MA28140 Datasheet, PDF (23/72 Pages) Dynex Semiconductor – Packet Telecommand Decoder

MA28140
The CPDU Packet is checked in two steps: the clean
validation process and the legal validation process. The clean
verification process performs the following tests:
- correct CRC (last two octets of the Packet contain a 16-bit
CRC calculated using the same algorithm as used for the TC
transfer frame, see section on transfer frame) to verify that
there is no error in the Packet.
- the TC Segment Segmentation Flags (in Segment
Header) are equal to 11.
- the CPDU Packet length is checked to be an even
number of octets, greater than or equal to 10 octets and less
than or equal to 248 octets: 10 octets ≤ TC Packet length =
even number of octets ≤ 248 octets. The CPDU Packet length
is read from Packet Header octets 5 and 6.
- consistency between the actual number of octets making
up the CPDU Packet and the Packet length field. To achieve
this, the Packet Header octet 5 is checked to be zero and the
Packet Header octet 6 is checked to be consistent with the
effective packet length.
At this level, if the packet is found to be error-free, it is
declared clean and the process continues. Otherwise, the
complete CPDU packet is erased.
The legal verification process performs the lollowing tests
on the Packet Header (see ref 1, Section 8):
- the first octet of the Packet Header (version number &
type bit & data fields header flag & 3 MSBs of Application
Process Identifier) is compared with the value programmed in
ROM at address 006.
- the second octet (8 LSBs of Application Process
Identifier) is compared with the value programmed in ROM at
address 007.
- in the third octet (sequence flags & 6 MSBs of packet
name or sequence count), only the sequence flags field is
checked by the PTD to be equal to 11. The packet name or
sequence count is not verified, it is only reported in the CPDU
status.
- the fourth octet (8 LSBs of packet name or sequence
count) is not tested since the packet name or sequence count
is not verified. It is only reported in the CPDU Status.
If the above check succeeds, the TC Packet is declared
legal and its Application Data (command instructions) read out
and executed as described in the next subsection. If the check
fails, the Packet is erased.
Processing the Application Data
The CPDU receives a segment from the segment layer and
stores it for further processing in the CPDU buffer provided in
RAM. At the same time, the clean process is performed. This
segment duplication is necessary due to delayed command
execution. The duration of the transfer is equal to:
Td = Nb * Tacc
where Nb is the number of octets of the TC segment
(including AU tail), and Tacc is the duration of three RAM
accesses Read - Write - Read (the last read is used for
computing the CRC with data effectively stored in RAM). Tacc
can be estimated to 20*Tck (5 µs for a clock frequency of 4
MHz).
The application data of the CPDU packet should consist of
at least one command instruction in the form of one double
octet, or several of such double-octet command instructions,
up to the maximum capacity.
Each double octet should be formatted as follows:
• first octet: specifies one of 256 Command Pulse outputs.
The command distribution shall be made by an external
demultiplexer (256 possible command pulse outputs).
• Second octet: specifies the duration of the Command
Pulse to be issued on the specified output as follows:
- the 5 MSBs are ignored by the CPDU,
- the 3 LSBs specify the duration of the Command
Pulse, which is equal to about 2^X multiplied by D
where X is the value of the 3 LSBs and
D = 40960 clock periods for CPDUDIV=0,
D = 8192 clock periods for CPDUDIV=1.
(see section 5.5 for exact figures.)
When there is more than one command instruction in the
CPDU Packet, each instruction is executed one after the other
in the same sequence as in the packet.
The maximum capacity of the CPDU packet is:
• 248 octets corresponding to 120 command instructions if
the Internal or External Authentication Unit is disabled
(AUDIS=1) or if the External Authentication Unit is enabled
(AUEXT=1 and AUDIS=0) and AUTSL=1.
• 238 octets corresponding to 115 command instructions if
the Internal Authentication Unit is enabled (AUEXT=0 and
AUDIS=0) or if the External Authentication Unit is enabled
(AUEXT=1 and AUDIS=0) and AUTSL=0.
For the calibrated pulses being output on the CPDUEN pin,
the pulse amplification shall be made by external hardware.
The CPDU provides a 16 bit status, CPDUS, that can be
fetched by the telemetry system. The different fields of the
CPDU status are detailed later.
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