The significance of time stamps¶

Signals can be retrieved at and from devices in different ways. While a device is being developed, it is often possible to access internal values via special interfaces (diagnostic port, direct memory access). But values can also be transported by their own dedicated lines. In addition, a large part of the signals of modern control units is also available by cyclical supply via different bus systems.

It is common to all possibilities of signal scanning that it can be carried out only in either a time-discrete or a value-discrete manner. Idealized signal flows are often assumed when specifications are written at an early stage of development.

In reality, however, it is not possible, due to the multitude of access possibilities, to measure an exact signal value at any point in time. With measuring on bus systems you rather depend on waiting for a next message to occur in which a new “measured value” of a signal is coded. But also in the case of direct memory access to control units there is the restriction that, partly for technical reasons, signals can be read only at certain intervals. The total of signal flows in and around a device can be better represented by a table with the lines showing individual points in time while the columns contain the measured values of individual signals at those points. Owing to the above-mentioned technical restrictions many lines in that table will contain no measured values.

The essential problem arising from the presence of empty spaces is the notion of simultaneity for writing specifications and automatically evaluating measured signal flows. Such a term is often used only implicitly in specifications where it is said, for example, that “the control unit must put the signal “SystemAwake” on “HIGH” as soon as there is a high-level at the “WakeUp” input”. For evaluating a measured signal flow the time relation between the two correlated signal values must be made very clear.

In general, an expression is evaluated at any time when at least one of the referenced signals has a value. This results in some peculiarities, which are described in more detail in Pitfalls when using trace analysis.

Monotony of the time axis

Chronologically ordered time-value pairs are a prerequisite for carrying out analyses. Therefore, an internal monotony test of the time axis, which is explained in more detail below, takes place before the actual analyses.

As a rule, the time axis of signals must be strictly monotonically increasing. In addition, consecutive repeated time stamps are permitted, provided that the values of the signal at these time stamps do not differ. In such a case, the repeated time-value pairs are removed before further processing. Repeated time stamps with different values, on the other hand, are only allowed if a jump is modeled this way. In that case, the first value must be the same as the value held by the previous time-value pair. If this condition is met, the first value is removed. Otherwise, an error will occur when reading the signal. (Strictly) monotonously falling sections of the time axis (i.e. “jumps into the past”) are not allowed, too.

During bus logging several messages may arrive at the same time, which can result in different raw data at one time. Therefore, in contrast to the restrictions described above, repeated time stamps with different values are permitted for special signals that are marked as “Bus Events” in the recording.