MA28140 Datasheet, PDF (6/72 Pages) Dynex Semiconductor – Packet Telecommand Decoder

MA28140
4. PTD FUNCTIONAL DESCRIPTION
4.1 CODING LAYER
Overview of the Layer
The coding layer provides the forward error correction
capability and synchronisation services used by the Transfer
layer. Each Transfer Frame is encoded/embedded in one
CLTU (Command Link Transmission Unit), which is the
protocol-data unit of the coding layer. At the receiving end of
the Coding Layer, a “dirty” symbol stream (plus control
information on whether the physical channel is active or
inactive) is received from the layer below. Searching for the
Start Sequence, the coding layer finds the beginning of a
CLTU and decodes the TC Codeblocks. As long as no errors
are detected, or errors are detected and corrected, the coding
layer passes “clean” octets of data to the Transfer layer.
Should any codeblock contain an uncorrectable error, this
Codeblock is abandoned and considered as Tail Sequence, no
further data is passed to the layer above and the Coding Layer
returns to a Start Sequence searching mode until it detects
one. The coding layer also generates part of the CLCW and
FAR status.
The PTD can handle up to 6 TC input interfaces, the data
bit rate on these inputs should not exceed 50 Kbits per second
when using the Authentication Unit. If the Authenication unit is
not used the symbol rate could exceed 200kBits/sec (not
guaranteed).
Standard Data Structures Within the Layer
A CLTU is made up of three distinct protocol data
elements:
- one 16-bit Start Sequence,
- one or more TC Codeblocks of a fixed length of 8 octets
to encode the protocol data unit from the layer above,
- one Tail Sequence of length equal to that of the TC
Codeblock, i.e. 8 octets.
Start
First
Sequence Codeblock
••••••••••
Last
Tail
Codeblock Sequence
16 Bits
Variable Number of Codeblocks 8 Octets
The Start Sequence marks the beginning of the TC
Codeblock field within a CLTU. It consists of a 16-bit
synchronisation pattern represented in hexadecimal as EB90,
where the first transmitted octet is EB.
The TC Codeblock field consists of one or more TC
Codeblocks. The codeblock length of received data is fixed
and set to 8 octets (information field: 7 octets).
The Tail Sequence marks the end of the TC Codeblock
Field within a CLTU. The length of the Tail Sequence is that of
a TC Codeblock. Reference 1 specifies that its pattern should
be alternating “zeros” and “ones”, ending with a “one” (55 ....
55 in hexadecimal), but any double error codeblock, or single
error codeblock with filler bit equal to 1 will be interpreted as
Tail Sequence by the PTD.
Synchronization and TC Input Selection
Synchronization is performed by searching for the Start
Sequence simultaneously on all active TC inputs. The Start
Sequence detection allows one bit error anywhere in the 16-bit
pattern. Furthermore due to NRZ coding ambiguity on the
incoming bit stream, it is possible to detect the inverted Start
Sequence pattern in order to choose between positive or
negative representation for further NRZ data processing. If an
inverted Start Sequence is detected, the following bit stream is
inverted until the Tail Sequence is encountered.
Two different modes to perform the TC channel selection
are supported, selectable with the PRIOR configuration input:
Standard Mode (PRIOR = 0), in which all TC inputs TC0 to
TC5 have the same priority, and the search for a Start
Sequence is performed on all active TC channels
simultaneously.
Priority Mode (PRIOR = 1), in which two inputs are
assigned an absolute priority.
Note: This mode is not compliant with Ref. 1, and is
intended for applications with specific requirements on
unconditional access to the TC decoder. If this mode is used,
a thorough analysis of potential failure modes and the built-in
timeout mechanisms is recommended.
Standard Mode
The TC input selection locks the selection multiplexer on
the first TC channel where the Start Sequence is found. The
selection mechanism is restarted once a Tail Sequence or a
codeblock rejection has been detected. Furthermore, as a
protection mechanism in case of RF receiver breakdown, a
timeout mechanism is provided; if the TC channel clock is not
detected during a certain time, the TC selection mechanism is
reactivated in order not to remain lacked on a Channel without
a clock signal.
The timeout value between two successive edges of the
TC channel clock is: 3932160
tCK < TC clock timeout < 4587520 tCK, with tCK being the
system clock period. With a
system clock frequency fCK of 4MHz this equals 0.98s
<TC clock timeout < 1.15s.
Information Field
7 Octets
P0 (MSB)
P6 P7 (LSB)
Error Control Field Filler Bit
7 parity bits
1 Octet
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