CC2520 DATASHEET
2.4 GHZ IEEE 802.15.4/ZIGBEE® RF TRANSCEIVER
SWRS068 – DECEMBER 2007
Tips and Tricks
•
The source address table can be modified safely during frame reception. If one address replaces another
while the receiver is active, the corresponding enable bit should be turned off during the modification.
This will prevent CC2520 from using a combination of old and new values, because it will only consider
entries that are enabled throughout the whole source matching process.
The following measures can be taken to avoid that the next received frame overwrites the results from
source address matching:
•
Use the appended SRCRESINDEX result instead of the value written to RAM (this is the recommended
approach).
•
Read the results from RAM before RX_FRM_ACCEPTED occurs in the next received frame. For the
shortest frame type this will happen after the sequence number, so the total available time (absolute
worst-case with a small safety margin) becomes:
16 µs (required preamble) + 32 µs (SFD) + 128 µs (4 bytes) = 176 µs
•
To increase the available time, clear the FSMCTRL.RX2RX_TIME_OFF bit. This will add another
192
µs,
for a total of
368 µs.
This will also reduce the risk of RX overflow.
20.3.4 Frame Check Sequence
In receive mode the FCS is verified by hardware if FRMCTRL0.AUTOCRC is enabled. The user is normally
only interested in the correctness of the FCS, not the FCS sequence itself. The FCS sequence itself is
therefore not written to the RX FIFO during receive. Instead, when FRMCTRL0.AUTOCRC is set the two
FCS bytes are replaced by other more useful values. Which values that are substituted for the FCS
sequence is configurable in the FRMCTRL0 register.
Figure 25: Data in RX FIFO for different settings.
Field descriptions:
•
The
RSSI
value is measured over the first 8 symbols following the SFD.
•
The
CRC_OK
bit indicates whether the FCS is correct (1) or incorrect (0). When incorrect, software is
responsible for discarding the frame.
•
The
correlation value
is the average correlation value over the 8 first symbols following the SFD.
•
SRCRESINDEX
is the same value that is written to RAM after completion of source address matching.
Calculation of the LQI value used by IEEE 802.15.4 is described in section 20.5.
80
WWW.TI.COM
TI [ TEXAS INSTRUMENTS ]
