MICRF507 datasheet(34/47 Pages) MICREL

34 / 47 page

Micrel, Inc.

MICRF507

October 2, 2013

Revision 2.2

Message Coding and Formatting

DC Balanced Line Coding

Line coding, diagrammed in Figure 24, is used when a

communication

channel

imposes

constraints

bit

sequences. An encoding stage that ensures DC balance

(equal numbers of ‘1’ and ‘0’ bits and no long consecutive

runs of either) in the encoded bit stream allows the use of

VCO modulation without restrictions on the message bit

stream. Of the coding schemes which meet this need,

Manchester encoding and 3B4B are among the most

commonly used. Programming and performance of the

MICRF507 are based on the encoded bit rate.

In Manchester encoding, each message bit maps to a

word made of two encoded bits as shown in Table 22. The

encoded bit rate is twice the message bit rate so the

encoding overhead is 100%. When selecting PLL loop

filter it is important to note that frequency content of the

encoded bit stream extends down to one-fourth of the

encoded bit rate. That is the case of a 0101… message

bit sequence, which results in a 100110011001… encoded

bit sequence.

Data

Word

“0”

“10”

“1”

“01”

Table 22. Manchester Encoding

Another encoding method, which is much more efficient

than Manchester coding, is 3B4B, where three message

data bits are encoded into a four-line-bit word. The

encoded bit rate is three-quarter times the message bit

rate. For perfect DC balance, a four bit word would have to

have two ‘1’ line bits and two ‘0’ line bits. Only six such

words are possible. Special steps are therefore needed to

deal with the remaining two encodings: whenever 000 and

111 data appear, toggle a flag that remembers whether the

last encoded word was taken from the “Word A” column

and select the respective word from the other column

shown in Table 23.

Data

Word A

Word B

000

1011

0100

001

1100

010

0011

011

1010

100

0101

101

1001

110

0110

111

1101

0010

Table 23. 3B4B Encoding

Data bits

Encoded words

Comments

000 000 000 000

000

1011 0100 1011

0100 1011

A Flag indicates

if “Word A” has

been used

111 111 010 110

000

1101 0010 0011

0110 1011

A Flag indicates

if “Word A” has

been used

Table 24. Example of 3B4B Encoding

Message Formatting: Preamble

Messages are typically preceded by a header consisting of

24 bits in an alternating pattern: 0101… Such a header

can permit the following actions by the receiver prior to the

arrival of information-carrying bits:

•

Reading of RSSI

•

Bit synchronizer lock-up

•

Using the FEE to null out frequency offset

Figure 24. Link Architecture with Encoding

MICRF507 Datasheet(PDF) 34 Page - Micrel Semiconductor

MICRF507 Datasheet(HTML) 34 Page - Micrel Semiconductor

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Datasheet Download

Link URL