Single-Chip Low Cost Low Power RF-Transceiver (Rev. B

CC2500The fraction of a symbol period used tochange the phase can be modified with theDEVIATN.DEVIATION_M setting. This isequivalent to changing the shaping of thesymbol.The MSK modulation format implemented inCC2500 inverts the sync word and datacompared to e.g. signal generators.16.3 Amplitude ModulationThe supported amplitude modulation On-OffKeying (OOK) simply turns on or off the PA tomodulate 1 and 0 respectively.17 Received Signal Qualifiers and Link Quality InformationCC2500 has several qualifiers that can be usedto increase the likelihood that a valid syncword is detected.17.1 Sync Word QualifierIf sync word detection in RX is enabled inregister MDMCFG2 the CC2500 will not startfilling the RX FIFO and perform the packetfiltering described in Section 15.3 before avalid sync word has been detected. The syncword qualifier mode is set byMDMCFG2.SYNC_MODE and is summarized inTable 24. Carrier sense in Table 24 isdescribed in Section 17.4.MDMCFG2.SYNC_MODESync word qualifier mode000 No preamble/sync001 15/16 sync word bits detected010 16/16 sync word bits detected011 30/32 sync word bits detected100 No preamble/sync, carrier senseabove threshold101 15/16 + carrier sense above threshold110 16/16 + carrier sense above threshold111 30/32 + carrier sense above thresholdTable 24: Sync Word Qualifier Mode17.2 Preamble Quality Threshold (PQT)The Preamble Quality Threshold (PQT) syncwordqualifier adds the requirement that thereceived sync word must be preceded with apreamble with a quality above a programmedthreshold.Another use of the preamble quality thresholdis as a qualifier for the optional RX terminationtimer. See Section 19.7 on page 44 for details.The preamble quality estimator increases aninternal counter by one each time a bit isreceived that is different from the previous bit,and decreases the counter by 8 each time abit is received that is the same as the last bit.The threshold is configured with the registerfield PKTCTRL1.PQT. A threshold of 4·PQT forthis counter is used to gate sync worddetection. By setting the value to zero, thepreamble quality qualifier of the sync word isdisabled.A “Preamble Quality Reached” signal can beobserved on one of the GDO pins by settingIOCFGx.GDOx_CFG=8. It is also possible todetermine if preamble quality is reached bychecking the PQT_REACHED bit in thePKTSTATUS register. This signal / bit assertswhen the received signal exceeds the PQT.17.3 RSSIThe RSSI value is an estimate of the signallevel in the chosen channel. This value isbased on the current gain setting in the RXchain and the measured signal level in thechannel.In RX mode, the RSSI value can be readcontinuously from the RSSI status registeruntil the demodulator detects a sync word(when sync word detection is enabled). At thatpoint the RSSI readout value is frozen until thenext time the chip enters the RX state. TheRSSI value is in dBm with ½dB resolution. TheRSSI update rate, f RSSI , depends on thereceiver filter bandwidth (BW channel defined inSection 13) and AGCCTRL0.FILTER_LENGTH.fRSSI2 ⋅ BW=8⋅2channelFILTER _ LENGTHIf PKTCTRL1.APPEND_STATUS is enabled theRSSI value at sync word detection isSWRS040B Page 35 of 92