Single-Chip Low Cost Low Power RF-Transceiver (Rev. A)

CC2500Carrier Sense (CS) can be used as a syncword qualifier that requires the signal level tobe higher than the threshold for a sync wordsearch to be performed. The signal can alsobe observed on one of the GDO pins and inthe status register bit PKTSTATUS.CS.Other uses of Carrier Sense include the TX-If-CCA function (see Section 17.5 on page 33)and the optional fast RX termination (seeSection 19.7 on page 38).CS can be used to avoid interference from e.g.WLAN.17.4.1 CS Absolute ThresholdThe absolute threshold related to the RSSIvalue is given by:THRRSSI= MAGN _ TARGET +CARRIER _ SENSE _ ABS _ THR − GAINMAXThe maximum possible gain can be reducedusing the AGCCTRL2.MAX_LNA_GAIN andAGCCTRL2.MAX_DVGA_GAIN register fields.CARRIER_SENSE_ABS_THR is programmablein 1 dB steps from -7 dB to + 7dB. Table 26and Table 27 show the RSSI readout valuesat the CS threshold at 2.4 kbps and 250 kbpsdata rate respectively. The defaultCARRIER_SENSE_ABS_THR = 0 (0 dB) andMAGN_TARGET = 3 (33 dB) have been used.MAX_LNA_GAIN[2:0]MAX_DVGA_GAIN[1:0]00 01 10 11000 -96 -90 -84 -78.5001 -94.5 -89 -83 -77.5010 -92.5 -87 -81 -75011 -91 -85 -78.5 -73100 -87.5 -82 -76 -70101 -85 -79.5 -73.5 -67.5110 -83 -76.5 -70.5 -65111 -78 -72 -66 -60Table 27: Typical RSSI value in dBm at CSthreshold with default MAGN_TARGET at250 kbpsIf the threshold is set high, i.e. only strongsignals are wanted, the threshold should beadjusted upwards by first reducing theMAX_LNA_GAIN value and then theMAX_DVGA_GAIN value. This will reducepower consumption in the receiver front end,since the highest gain settings are avoided.The MAGN_TARGET setting is a compromisebetween blocker tolerance/selectivity andsensitivity. The value sets the desired signallevel in the channel into the demodulator.Increasing this value reduces the headroomfor blockers, and therefore close-in selectivity.MAX_LNA_GAIN[2:0]MAX_DVGA_GAIN[1:0]00 01 10 11000 -99 -93 -87 -81.5001 -97 -90.5 -85 -78.5010 -93.5 -87 -82 -76011 -91.5 -86 -80 -74100 -90.5 -84 -78 -72.5101 -88 -82.5 -76 -70110 -84.5 -78.5 -73 -67111 -82.5 -76 -70 -64Table 26: Typical RSSI value in dBm at CSthreshold with default MAGN_TARGET at 2.4kbps17.4.2 CS relative thresholdThe relative threshold detects sudden changesin the measured signal level. This setting is notdependent on the absolute signal level and isthus useful to detect signals in environmentswith a time varying noise floor. The registerfield AGCCTRL1.CARRIER_SENSE_REL_THRis used to enable/disable relative CS, and toselect threshold of 6 dB, 10 dB or 14 dB RSSIchange17.5 Clear Channel Assessment (CCA)The Clear Channel Assessment is used toindicate if the current channel is free or busy.The current CCA state is viewable on any ofthe GDO pins.MCSM1.CCA_MODE selects the mode to usewhen determining CCA.When the STX or SFSTXON command strobe isgiven while CC2500 is in the RX state, the TXstate is only entered if the clear channelrequirements are fulfilled. The chip willPRELIMINARY Data Sheet (Rev.1.2) SWRS040A Page 33 of 83