CC2420ZRTC Datasheet, PDF (39/96 Pages) Texas Instruments – 2.4 GHz IEEE 802.15.4 / ZigBee-ready RF Transceiver

Length byte
Data in RXFIFO
n
MPDU₁
MPDU₂
CC2420
MPDU
MPDU_n-2
RSSI
(signed)
CRC / Corr
Bit number 7 6 5 4 3 2 1 0
CRC
OK
Correlation value (unsigned)
Figure 21. Data in RXFIFO when MDMCTRL0.AUTOCRC is set
17 RF Data Buffering
CC2420 can be configured for different
transmit and receive modes, as set in the
MDMCTRL1.TX_MODE
and
MDMCTRL1.RX_MODE control bits.
Buffered mode (mode 0) will be used for
normal operation of CC2420, while other
modes are available for test purposes.
17.1 Buffered transmit mode
In buffered transmit mode (TX_MODE 0),
the 128 byte TXFIFO, located in CC2420
RAM, is used to buffer data before
transmission. A preamble sequence
(defined in the Frame Format section
below) is automatically inserted before the
length field during transmission. The
length field must always be the first byte
written to the transmit buffer for all frames.
A TXFIFO underflow is issued if too few
bytes are written to the TXFIFO.
Transmission is then automatically
stopped. The underflow is indicated in the
TX_UNDERFLOW status bit, which is
returned during each address byte and
each byte written to the TXFIFO. The
underflow bit is only cleared by issuing a
SFLUSHTX command strobe.
The TXFIFO can only contain one data
frame at a given time.
After complete transmission of a data
frame, the TXFIFO is automatically refilled
with the last transmitted frame. Issuing a
new STXON or STXONCCA command
strobe will then cause CC2420 to retransmit
the last frame.
Writing one or multiple bytes to the
TXFIFO is described in the FIFO access
section on page 31. Reading data from the
TXFIFO is possible with RAM access, but
this does not remove the byte from the
FIFO.
Writing to the TXFIFO after a frame has
been transmitted will cause the TXFIFO to
be automatically flushed before the new
byte is written. The only exception is if a
TXFIFO underflow has occurred, then a
SFLUSHTX command strobe is required.
Transmission is enabled by issuing a
STXON or STXONCCA command strobe.
See the Radio control state machine
section on page 43 for an illustration of
how the transmit command strobes affect
the state of CC2420. The STXONCCA strobe
is ignored if the channel is busy. See the
Clear Channel Assessment section on
page 50 for details on CCA.
The preamble sequence is started 12
symbol periods after the command strobe.
After the programmable start of frame
delimiter has been transmitted, data is
fetched from the TXFIFO.
17.2 Buffered receive mode
In buffered receive mode (RX_MODE 0),
the 128 byte RXFIFO, located in CC2420
RAM, is used to buffer data received by
the demodulator. Accessing data in the
RXFIFO is described in the FIFO access
section on page 31.
The FIFO and FIFOP pins are used to
assist the microcontroller in supervising
the RXFIFO. Please note that the FIFO
and FIFOP pins are only related to the
RXFIFO, even if CC2420 is in transmit
mode.
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