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CC2500-RTR1 数据表(PDF) 33 Page - Texas Instruments |
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CC2500-RTR1 数据表(HTML) 33 Page - Texas Instruments |
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33 / 84 page  CC2500 PRELIMINARY Data Sheet (Rev.1.2) SWRS040A Page 33 of 83 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 on one of the GDO pins and in the 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 (see Section 19.7 on page 38). CS can be used to avoid interference from e.g. WLAN. 17.4.1 CS Absolute Threshold The absolute threshold related to the RSSI value is given by: MAX RSSI GAIN THR ABS SENSE CARRIER TARGET MAGN THR − + = _ _ _ _ The maximum possible gain can be reduced using the AGCCTRL2.MAX_LNA_GAIN and AGCCTRL2.MAX_DVGA_GAIN register fields. CARRIER_SENSE_ABS_THR is programmable in 1 dB steps from -7 dB to + 7dB. Table 26 and Table 27 show the RSSI readout values at the CS threshold at 2.4 kbps and 250 kbps data rate respectively. The default CARRIER_SENSE_ABS_THR = 0 (0 dB) and MAGN_TARGET = 3 (33 dB) have been used. MAX_DVGA_GAIN[1:0] 00 01 10 11 000 -99 -93 -87 -81.5 001 -97 -90.5 -85 -78.5 010 -93.5 -87 -82 -76 011 -91.5 -86 -80 -74 100 -90.5 -84 -78 -72.5 101 -88 -82.5 -76 -70 110 -84.5 -78.5 -73 -67 111 -82.5 -76 -70 -64 Table 26: Typical RSSI value in dBm at CS threshold with default MAGN_TARGET at 2.4 kbps MAX_DVGA_GAIN[1:0] 00 01 10 11 000 -96 -90 -84 -78.5 001 -94.5 -89 -83 -77.5 010 -92.5 -87 -81 -75 011 -91 -85 -78.5 -73 100 -87.5 -82 -76 -70 101 -85 -79.5 -73.5 -67.5 110 -83 -76.5 -70.5 -65 111 -78 -72 -66 -60 Table 27: Typical RSSI value in dBm at CS threshold with default MAGN_TARGET at 250 kbps If the threshold is set high, i.e. only strong signals are wanted, the threshold should be adjusted upwards by first reducing the MAX_LNA_GAIN value and then the MAX_DVGA_GAIN value. This will reduce power consumption in the receiver front end, since the highest gain settings are avoided. The MAGN_TARGET setting is a compromise between blocker tolerance/selectivity and sensitivity. The value sets the desired signal level in the channel into the demodulator. Increasing this value reduces the headroom for blockers, and therefore close-in selectivity. 17.4.2 CS relative threshold The relative threshold detects sudden changes in the measured signal level. This setting is not dependent on the absolute signal level and is thus useful to detect signals in environments with a time varying noise floor. The register field AGCCTRL1.CARRIER_SENSE_REL_THR is used to enable/disable relative CS, and to select threshold of 6 dB, 10 dB or 14 dB RSSI change 17.5 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 on any of the GDO pins. MCSM1.CCA_MODE selects the mode to use when determining CCA. When the STX or SFSTXON command strobe is given while CC2500 is in the RX state, the TX state is only entered if the clear channel requirements are fulfilled. The chip will |
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