Texas Instruments CC2500 User Manual

CC2500
S
WRS040
C
Page
30
of
89
The preamble pattern is an alternating
sequence of ones and zeros (
1
01010101…).
T
he 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 programme
d 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 th
e
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
a 32 bit sync word by using
MDMCFG2.SYNC_MODE
=3
or 7. The syn
c 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 packet
s
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
register.
In 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
register is
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 t
o 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 len
gth 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 s
tart 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 th
e 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 au
tomation 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 be
active. On the TX side, the
PKTLEN
register is
set to
mod(length,
256)
.
On the RX side the
MCU reads o
ut enough bytes to interpret the
length field in the packet and sets the
PKTLEN
register to
mod(length,
256)
.
When less than
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 by
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
.

Texas Instruments CC2500 Specifications

General

Brand	Texas Instruments
Model	CC2500
Category	Transceiver
Language	English