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

CC2500If PKTCTRL1.APPEND_STATUS is enabled theRSSI value at sync word detection isautomatically added to the first byte appendedafter the data.The RSSI value read from the RSSI statusregister is a 2’s complement number. Thefollowing procedure can be used to convert theRSSI reading to an absolute power level(RSSI_dBm).1) Read the RSSI status register2) Convert the reading from a hexadecimalnumber to a decimal number (RSSI_dec)3) If RSSI_dec ≥ 128 then RSSI_dBm =(RSSI_dec - 256)/2 – RSSI_offset4) Else if RSSI_dec < 128 then RSSI_dBm =(RSSI_dec)/2 – RSSI_offsetTable 25 provides typical values for theRSSI_offset.Figure 13 shows typical plots of RSSI readingsas a function of input power level for differentdata rates.Data rate[kbps]RSSI_offset [decimal]2.4 7110 69250 72500 72Table 25: Typical RSSI_offset values0.0-10.0-20.0-30.0RSSI readout [dBm]-40.0-50.0-60.0-70.0-80.0-90.0-100.0-110.0-120.0-120 -110 -100 -90 -80 -70 -60 -50 -40 -30 -20 -10 0Input power [dBm]2.4 kbps 10 kbps 250 kbps 250 kbps, reduced current 500 kbpsFigure 13: Typical RSSI value vs. input power level for some typical data rates17.4 Carrier Sense (CS)The Carrier Sense flag is used as a sync wordqualifier and for CCA. The CS flag can be setbased on two conditions, which can beindividually adjusted:• CS is asserted when the RSSI is above aprogrammable absolute threshold, and deassertedwhen RSSI is below the samethreshold (with hysteresis).• CS is asserted when the RSSI hasincreased with a programmable number ofdB from one RSSI sample to the next, andde-asserted when RSSI has decreasedwith the same number of dB. This settingis not dependent on the absolute signallevel and is thus useful to detect signals inenvironments with a time varying noisefloor.PRELIMINARY Data Sheet (Rev.1.2) SWRS040A Page 32 of 83