Document number
205065
Version
Rev. N
Issue date
2019-02-04
Sirius OBC and TCM User Manual
Page
36
of
174
5.4. SCET
5.4.1. Description
The main purpose of the SCET IP and driver is to track the time since power on and to act
as a source of timestamps. The SCET has also been enhanced with General purpose
triggers and PPS signaling.
The SCET counts in seconds and subseconds, with a subsecond being 2
-16
th of a second,
roughly equivalent to 15.3 µs.
5.4.2. General purpose triggers
To be able to provide more accurate time stamping on external events, the SCET has a
number of general purpose triggers. When a trigger fires, the SCET will sample a subset (24
bits) of the current clock for later software readout, matching the external event to the SCET
time regardless of current software state. The exact functionality connected to each general
purpose trigger and the number available is dependent on the system mapping of the SCET,
e.g. in a System-On-Chip (SoC).
5.4.3. Pulse-Per-Second (PPS) signals
The SCET block is designed to be included in several different units in a system and for time
synchronization between these SCETs; each SCET has the ability to receive and/or transmit
PPS signals using two PPS signals which is intended for off-chip use. The first signal, pps0,
is an input only and intended to be used with a time-aware component such as a GPS
device for synchronizing the SCET counter to real time. The second signal, pps1, is
bidirectional and intended for use in a multi-drop PPS network. One unit in a system can act
as master on the multi-drop PPS network with the other units as slaves, with the ability to
switch master depending on the redundancy concept used.
When the SCET synchronizes the time counter with a PPS signal, it will also monitor this
PPS signal to make sure it arrives as expected within a user set timeframe (PPS threshold).
If input PPS is lost, it requires software interaction to resynchronize to the incoming PPS
pulse. This is to minimize the risk for sudden glitches in the SCET counter depending on the
incoming PPS accuracy and availability. The PPS monitoring will issue interrupts in bare-
metal or messages on the SCET message queue in RTEMS to notify the application if the
PPS has arrived, been lost or been found.
Exp PPS
OK PPS
Missed PPS
Missed PPS
Configured
threshold
(N + 1) s
(N + 2) s
Time
Figure 4 PPS Threshold configuration
