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

CC250015 Packet Handling Hardware SupportThe CC2500 has built-in hardware support forpacket oriented radio protocols.In transmit mode, the packet handler will addthe following elements to the packet stored inthe TX FIFO:• A programmable number of preamblebytes.• A two byte synchronization (sync) word.Can be duplicated to give a 4-byte syncword.• Optionally whiten the data with a PN9sequence.• Optionally Interleave and Forward ErrorCode the data.• Optionally compute and add a CRCchecksum over the data field.• The recommended setting is 4-bytepreamble and 4-byte sync word except for500 kbps data rate where therecommended preamble length is 8 bytes.In receive mode, the packet handling supportwill de-construct the data packet:• Preamble detection.• Sync word detection.• Optional one byte address check.• Optionally compute and check CRC.• Optionally append two status bytes (seeTable 21 and Table 22) with RSSI value,Link Quality Indication and CRC status.Bit Field name Description7:0 RSSI RSSI valueTable 21: Received packet status byte 1(first byte appended after the data)Bit Field name Description7 CRC_OK 1: CRC for received data OK (orCRC disabled)0: CRC error in received data6:0 LQI The Link Quality Indicatorestimates how easily a receivedsignal can be demodulatedTable 22: Received packet status byte 2(second byte appended after the data)Note that register fields that control the packethandling features should only be altered whenCC2500 is in the IDLE state.15.1 Data WhiteningFrom a radio perspective, the ideal over the airdata are random and DC free. This results inthe smoothest power distribution over theoccupied bandwidth. This also gives theregulation loops in the receiver uniformoperation conditions (no data dependencies).Real world data often contain long sequencesof zeros and ones. Performance can then beimproved by whitening the data beforetransmitting, and de-whitening in the receiver.With CC2500, this can be done automaticallyby setting PKTCTRL0.WHITE_DATA=1. Alldata, except the preamble and the sync word,are then XOR-ed with a 9-bit pseudo-random(PN9) sequence before being transmitted asshown in Figure 10. At the receiver end, thedata are XOR-ed with the same pseudorandomsequence. This way, the whitening isreversed, and the original data appear in thereceiver.Data whitening can only be used whenPKTCTRL0.CC2400_EN = 0 (default).PRELIMINARY Data Sheet (Rev.1.2) SWRS040A Page 26 of 83