CC1100-RTR1 datasheet(46/100 Pages) TI1

46 / 100 page

CC1100

SWRS038D

Page 46 of 92

19.7 RX Termination Timer

CC1100 has optional functions for automatic

termination of RX after a programmable time.

The main use for this functionality is wake-on-

radio (WOR), but it may be useful for other

applications. The termination timer starts when

in RX state. The timeout is programmable with

the MCSM2.RX_TIME setting. When the timer

expires, the radio controller will check the

condition for staying in RX; if the condition is

not met, RX will terminate.

The programmable conditions are:

•

MCSM2.RX_TIME_QUAL=0

Continue

receive if sync word has been found

•

MCSM2.RX_TIME_QUAL=1

Continue

receive if sync word has been found or

preamble quality is above threshold (PQT)

If the system can expect the transmission to

have started when enabling the receiver, the

MCSM2.RX_TIME_RSSI

function can be used.

The radio controller will then terminate RX if

the first valid carrier sense sample indicates

no carrier (RSSI below threshold). See Section

17.4 on page 39 for details on Carrier Sense.

For ASK/OOK modulation, lack of carrier

sense is only considered valid after eight

symbol

periods.

Thus,

the

MCSM2.RX_TIME_RSSI

function can be used

in ASK/OOK mode when the distance between

“1” symbols is 8 or less.

If RX terminates due to no carrier sense when

the MCSM2.RX_TIME_RSSI function is used,

or if no sync word was found when using the

MCSM2.RX_TIME

timeout function, the chip

will always go back to IDLE if WOR is disabled

and back to SLEEP if WOR is enabled.

Otherwise, the MCSM1.RXOFF_MODE setting

determines the state to go to when RX ends.

This means that the chip will not automatically

go back to SLEEP once a sync word has been

received. It is therefore recommended to

always wake up the microcontroller on sync

word detection when using WOR mode. This

can be done by selecting output signal 6 (see

Table 34 on page 56) on one of the

programmable

GDO

output

pins,

and

programming the microcontroller to wake up

on an edge-triggered interrupt from this GDO

pin.

Data FIFO

The

CC1100 contains two 64 byte FIFOs, one

for received data and one for data to be

transmitted. The SPI interface is used to read

from the RX FIFO and write to the TX FIFO.

Section 10.5 contains details on the SPI FIFO

access. The FIFO controller will detect

overflow in the RX FIFO and underflow in the

TX FIFO.

When writing to the TX FIFO it is the

responsibility of the MCU to avoid TX FIFO

overflow. A TX FIFO overflow will result in an

error in the TX FIFO content.

Likewise, when reading the RX FIFO the MCU

must avoid reading the RX FIFO past its empty

value, since an RX FIFO underflow will result

in an error in the data read out of the RX FIFO.

The chip status byte that is available on the

SO pin while transferring the SPI header

contains the fill grade of the RX FIFO if the

access is a read operation and the fill grade of

the TX FIFO if the access is a write operation.

Section 10.1 on page 26 contains more details

on this.

The number of bytes in the RX FIFO and TX

FIFO can be read from the status registers

RXBYTES.NUM_RXBYTES

and

TXBYTES.NUM_TXBYTES

respectively. If a

received data byte is written to the RX FIFO at

the exact same time as the last byte in the RX

FIFO is read over the SPI interface, the RX

FIFO pointer is not properly updated and the

last read byte is duplicated. To avoid this

problem one should never empty the RX FIFO

before the last byte of the packet is received.

For packet lengths less than 64 bytes it is

recommended to wait until the complete

packet has been received before reading it out

of the RX FIFO.

If the packet length is larger than 64 bytes the

MCU must determine how many bytes can be

read

from

the

FIFO

(RXBYTES.NUM_RXBYTES-1) and the following

software routine can be used:

1. Read

RXBYTES.NUM_RXBYTES

repeatedly at a rate guaranteed to be at

least twice that of which RF bytes are

received until the same value is returned

twice; store value in n

2. If n < # of bytes remaining in packet, read

-1 bytes from the RX FIFO.

CC1100-RTR1 Datasheet(PDF) 46 Page - Texas Instruments

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