CC2500-RTR1 datasheet(30/96 Pages) TI

30 / 96 page

CC2500

SWRS040C

Page 30 of 89

The

preamble

pattern

alternating

sequence of ones and zeros (101010101…).

The minimum length of the preamble is

programmable.

When

enabling

TX,

the

modulator will start transmitting the preamble.

When the programmed number of preamble

bytes has been transmitted, the modulator will

send the sync word and then data from the TX

FIFO if data is available. If the TX FIFO is

empty, the modulator will continue to send

preamble bytes until the first byte is written to

the TX FIFO. The modulator will then send the

sync word and then the data bytes. The

number of preamble bytes is programmed with

the MDMCFG1.NUM_PREAMBLE value.

The synchronization word is a two-byte value

set in the SYNC1 and SYNC0 registers. The

sync word provides byte synchronization of the

incoming packet. A one-byte sync word can be

emulated by setting the SYNC1 value to the

preamble pattern. It is also possible to emulate

bit

sync

word

using

MDMCFG2.SYNC_MODE

=3 or 7. The sync word

will then be repeated twice.

CC2500 supports both fixed packet length

protocols and variable packet length protocols.

Variable or fixed packet length mode can be

used for packets up to 255 bytes. For longer

packets, infinite packet length mode must be

used.

Fixed packet length mode is selected by

setting PKTCTRL0.LENGTH_CONFIG=0. The

desired packet length is set by the PKTLEN

variable

packet

length

mode,

PKTCTRL0.LENGTH_CONFIG

=1,

the packet

length is configured by the first byte after the

sync word. The packet length is defined as the

payload data, excluding the length byte and

the optional CRC. The PKTLEN

used to set the maximum packet length

allowed in RX. Any packet received with a

length byte with a value greater than PKTLEN

will be discarded.

With

PKTCTRL0.LENGTH_CONFIG

=2,

the

packet length is set to infinite and transmission

and reception will continue until turned off

manually. As described in the next section, this

can be used to support packet formats with

different length configuration than natively

supported by

CC2500. One should make sure

that TX mode is not turned off during the

transmission of the first half of any byte. Refer

to the

CC2500

Errata Notes [1] for more

details.

Note

that

the

minimum

packet

length

supported (excluding the optional length byte

and CRC) is one byte of payload data.

15.2.1 Arbitrary Length Field Configuration

The packet length register, PKTLEN, can be

reprogrammed during receive and transmit. In

combination with fixed packet length mode

(PKTCTRL0.LENGTH_CONFIG=0) this opens

the possibility to have a different length field

configuration

than

supported

for

variable

length packets (in variable packet length mode

the length byte is the first byte after the sync

word). At the start of reception, the packet

length is set to a large value. The MCU reads

out enough bytes to interpret the length field in

the packet. Then the PKTLEN value is set

according to this value. The end of packet will

occur when the byte counter in the packet

handler is equal to the PKTLEN register. Thus,

the MCU must be able to program the correct

length, before the internal counter reaches the

packet length.

15.2.2 Packet Length > 256 bytes

Also the packet automation control register,

PKTCTRL0,

can be reprogrammed during TX

and RX. This opens the possibility to transmit

and receive packets that are longer than 256

bytes and still be able to use the packet

handling hardware support. At the start of the

packet,

the

infinite

packet

length

mode

(PKTCTRL0.LENGTH_CONFIG=2)

must

active. On the TX side, the PKTLEN register is

set to mod(length,256). On the RX side the

MCU reads out enough bytes to interpret the

length field in the packet and sets the PKTLEN

256 bytes remains of the packet the MCU

disables infinite packet length mode and

activates fixed packet length mode. When the

internal byte counter reaches the PKTLEN

value, the transmission or reception ends (the

radio

enters

the

state

determined

TXOFF_MODE

or RXOFF_MODE). Automatic

CRC appending/checking can also be used

(by setting PKTCTRL0.CRC_EN=1).

When for example a 600-byte packet is to be

transmitted, the MCU should do the following

(see also Figure 12):



Set PKTCTRL0.LENGTH_CONFIG=2.

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