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CC1101The preamble pattern is an alternatingsequence of ones and zeros (10101010…).The minimum length of the preamble isprogrammable. When enabling TX, themodulator will start transmitting the preamble.When the programmed number of preamblebytes has been transmitted, the modulator willsend the sync word and then data from the TXFIFO if data is available. If the TX FIFO isempty, the modulator will continue to sendpreamble bytes until the first byte is written tothe TX FIFO. The modulator will then send thesync word and then the data bytes. Thenumber of preamble bytes is programmed withthe MDMCFG1.NUM_PREAMBLE value.The synchronization word is a two-byte valueset in the SYNC1 and SYNC0 registers. Thesync word provides byte synchronization of theincoming packet. A one-byte synch word canbe emulated by setting the SYNC1 value to thepreamble pattern. It is also possible to emulatea 32 bit sync word by usingMDMCFG2.SYNC_MODE set to 3 or 7. The syncword will then be repeated twice.CC1101 supports both constant packet lengthprotocols and variable length protocols.Variable or fixed packet length mode can beused for packets up to 255 bytes. For longerpackets, infinite packet length mode must beused.Fixed packet length mode is selected bysetting PKTCTRL0.LENGTH_CONFIG=0. Thedesired packet length is set by the PKTLENregister.In variable packet length mode,PKTCTRL0.LENGTH_CONFIG=1, the packetlength is configured by the first byte after thesync word. The packet length is defined as thepayload data, excluding the length byte andthe optional CRC. The PKTLEN register isused to set the maximum packet lengthallowed in RX. Any packet received with alength byte with a value greater than PKTLENwill be discarded.With PKTCTRL0.LENGTH_CONFIG=2, thepacket length is set to infinite and transmissionand reception will continue until turned offmanually. As described in the next section, thiscan be used to support packet formats withdifferent length configuration than nativelysupported by CC1101. One should make surethat TX mode is not turned off during thetransmission of the first half of any byte. Referto the CC1101 Errata Notes [1] for more details.Note that the minimum packet lengthsupported (excluding the optional length byteand CRC) is one byte of payload data.15.2.1 Arbitrary Length Field ConfigurationThe packet length register, PKTLEN, can bereprogrammed during receive and transmit. Incombination with fixed packet length mode(PKTCTRL0.LENGTH_CONFIG=0) this opensthe possibility to have a different length fieldconfiguration than supported for variablelength packets (in variable packet length modethe length byte is the first byte after the syncword). At the start of reception, the packetlength is set to a large value. The MCU readsout enough bytes to interpret the length field inthe packet. Then the PKTLEN value is setaccording to this value. The end of packet willoccur when the byte counter in the packethandler is equal to the PKTLEN register. Thus,the MCU must be able to program the correctlength, before the internal counter reaches thepacket length.15.2.2 Packet Length > 255Also the packet automation control register,PKTCTRL0, can be reprogrammed during TXand RX. This opens the possibility to transmitand receive packets that are longer than 256bytes and still be able to use the packethandling hardware support. At the start of thepacket, the infinite packet length mode(PKTCTRL0.LENGTH_CONFIG=2) must beactive. On the TX side, the PKTLEN register isset to mod(length, 256). On the RX side theMCU reads out enough bytes to interpret thelength field in the packet and sets the PKTLENregister to mod(length, 256). When less than256 bytes remains of the packet the MCUdisables infinite packet length mode andactivates fixed packet length mode. When theinternal byte counter reaches the PKTLENvalue, the transmission or reception ends (theradio enters the state determined byTXOFF_MODE or RXOFF_MODE). AutomaticCRC appending/checking can also be used(by setting PKTCTRL0.CRC_EN=1).When for example a 600-byte packet is to betransmitted, the MCU should do the following(see also Figure 12)• Set PKTCTRL0.LENGTH_CONFIG=2.• Pre-program the PKTLEN register tomod(600, 256) = 88.SWRS061C Page 33 of 94