CC1110-RTR1 datasheet(166/203 Pages) TI

166 / 203 page

CC1110

CC1110 PRELIMINARY Datasheet (Rev. 1.01) SWRS033A

Page 166 of 202

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 dB with 0.5dB resolution.

If PKTCTRL1.APPEND_STATUS is enabled, a

snapshot of the RSSI during the first 8 bytes of

the packet is automatically added to the end of

each received packet.

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).

15.10.3 Carrier Sense (CS)

The Carrier Sense flag is used as a sync word

qualifier and for CCA. The CS flag can be set

based on two conditions, which can be

individually adjusted:

•

CS is asserted when the RSSI is above a

programmable absolute threshold, and de-

asserted when RSSI is below the same

threshold (with hysteresis).

•

CS is asserted when the RSSI has

increased with a programmable number of

dB from one RSSI sample to the next, and

de-asserted when RSSI has decreased

with the same number of dB. This setting

is not dependent on the absolute signal

level and is thus useful to detect signals in

environments with time varying noise floor.

Carrier Sense (CS) can be used as a sync

word qualifier that requires the signal level to

be higher than the threshold for a sync word

search to be performed. The signal can also

be observed in the status register bit

PKTSTATUS.CS.

Other uses of Carrier Sense include the TX-If-

CCA function (see section 15.10.4 on page

166) and the optional fast RX termination (see

section See section 15.12.3 on page 169 for

details.).

15.10.4 Clear Channel Assessment (CCA)

The Clear Channel Assessment is used to

indicate if the current channel is free or busy.

The current CCA state is viewable in the

PKTSTATUS register

MCSM1.CCA_MODE selects the mode to use

when determining CCA.

When the STX or SFSTXON command strobe is

given while

CC1110 is in the RX state, the TX

state is only entered if the clear channel

requirements are fulfilled. The chip will

otherwise remain in RX. This feature is called

TX if CCA.

Four CCA requirements can be programmed:

•

Always (CCA disabled, always goes to TX)

•

If RSSI is below threshold

•

Unless currently receiving a packet

•

Both the above (RSSI below threshold and

not currently receiving a packet)

15.10.5 Link Quality Indicator (LQI)

The Link Quality Indicator is a metric of the

current quality of the received signal. If

PKTCTRL1.APPEND_STATUS is enabled, the

value is automatically appended to the end of

each received packet. The value can also be

read from the LQI status register. The LQI is

calculated over the 64 symbols following the

sync word (first 8 packet bytes for 2-ary

modulation). LQI is best used as a relative

measurement of the link quality, since the

value is dependent on the modulation format.

15.11 Forward Error Correction with

Interleaving

CC1110 has built-in support for Forward Error

Correction (FEC). To enable this option, set

MDMCFG1.FEC_EN to 1. FEC is employed on

the data field and CRC word in order to reduce

the gross bit error rate when operating near

the sensitivity limit. Redundancy is added to

the transmitted data in such a way that the

receiver can restore the original data in the

presence of some bit errors.

The use of FEC allows correct reception at a

lower SNR, thus extending communication

range. Alternatively, for a given SNR, using

FEC decreases the bit error rate (BER). As the

packet error rate (PER) is related to BER by:

length

packet

BER

PER

)

(

−

a lower BER can be used to allow significantly

longer packets, or a higher percentage of

packets of a given length, to be transmitted

successfully. Finally, in realistic ISM radio

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