Time Glossary#

Time Feature#

score::time#: The Time feature. It provides applications with access to several time bases — local, vehicle and absolute — through a uniform Clock interface, and owns the synchronization of the synchronized time bases.

Core Time Model#

Clock#: A source of time. A clock produces a progressing sequence of TimePoint values for a given clock domain. All clocks are read through a uniform interface, independent of the underlying time base.
Clock domain#: The kind of time a Clock represents (e.g. Vehicle Time or the Local Clock variants). The domain is selected explicitly by the application; each domain has its own TimePoint type and status concept, so time values of different domains are incompatible and cannot be mixed unintentionally.
TimePoint#: A specific point in time issued by a Clock. TimePoints of one clock domain are ordered (the relations equal and less than are defined), so two TimePoints of the same domain can be subtracted to yield a TimeSpan.
TimeSpan#: A duration, i.e. the difference between two TimePoint values of the same clock domain. Supported operations include TimePoint - TimePoint, TimePoint + TimeSpan, TimeSpan + TimeSpan, scaling by a factor, equality and comparison. Subtraction saturates to zero (negative TimeSpans are not produced).
Snapshot#: The value returned when reading a Clock. It bundles a TimePoint with the domain-specific status concept (e.g. Vehicle Time status, or no status for the local clocks), allowing a caller to read the time and judge its quality in a single call.
Epoch#: The reference TimePoint at which a Clock starts counting. The semantics of the epoch are a documented property of the clock (e.g. the Unix system clock epoch is 1970-01-01 00:00:00 UTC).
Resolution#: The smallest time difference an individual TimePoint can represent. For an ideal clock the resolution equals the reciprocal of the frequency, but in practice it may be coarser.
Frequency#: The rate at which a Clock updates the TimePoint values it issues.
Monotonic#: Property of a Clock whose successive TimePoints never decrease (TP[n+1] >= TP[n]). A strictly monotonic clock never repeats a value. The System Clock is not monotonic (it may jump backward).
Steady#: Property of a Clock whose TimePoints advance in fixed increments of exactly 1 / frequency, without jumps or rate adjustments.
Delay Tag#: A timestamp of an event — a datagram, frame, message, or any produced value — expressed in the Vehicle Time base. Comparing a Delay Tag against a later Vehicle Time yields the time elapsed since the event, for example to compensate a transmission delay.

Clocks and Clock Domains#

Naming convention: a “… Time” term names a clock domain — a time base, i.e. the kind of time and its properties (epoch, monotony, status concept, synchronization source). A “… Clock” term names the Clock interface that provides that time base to applications (the now operation, plus initialization, availability and subscription where the time base requires it). The local time bases keep their conventional names (Steady Clock, System Clock, High-Resolution Steady Clock) even though they are all provided by the single Local Clock interface.

Vehicle Time#: The clock domain (time base) representing the vehicle-wide synchronized time, driven by the external Grand Master via the gPTP. Reading it yields a Snapshot carrying a Vehicle Time status. It is exposed to applications through the Vehicle Clock.
Vehicle Clock#: The Clock interface that provides Vehicle Time. Because that time base depends on external synchronization, the interface additionally offers initialization, availability checks and subscription to synchronization events, on top of reading the time.
Local Clock#: The Clock interface that provides the local, non-synchronized time bases — Steady Clock, System Clock and High-Resolution Steady Clock. They need no initialization and are always available.
Steady Clock#: A monotonic, non-adjustable local time base. It never goes backward, which makes it the standard choice for measuring elapsed time and computing timeouts.
System Clock#: A wall-clock (UTC-based) local time base — the OS CLOCK_REALTIME, also reachable through standard POSIX (clock_gettime) and C++ (std::chrono::system_clock) APIs. It may jump or be adjusted, so it is used for calendar timestamps, not for measuring elapsed time. Its value is kept aligned to Absolute Time by score::time; consumers reading it thus obtain the absolute time, but as QM data without the accuracy qualifier and security qualifier.
High-Resolution Steady Clock#: A monotonic, nanosecond-resolution local time base optimized for low-overhead timing. Used for tight timing loops and deadline checks.
Absolute Time#: The clock domain (time base) representing an external absolute time source (e.g. UTC from GPS). Reading it yields a Snapshot carrying an Absolute Time status. It is exposed to applications through the Absolute Clock. Its transmission delay is compensated using a Delay Tag.
Absolute Clock#: The Clock interface that provides Absolute Time.

Time Quality and Status#

Vehicle Time status#: The status concept attached to a Vehicle Time Snapshot. It indicates the reliability of the time value — for example whether it is synchronized, whether a timeout occurred, and whether the time leaped to the future or the past — together with a rate-deviation measurement. It also carries the accuracy qualifier and the Time point qualifier, letting a caller decide both whether the time value is reliable and whether it may be treated as ASIL-B data.
Accuracy qualifier#: An indication of how accurate a TimePoint is, i.e. how close it is to the corresponding point in the reference time base. The accuracy qualifier is a property of the clock domain and is reflected in the Snapshot returned by the Clock interface.
Security qualifier#: An indication of the security level of a TimePoint, i.e. whether it may be treated as trustworthy data or not.
Time point qualifier#: An indication of the integrity level of a TimePoint, i.e. whether it may be treated as ASIL-B data or only as QM data.
Absolute Time status#: The status concept attached to an Absolute Time Snapshot. It indicates the accuracy qualifier and the security qualifier of the time value, letting a caller decide whether the value is reliable enough for its use case.

Synchronization Infrastructure#

PTP#: Precision Time Protocol - a protocol (IEEE 1588) used to synchronize clocks in a network.
gPTP#: Generalized Precision Time Protocol, the IEEE 802.1AS profile of the PTP used for in-vehicle Ethernet time synchronization.
Syntonization#: Alignment of clock frequency (rate) between nodes, as opposed to synchronization which aligns the absolute TimePoint. Both are required for long-term timing consistency.
Grand Master#: The external, network-wide time source (the PTP Grand Master) that the system synchronizes to using the gPTP.