Texas Instruments CC2500 User Manual

CC2500
S
WRS040
C
Page
33
of
89
bytes that contain CRC status, link quality
indication and RSSI value.
15.7
Packet Handling in Firmware
When implementing a packet oriented radio
protocol in firmware, the MCU needs to kno
w
when a packet has been received/transmitted.
Additionally, for packets longer than 64 bytes
the RX FIFO needs to be read whi
le in RX
and
the TX FIFO needs to be refilled while in TX.
This means that the MCU needs to know the
number of bytes that can be r
ead from or
written to the RX FIFO and TX FIFO
respectively. There are two possible solutions
to get the necessary status information:
a) Interrupt driven solution
In both RX and TX one can use one of the
GDO
pins to give an interrupt when a sync word has
been received/transmitted and/or when a
complete packet has been
received/transmitted (
IOCFGx
=
0x06). In
addition, there are
two
configurations for the
IOCFGx
register
that are associated with the
RX
FIFO (
IOCFGx
=
0x00 and
IOCFGx
=
0x01)
and two that are asso
ciated with the TX
FIFO
(
IOCFGx
=
0x02 and
IOCFG
=
0x03) that can be
used as interrupt sources to provide
information on how many bytes are in the RX
FIFO and TX FIFO respectively. See
Table
33
.
b) SPI polling
The
PKTS
TATUS
register can be polled at a
given rate to get information about the current
GDO2
and
GDO0
values respectively. The
RXBYTES
and
TXBYTES
registers can be
polled at a given rate to get information about
the number of bytes in the RX FIFO and TX
FIFO res
pectively. Alternatively, the number of
bytes in the RX FIFO and TX FIFO can be
read from the chip status byte returned on the
MISO line each time a header byte, data byte,
or command strobe is sent on the SPI bus.
It is recommended to employ an interrupt
driven solution as high rate SPI polling will
reduce the RX sensitivity. Furthermore, as
explained in
Section
10.3
and the
CC2500
Errata Note
s
[1]
, when using SPI po
lling there
is a small, but finite, probability that a single
read from registers
PKTSTATUS
,
RXBYTES
and
TXBYTES
is being corrupt. The same is the
case when reading the chip status byte.
Refer to the TI website for SW examples
(
[6]
and
[7]
)
.
16
Modulation Formats
CC2500
supports amplitude, frequency and
phase shift modulation formats. The desired
modulation format is set in the
MDMCFG2.MOD
_FORMAT
register.
Optionally, the data stream can be Manchester
coded by the modulator and decoded by the
demodulator. This option is enabled by setting
MDMCFG2.MANCHESTER_EN
=1
. Manchester
encoding is not supported at the same
time as
using the FEC/Interleaver option.
16.1
Frequency Shift Keying
2
-
FSK can optionally be shaped by a
Gaussian filter with BT=1, producing a GFSK
modulated signal.
The frequency deviation is programmed with
the
DEVIATION_M
and
DEVIATION_E
values
in the
DEVIATN
register. The value has an
exponent/mantissa form, and the resultant
deviation is given by:
E
DEVIATION
xosc
dev
M
DEVIATION
f
f
_
17
2
)
_
8
(
2




The symbol encoding is shown in
Table
23
.
Format
Symbol
Coding
2
-
FSK/
GFSK
‘0’
–
Deviation
‘1’
+ Deviation
Table
23
: Symbol
E
ncoding for
2
-
FSK
/GFSK
M
odulation
16.2
Minimum Shift Keying
When using MSK
1
, the complete transmission
(preamble, syn
c word and payload) will be
MSK modulated.
Phase shifts are performed with a constant
transition time.
1
Identical to offset QPSK with half
-
sine
shaping (data coding may differ)

Texas Instruments CC2500 Specifications

General

Brand	Texas Instruments
Model	CC2500
Category	Transceiver
Language	English