CC2400-RTR1 datasheet(41/84 Pages) TI2

41 / 84 page

CC2400

SWRS042A

Page 41 of 83

31 Demodulator, Bit Synchronizer and Data Decision

The block diagram for the demodulator,

data slicer and bit synchronizer is shown

in Figure 20. The built-in bit synchronizer

extracts the data rate and performs data

decision. The data decision is done using

over-sampling and digital filtering of the

incoming

signal.

This

improves

the

reliability of the data transmission and

provides a synchronous clock in the un-

buffered mode. Using the buffered mode

simplifies the data interface further, as

data can be written and read byte-for-byte

in bursts from the FIFO.

The suggested preamble is a 32 bit

‘(0)10101…’ bit pattern, the same as used

by the packet handling support, see page

29.

This

necessary

for

the

bit

synchronizer to synchronize with the

coding correctly.

The data slicer performs the bit decision.

Ideally the two received FSK frequencies

are placed symmetrically around the IF

frequency. However, if there is some

frequency error between the transmitter

and the receiver, the bit decision level

should be adjusted accordingly. In

CC2400

this is done automatically by measuring

the two frequencies and by using the

average value as the decision level.

The digital data slicer in

CC2400 uses an

average value of the minimum and

maximum frequency deviation detected as

the

comparison

level.

The

MDMTST0.AFC_DELTA

set the expected deviation of the incoming

signal. Once a shift in the received

frequency larger than half the expected

separation is detected, a bit transition is

recorded and the average value to be

used by the data slicer is calculated.

The actual number of samples used to find

the averaging value can be programmed

and set higher for better data decision

accuracy. This is controlled by the

AFC_SETTLING[1:0]

bits. If RX data is

present in the channel when the RX chain

is turned on, then the data slicing estimate

will usually give correct results after 4 bits.

The data slicing accuracy will increase

after

this,

depending

the

AFC_SETTLING[1:0]

bits. If the start of

a transmission occurs after the RX chain

is turned on, the minimum number of bit

transitions (or preamble bits) before

correct data slicing will depend on the

AFC_SETTLING[1:0]

bits, as shown in

Table 17. The recommended setting is

11b, requiring 16 data bits of preamble to

fill the averaging filter completely.

The internally calculated average FSK

frequency value gives a measure for the

frequency offset of the receiver compared

to the transmitter. The frequency offset

can

read

from

RSSI.RX_FREQ_OFFSET[7:0]

This

information can also be used for an

automatic frequency control, as described

at page 43.

Figure 20. Demodulator block diagram

Digital IF

filtering

Average

filter

Data

filter

Decimator

Frequency

detector

Data slicer

comparator

Bit

synchronizer

and data

decoder

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