Texas Instruments 3138 155 232931 - Manual (Page 35)

CC2420
SWRS041B Page 35 of 89
Preamble SFD Length
Data transmitted
over RF
SFD Pin
SF
D
tra
ns
m
itte
d
La
st
M
PD
U
by
te
tr
an
sm
itte
d
or
TX
u
nd
er
flo
w
MAC Protocol Data Unit (MPDU)
S T X
O N c
om
m
an
d
str
ob
e
12 symbol periods Automatically generated
preamble and SFD
Data fetched
from TXFIFO
CRC generated
by CC2420
Figure 15. Pin activity example during transmit
14.5 General control and status pins
In receive mode, the FIFOP pin can be
used to interrupt the microcontroller when
a threshold has been exceeded or a
complete frame has been received. This
pin should then be connected to a
microcontroller interrupt pin.
In receive mode, the FIFO pin can be
used to detect if there is data at all in the
receive FIFO.
The SFD pin can be used to extract the
timing information of transmitted and
received data frames. The SFD pin will go
active when a start of frame delimiter has
been completely detected / transmitted.
The SFD pin should preferably be
connected to a timer capture pin on the
microcontroller.
For debug purposes, the SFD and CCA
pins can be used to monitor several status
signals as selected by the IOCFG1
register. See Table 12 and Table 13 for
available signals.
The polarity of FIFO, FIFOP, SFD and CCA
can be controlled by the IOCFG0 register
(address 0x1C).
15 Demodulator, Symbol Synchroniser and Data Decision
The block diagram for the CC2420
demodulator is shown in Figure 16.
Channel filtering and frequency offset
compensation is performed digitally. The
signal level in the channel is estimated to
generate the RSSI level (see the RSSI /
Energy Detection section on page 48 for
more information). Data filtering is also
included for enhanced performance.
With the ±40 ppm frequency accuracy
requirement from [1], a compliant receiver
must be able to compensate for up to 80
ppm or 200 kHz. The
CC2420 demodulator
tolerates up to 300 kHz offset without
significant degradation of the receiver
performance.
Soft decision is used at the chip level, i.e.
the demodulator does not make a decision
for each chip, only for each received
symbol. De-spreading is performed using
over sampled symbol correlators. Symbol
synchronisation is achieved by a
continuous start of frame delimiter (SFD)
search.
When a SFD is detected, data is written to
the RXFIFO and may be read out by the
microcontroller at a lower bit rate than the
250 kbps generated by the receiver.
The CC2420 demodulator also handles
symbol rate errors in excess of 120 ppm
without performance degradation.
Resynchronisation is performed
continuously to adjust for error in the
incoming symbol rate.
The RXCTRL1.RXBPF_LOCUR control bit
should be written to 1.
The MDMCTRL1.CORR_THR control bits
are by default set to 20 defining the
threshold for detecting IEEE 802.15.4 start
of frame delimiters.