2.4 Gating Blocks

Figure 2.16: Gating Blocks: Each gating block can use an arbitrary combination of inputs to trigger its state machine. The outputs can be individually inverted and routed to the AND-gate feeding the trigger blocks.

In order to decrease the amount of data transmitted to the PC, the Ndigo6G-12 includes four independent gate and delay units.

They are configured using ndigo6g12_configuration::gating_block and (specifically for the TDC channels) ndigo6g12_tdc_channel::gating_block.

A gate and delay unit creates a gate window starting and closing at specified times after a trigger event (as configured by the user with ndigo6g12_gating_block::start and stop).

Concretely, if a trigger event is detected, a timer starts. After the timer reaches the time corresponding to start, the gate will activate. After the timer reaches the time corresponding to stop, it will inactivate.

This behavior may be influenced by the retrigger feature. With this feature enabled, another trigger signal will reset the timer to zero. That means, if a second trigger is detected before the gate is activated, the time until it activates is extended. If, however, the gate was already active, the time until it inactivates will be extended.

Attention

A bug in Firmware Rev. \(\le\)1.24120 causes the retrigger feature to reset the gate logic entirely (i.e, the state of the gate will inactivate after a retrigger event).

Depending on ndigo6g12_gating_block::negate, an active gate will be open (signal detection enabled) or closed (signal detection disabled).

Each gating block can use an arbitrary combination of inputs which trigger it. This is configured using ndigo6g12_gating_block::sources.

trigger_blocks can use the gate signal to suppress data acquisition, that is, only data that fulfills zero suppression specifications occurring in an open gate window is written to the PC.

Figure 2.17 shows the functionality of the gate timing and delay unit.

Figure 2.17: Gate and delay functionality: When a trigger occurs, the gate opens after a set period of time “Gate Start” and closes when it reaches “Gate Stop”. A second trigger event may influence this behavior if retriggering is enabled.

2.4.1 Examples

Example 1: Suppression of Noise After Starting an Acquisition

In mass spectrometer and other experiments, noise while starting data acquisition can result in undesired trigger events during start-up time. To prevent noise in the output data, a gating block could be used to suppress all triggers during start-up.

The following example illustrates the use of a gating block (in the following, gating_block[0]) to prevent recording noise:

• Set up the GATE input to trigger on each acquisition start, that is, trigger[NDIGO6G12_TRIGGER_GATE] is configured corresponding to the input signal (e.g., configuring the polarity).

• NDIGO6G12_TRIGGER_SOURCE_GATE is selected as input source of gating_block[0].source and the gating_block[0].start parameter is set to 0.

• The gating_block[0].stop parameter is set to the desired length (in multiples of 5 ns).

• gating_block[0].negate is set to true.

Now, gating_block[0] will output a LOW pulse of the desired length (that is, the gate is closed during start-up time) whenever there is a pulse on the GATE input.

Now, select the above gate for the trigger bock you want to use for triggering data acquisition, e.g., trigger_block[0]:

• Set trigger_block[0].sources e.g.,

  config.trigger_block[0].sources = NDIGO6G12_TRIGGER_SOURCE_A0 | NDIGO6G12_TRIGGER_SOURCE_D0
  

  uses the ADC input channels A and D as sources.

• Set NDIGO6G12_TRIGGER_GATE_0 as trigger_block[0].gates.

  config.trigger_block[0].gates = NDIGO6G12_TRIGGER_GATE_0
  

Now, recording of data is suppressed for an initial start-up time.

2.4.2 Example 2: Delayed Trigger

To sample a short window at a specified time after a trigger event on a channel, a gating block can be used to create a delayed trigger. To do this, one of the triggers of the channel of interest is configured to the desired parameters by selecting the threshold, setting the edge polarity and enabling edge triggering.

Instead of directly using this trigger as an input to the trigger block’s input matrix, the trigger is selected as an input to a gating block. The block is configured with start = delay (in multiples 5 ns) and stop = start+1, negate = false. This causes the gating block to produce a one clock cycle pulse on its output after the specified delay.

To send this pulse to the trigger block, the gating block must be enabled in the trigger block’s AND matrix and the ONE trigger source must be selected.