
Always (CCA disabled, always goes to TX)
If RSSI is below threshold
Unless currently receiving a packet
Both the above (RSSI below threshold and not currently receiving a packet)
If RSSI is below threshold and ETSI LBT [2] requirements are met
11
PKT_CFG2.CCA_MODE = 100b
(LBT) the radio will try to enter TX mode again automatically
until the channel is clear and TX mode is being entered