Aerospace and Electronic Systems Magazine August 2016 - 14
Design of Telecommand Software for the Mars Orbiter Mission
Figure 4.
Telecommand core card.
The Watch Dog Timer is implemented in hardware, which
resets the processor periodically if the software failed to reset it
within a particular period. The software also gets system identification (ID) to distinguish between main and redundant systems.
Therefore, the software is the same in the main and redundant systems but configures itself differently by seeing the system ID.
MT c P SOFTWARE ARCHITECTURE
The on-board software execution is under the control of a Real
Time Executive (RTE). The RTE is an infinite loop repeated every
major cycle comprising of four minor cycles. The RTE executes
the MTcP software functionalities in round robin fashion in the
RTE cycle. The MTcP software architecture is general purpose,
flexible, and robust. A modified water fall model [17], [18] shown
in Figure 5 is used for the development of MTcP software.
Figure 5.
Modified waterfall model for MTcP software design.
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Figure 6.
RTE for MTcP software.
The modified waterfall model is used to maintain the design
configuration of the component developments. Each phase has
a specific deliverable and a review process. It serves to robustly
check operations to validate software changes still resulting in
maintained or improved operations.
The MTcP software consists of a power-on initialization routine
and a major cycle loop, which runs infinitely. The major cycle loop
consists of four minor cycles. Various MTcP functionalities are time
sliced/shared within these four minor cycles. The software architecture is shown in Figure 6. Clock driven cyclic scheduling [19] is
used with minor cycle time of 16 ms. The minor cycle time is chosen
based on software performance under a full load test. Four minor cycles are used as it is sufficient for all the tasks to complete and, also,
the generation of the timing reference is easy as it is a power of two.
The various tasks scheduled in different minor cycles are as
shown in Table 1. All the tasks within a minor cycle must not take
more than 75% of the maximum allotted time according to ISRO
Quality Assurance (QA) guidelines. The minor cycle time is optimized to reduce context switches and to satisfy the time deadlines
for various functionalities. For example, telemetry frame rate puts
a constraint on processing telemetry data within a frame time. EBC
may have conditions programmed to check data from a same teIEEE A&E SYSTEMS MAGAZINE
AUGUST 2016
Table of Contents for the Digital Edition of Aerospace and Electronic Systems Magazine August 2016