micrf507ymltr Micrel Semiconductor, micrf507ymltr Datasheet - Page 34

• Manufacturer Part Number: micrf507ymltr
• Description: 470mhz To 510mhz Low-power Fsk Transceiver With +10dbm Power Amplifier
• Manufacturer: Micrel Semiconductor

Message Coding and Formatting
DC Balanced Line Coding
Line coding, diagrammed in Figure 24, is used when a
communication channel imposes constraints on 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.
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.
March 2010
Micrel, Inc.
Data
“0”
“1”
Table 22. Manchester Encoding
Word
“10”
“01”
Figure 24. Link Architecture with Encoding
34
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:
Data bits
000 000 000 000
000
111 111 010 110
000
•
•
•
Reading of RSSI
Bit synchronizer lock-up
Using the FEE to null out frequency offset
Data
000
001
010
011
100
101
110
111
Table 24. Example of 3B4B Encoding
Table 23. 3B4B Encoding
Encoded words
1011 0100 1011
0100 1011
1101 0010 0011
0110 1011
Word A
1011
1100
0011
1010
0101
1001
0110
1101
Comments
A Flag indicates
if “Word A” has
been used
A Flag indicates
if “Word A” has
been used
M9999-032210-B
Word B
0100
0010
MICRF507