CC2520_12 Datasheet, PDF (77/133 Pages) Texas Instruments – 2.4 GHZ IEEE 802.15.4/ZIGBEE® RF TRANSCEIVER

CC2520 DATASHEET
2.4 GHZ IEEE 802.15.4/ZIGBEE® RF TRANSCEIVER
SWRS068 – DECEMBER 2007
• Set FRMFILT1.ACCEPT_FT2_ACK after successfully starting a transmission with acknowledgment
request, and clear the bit again after the acknowledgment frame has been received, or the timeout
has been reached.
• Keep the bit cleared otherwise.
It is not necessary to turn off the receiver while changing the values of the FRMFILT0/1 registers and the
local address information stored in RAM. However, if the changes take place between reception of the SFD
byte and the source PAN ID (i.e. between the SFD and RX_FRM_ACCEPTED exceptions), the modified
values must be considered as don’t care for that particular frame (CC2520 will use either the old or the new
value).
Note that it is possible to make CC2520 ignore all IEEE 802.15.4 incoming frames by setting
MDMTEST1.MODULATION_MODE=’1’.
20.3.3 Source Address Matching
CC2520 supports matching of the source address in received frames against a table stored in the on-chip
memory. The table is 96 bytes long, and hence it can contain up to:
• 24 short addresses (2 + 2 bytes each)
• 12 IEEE extended addresses (8 bytes each).
Source address matching will only be performed when frame filtering is also enabled, and the received
frame has been accepted. The function is controlled by:
• The SRCMATCH, SRCSHORTEN0, SRCSHORTEN1, SRCSHORTEN2, SRCEXTEN0,
SRCEXTEN1 and SRCEXTEN2 registers
• The source address table in RAM.
Applications
Automatic acknowledgment transmission with correct setting of the frame pending bit: When using indirect
frame transmission, the devices will send data requests to poll frames stored on the coordinator. To indicate
whether it actually has a frame stored for the device, the coordinator must set or clear the frame pending bit
in the returned acknowledgment frame. On most 8- and 16-bit MCUs, however, there is not enough time to
determine this, and so the coordinator ends up setting the pending bit regardless of whether there are
pending frames for the device (as required by IEEE 802.15.4 [2]). This is wasteful in terms of power
consumption, because the polling device will have to keep its receiver enabled for a considerable period of
time, even if there are no frames for it. By loading the destination addresses in the indirect frame queue into
the source address table and enabling the AUTOPEND function, CC2520 will set the pending bit in outgoing
acknowledgment frames automatically. This way the operation is no longer timing critical, as the effort done
by the microcontroller is when adding or removing frames in the indirect frame queue and updating the
source address table accordingly.
Security material look-up: To reduce the time needed to process secured frames, the source address table
can be set up so the entries match the table of security keys on the microcontroller. A second level of
masking on the table entries allows this application to be combined with automatic setting of the pending bit
in acknowledgment frames.
Other applications: The two previous applications are the main targets for the source address matching
function. However, for proprietary protocols that only rely on the basic IEEE 802.15.4 frame format, there
are several other useful applications. For instance, by using it together with the exception binding
mechanism, it is possible to create firewall functionality where only a specified set of nodes will be
acknowledged.
The Source Address Table
The source address table begins at address 0x380 in RAM as shown in Figure 11. The space is shared
between short and extended addresses, and the SRCSHORTEN0/1/2 and SRCEXTEN0/1/2 registers are
used to control which entries are enabled. All values in the table are little-endian (as in the received frames).
• A short address entry starts with the 16-bit PAN ID followed by the 16-bit short address. These
entries are stored at address 0x380 + (4 × n), where n is a number between 0 and 23.
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