Document number
205065
Version
Rev. N
Issue date
2019-02-04
Sirius OBC and TCM User Manual
Page
37
of
174
To differentiate between the uses of the PPS signal synchronization methods, the SCET can
be said to operate in a number of different modes: Free-running, Master, Master with time
synchronization and Slave. Please see the explanations below and 5.4.5.1 for an
implementation description.
5.4.3.1. Free-running mode
In this mode, the SCET doesn't use any PPS signals at all. It simply counts the current time
since power on without correlation with anyone else.
5.4.3.2. Master mode
In this mode, the SCET is still counting on its own, but now it also emits a pulse on pps1 for
every second tick, acting as a master on the bidirectional multi-drop PPS network.
5.4.3.3. Master mode with time synchronization
This mode is the same as the previous master mode, with the addition of also synchronizing
the time counter with the incoming pps0 signal. Should the PPS signal on pps0 disappear
for some reason, it will revert back to normal master mode and continue issuing PPS signals
on pps1.
5.4.3.4. Slave mode
In this mode, the SCET will synchronize the time counter with pps1, using the bidirectional
multi-drop PPS network as an input. Should the PPS pulse disappear for some reason, it will
revert to free running mode.
5.4.4. RTEMS API
This API represents the driver interface of the module from an RTEMS user application's
perspective.
The driver functionality is accessed through the RTEMS POSIX API for ease of use. In case
of a failure on a function call, the
errno
value is set for determining the cause.
SCET counter accesses can be done by reading or writing to the device file, modifying the
second and subsecond counter values.
The SCET RTEMS driver also supports a number of different IOCTLs for other operations
which isn't specifically affecting the SCET counter registers.
For event signaling, the SCET driver has a number of message queues, allowing the
application to act upon different events.