CC1100-RTY1 datasheet(37/100 Pages) TI1

37 / 100 page

CC1100

SWRS038D

Page 37 of 92

Received Signal Qualifiers and Link Quality Information

CC1100 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

CC1100 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

set

MDMCFG2.SYNC_MODE

and is summarized in

Table 24. Carrier sense is described in Section

17.4.

MDMCFG2.

SYNC_MODE

Sync Word Qualifier 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

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 on page 46 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 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 synch 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

the

PKTSTATUS

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 in dBm with ½dB resolution. The RSSI

update rate, fRSSI, depends on the receiver

filter bandwidth (BWchannel defined in Section

13) and AGCCTRL0.FILTER_LENGTH.

LENGTH

FILTER

channel

RSSI

⋅

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

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

4) Else if RSSI_dec < 128 then RSSI_dBm =

(RSSI_dec)/2 – RSSI_offset

Table 25 gives typical values for the

RSSI_offset.

Figure 13 and Figure 14 shows typical plots of

RSSI reading as a function of input power

level for different data rates.

CC1100-RTY1 Datasheet(PDF) 37 Page - Texas Instruments

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