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The fraction of a symbol period used to
change the phase can be modified with the
DEVIATN.DEVIATION_M
equivalent to changing the shaping of the
symbol.
The MSK modulation format implemented in
CC2500
inverts the sync word and data
compared to e.g. signal generators.
17 Received Signal Qualifiers and Link Quality Information
CC2500 has several qualifiers that can be used
to increase the likelihood that a valid sync
word is detected.
17.1
Sync Word Qualifier
If sync word detection in RX is enabled in
register MDMCFG2 the CC2500 will not start
filling the RX FIFO and perform the packet
filtering described in Section 15.3 before a
valid sync word has been detected. The sync
word
qualifier
mode
MDMCFG2.SYNC_MODE and is summarized in
Table 24. Carrier sense in Table 24 is
described in Section 17.4.
MDMCFG2.
Sync Word Qualifier Mode
SYNC_MODE
000
No preamble/sync
001
15/16 sync word bits detected
010
16/16 sync word bits detected
011
30/32 sync word bits detected
100
No preamble/sync, carrier sense
above threshold
101
15/16 + carrier sense above threshold
110
16/16 + carrier sense above threshold
111
30/32 + carrier sense above threshold
Table 24: 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 a programmed
threshold.
Another use of the preamble quality threshold
is as a qualifier for the optional RX termination
timer. See Section 19.7 on page 43 for details.
setting.
This
is
is
set
by
SWRS040C
16.3
Amplitude Modulation
The supported amplitude modulation On-Off
Keying (OOK) simply turns on or off the PA to
modulate 1 and 0 respectively.
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.
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
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
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 ½dB resolution. The
RSSI update rate, f
receiver filter bandwidth (BW
Section 13) and AGCCTRL0.FILTER_LENGTH.
2
f
RSSI
8
2
If PKTCTRL1.APPEND_STATUS is enabled the
RSSI value at sync
CC2500
bit
in
the
, depends on the
RSSI
defined in
channel
BW
channel
FILTER
_
LENGTH
word detection is
Page 34 of 89
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