Timestamp

Bus: PCI Express, PCI-X
Instruments: Digitizers / Oscilloscopes, High-Speed Digital I/O

The Timestamp option is used to record trigger events relative to the beginning of the measurement, relative to a fixed time-zero point or synchronized to an external reset clock. The reset clock can come from a radio clock a GPS signal or from any other external machine.

Please note that the Timestamp option cannot be purchased separately but is included in the option Bundles.

How the Timestamp works

The Timestamp is implemented on the instrument card as a 64 bit counter that is running at the same speed as the used sampling rate. The counter is reset either by explicit software command or depending on the mode by the start of the card. On receiving the trigger event (or at the start and at the end of a gate interval when using Gated Sampling mode) the current counter value is stored in an extra FIFO Timestamp memory.

This function is designed as an enhancement to the Multiple Recording and the Gated Sampling options and can also used together with the ABA mode option. Timestamp can also be used without these options with ordinary measurements. If Gated Sampling mode is used, then both the start and end of a recorded segment are timestamped.

Each recorded timestamp consists of the number of samples that has been counted since the last counter reset. The actual time in relation to the reset command can be easily calcu­lated by the formula below:

Time = Timestamp value / (Sampling rate * Oversampling factor)

Please note that the timestamp calculation depends on the cur­rent sampling rate and the oversampling factor. Please have a look at the Clock chapter in the user manual for the specific instrument card to see how to read out the sampling rate and the oversampling factor

If you want to know the time between two timestamps, you can simply calculate this by the for­mula on the below:

Time between two timestamps = (Timestamp_n+1 – Timestamp_n ) / (Sampling rate * Oversampling factor)

Standard mode

In Standard mode the Timestamp counter is set to zero once by writing the TS_RESET command to the command register. After that command starts counting and continues even if the measurement stops. The timestamps of all the recorded trigger events are referenced to this common zero time. With this mode you can calculate the exact time difference between different recordings and also within one acqui­sition (if using Multiple Recording or Gated Sampling).

StartReset mode

In StartReset mode the timestamp counter is set to zero every time that the card is started. After starting the card the counter begins to count. All of the timestamps recorded during this measurement are referenced to this point. This mode is very useful for Multiple Recording and Gated Sampling .

Refclock mode (needs BaseXIO option)

The counter is split in HIGH and a LOW parts and an additional external signal, that affects both parts of the counter, needs to be fed in externally. The external reference clock signal will reset the LOW part of the counter and increase the HIGH part of the counter. The upper 32 bit of the timestamp value holds the number of the clock edges that have occurred on the external reference clock signal, the lower 32 bit will hold the timestamp counter with the position within the current reference clock period with the resolution of the sampling rate.

This mode can be used to obtain an absolute time reference when using an external radio clock or a GPS receiver. In that case the higher part is counting the seconds since the last reset and the lower part is counting the position inside the second using the currently sampling rate.

Please note that the BaseXIO option is required as this provides the input for the additional external signal.

The counting is initialized with the timestamp reset command. Both counter parts will then be set to zero.

To synchronize the external reference clock signal with the PC clock it is possible to perform a timestamp reset command which waits a spec­ified time for the occurrence of the external clock edge. As soon as the clock edge is found, the function stores the current PC time and date which can be used to get the absolute time. As the timestamp reference clock can also be used with other clocks that don’t need to be syn­chronized with the PC clock, the waiting time can be programmed using the SPC_TIMESTAMP_TIMEOUT register.