Texas Instruments 3138 155 232931 - Manual (Page 34)

CC2420
SWRS041B Page 34 of 89
Preamble SFD Length Data received over RF
SFD Pin
FIFO Pin
FIFOP Pin, if threshold
higher than frame length
FIFOP Pin, if threshold
lower than frame length
SF
D
de
te
cte
d
Le
ng
th
b
yte
re
ce
ive
d
La
st
M
PD
U
by
te
re
ce
ive
d
Preamble SFD Length Data received over RF
SFD Pin
FIFO Pin
FIFOP Pin
Ad
dr
es
s r
ec
og
nit
ion
co
m
ple
te
d
MAC Protocol Data Unit (MPDU) with correct address
MAC Protocol Data Unit (MPDU) with wrong address
Address
recognition OK
Address
recognition fails
Figure 13. Pin activity examples during receive
SI - ADDR
TXFIFO
PSDU0 PSDU1
-
SO
- - -
Length Status
- -
PSDU5 PSDU2 PSDU3 RSSI
CSn
SFD
SCLK
FIFOP
FIFO
-
PSDU4
FI
FO
g
oe
s l
ow
w
he
n
re
ad
ou
t o
f la
st
b
yt
e
sta
rts
FI
FO
P
re
m
ain
s
hig
h
as
lo
ng
a
s
nu
m
be
r o
f
by
te
s >
F
IF
OP
_T
HR
-
FCS/Corr
Figure 14. Example of pin activity when reading RXFIFO.
14.4 Transmit mode
During transmit the FIFO and FIFOP pins
are still only related to the RXFIFO. The
SFD
pin is however active during
transmission of a data frame, as shown in
Figure 15.
The SFD pin goes active when the SFD
field has been completely transmitted. It
goes inactive again when the complete
MPDU (as defined by the length field) has
been transmitted or if an underflow is
detected. See the RF Data Buffering
section on page 39 for more information
on TXFIFO underflow.
As can be seen from comparing Figure 13
and Figure 15, the SFD pin behaves very
similarly during reception and transmission
of a data frame. If the SFD pins of the
transmitter and the receiver are compared
during the transmission of a data frame, a
small delay of approximately 2 µs can be
seen because of bandwidth limitations in
both the transmitter and the receiver.