CC1110Fx / CC1111Fx
14.10 Received Signal Qualifiers and Link Quality Information
A “Preamble Quality Reached” signal can be
observed on P1_5, P1_6, or P1_7 by setting
IOCFGx.GDOx_CFG=1000. It is also possible
to determine if preamble quality is reached by
checking the PQT_REACHED bit in the
PKTSTATUS register. This signal / bit asserts
when the received signal exceeds the PQT.
CC1110Fx/CC1111Fx has several qualifiers that
can be used to increase the likelihood that a
valid sync word is detected.
14.10.1 Sync Word Qualifier
If sync word detection in RX is enabled in
register MDMCFG2 the CC1110Fx/CC1111Fx will not
start writing received data to the RFD register
and perform the packet filtering described in
Section 14.8.3 before a valid sync word has
been detected. The sync word qualifier mode
is set by MDMCFG2.SYNC_MODE and is
summarized in Table 68. Carrier sense in
Table 68 is described in Section 14.10.4.
14.10.3 RSSI
The RSSI value is an estimate of the signal
level in the chosen channel. This value is
based on the current gain setting in the RX
chain and the measured signal level in the
channel.
In RX mode, the RSSI value can be read
continuously from the RSSI status register until
the demodulator detects a sync word (when
sync word detection is enabled). At that point
the RSSI readout value is frozen until the next
time the chip enters the RX state. The RSSI
value is in dBm with ½ dB resolution. The
RSSI update rate, fRSSI, depends on the
receiver filter bandwidth (BWchannel defined in
MDMCFG2.
Sync Word Qualifier Mode
SYNC_MODE
000
001
010
011
100
No preamble/sync
15/16 sync word bits detected
16/16 sync word bits detected
30/32 sync word bits detected
No preamble/sync, carrier sense
above threshold
Section
14.6)
and
AGCCTRL0.FILTER_LENGTH.
101
110
111
15/16 + carrier sense above threshold
16/16 + carrier sense above threshold
30/32 + carrier sense above threshold
2⋅ BWchannel
fRSSI
=
8⋅2FILTER _ LENGTH
If PKTCTRL1.APPEND_STATUS is enabled the
RSSI value at sync word detection is
automatically added to the first byte appended
after the data payload.
Table 68: Sync Word Qualifier mode
14.10.2 Preamble Quality Threshold (PQT)
The Preamble Quality Threshold (PQT) sync-
word qualifier adds the requirement that the
received sync word must be preceded with a
preamble with a quality above a programmed
threshold.
The RSSI value read from the RSSI status
register is a 2’s complement number. The
following procedure can be used to convert the
RSSI reading to an absolute power level
(RSSI_dBm).
Another use of the preamble quality threshold
is as a qualifier for the optional RX termination
timer. See section 14.12.3 on page 203 for
details.
1) Read the RSSI status register
2) Convert the reading from a hexadecimal
number
to
a
decimal
number
(RSSI_dec)
The preamble quality estimator increases an
internal counter by one each time a bit is
received that is different from the previous bit,
and decreases the counter by 8 each time a bit
is received that is the same as the last bit. The
threshold is configured with the register field
PKTCTRL1.PQT. A threshold of 4·PQT for this
counter is used to gate sync word detection.
By setting the value to zero, the preamble
quality qualifier of the sync word is disabled.
3) If RSSI_dec ≥ 128 then RSSI_dBm =
(RSSI_dec - 256)/2 – RSSI_offset
4) Else if RSSI_dec < 128 then RSSI_dBm
= (RSSI_dec)/2 – RSSI_offset
Table 69 provides typical values for the
RSSI_offset.
Figure 51 and Figure 52 shows typical plots of
RSSI readings as a function of input power
level for different data rates.
SWRS033E
Page 196 of 239
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