Time stamp relative to frame duration
Size and location of time stamp
The time stamp information is provided in the packet descriptor of every frame. For the standard descriptor it can be retrieved using the NT_NET_GET_PKT_TIMESTAMP macro (see DN-0449). The size of the time stamp field is 64 bits.
Time stamp formats
There are three time stamp formats with different start times:
- Native UNIX format based on a 10 ns unit, with a 64-bit unit counter. The start time is January 1st 1970.
PCAP-μs format based on a 1000 ns (1 μs) unit, with a 32-bit unit counter in the MSBs of the time stamp descriptor field, and a 32-bit second counter in the LSBs of the time stamp descriptor field. The start time is January 1st 1970.
PCAP-ns format based on a 1 ns unit, with a 32-bit unit counter in the MSBs of the time stamp descriptor field, and a 32-bit second counter in the LSBs of the time stamp descriptor field, so the time resolution is 1 ns. The start time is January 1st 1970.
Time offset value
The accelerator time provided from an external time source, for instance a GPS signal, can be adjusted by an offset if required. This can be used, for instance, to compensate for delays in time synchronization cables.
Data path delays
RX data path delay values are available in the API to allow applications to compensate for these delays (see DN-0449).
The internal accelerator time, time offset and time stamp format are global settings for the accelerator, so all received frames use the same time source and format.
If the accelerator is time-synchronized to another accelerator, it is recommended that the time stamp format of the time slave accelerator is the same as the time stamp format in the time master accelerator.
If the traffic from two accelerators is SW-merged, the time stamp formats of the two accelerators must be the same.
PCAP-ns or PCAP-μs format can only be used if the PCAP packet descriptor is selected for all accelerators (see Restrictions on the PCAP packet descriptor).