CC2420-RTR1 datasheet(39/92 Pages) TI

39 / 92 page

CC2420

SWRS041B

Page 39 of 89

MPDU

Length byte

MPDU

n-2

RSSI

(signed)

CRC / Corr

Bit number

CRC

Correlation value (unsigned)

Data in RXFIFO

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.

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.

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.

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.

A TXFIFO underflow is issued if too few

bytes

are

written

the

TXFIFO.

Transmission

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 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.

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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