CC1101
SWRS061H Page 44 of 98
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
15/16 + carrier sense above threshold
16/16 + carrier sense above threshold
30/32 + carrier sense above threshold
Table 30: Sync Word Qualifier Mode
17.2 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 the
programmed threshold.
Another use of the preamble quality threshold
is as a qualifier for the optional RX termination
timer. See Section 19.7 for details.
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 eight 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.
A “Preamble Quality Reached” signal can be
observed on one of the GDO pins by setting
IOCFGx.GDOx_CFG=8. 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.
17.3 RSSI
The RSSI value is an estimate of the signal
power 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 given in dBm with a ½ dB
resolution. The RSSI update rate, f
RSSI
,
depends on the receiver filter bandwidth
(BW
channel
is defined in Section 13) and
AGCCTRL0.FILTER_LENGTH.
LENGTHFILTER
channel
RSSI
BW
f
_
28
2
If PKTCTRL1.APPEND_STATUS is enabled,
the last RSSI value of the packet is
automatically added to the first byte appended
after the payload.
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)
1) Read the RSSI status register
2) Convert the reading from a hexadecimal
number to a decimal number (RSSI_dec)
3) If RSSI_dec ≥ 128 then RSSI_dBm =
(RSSI_dec - 256)/2 – RSSI_offset
Note: It takes some time from the radio
enters RX mode until a valid RSSI value is
present in the RSSI register. Please see
DN505 [12] for details on how the RSSI
response time can be estimated.
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