New ZSync trigger behavior

Status until L1 23.02

The PQSC can send messages with a fixed size of 16 bits. The purpose of the message is not carried with the data itself, and the sequencer must know a priori which kind of message it is. A message always generates a trigger in the sequencer that can be waited for with the instruction waitZSyncTrigger. The payload of the message can be acquired in the sequencer with the instruction getFeedback (or alternatively getZSyncData), or a conditional playback can be executed with the instruction executeTableEntry (or alternatively playWaveZSync).

The PQSC sends two kind of messages. Start triggers are used to synchronize the execution of the sequencers across instruments. Each sequencer should wait for it at the beginning of the sequence, and its data payload is not used.

Feedback data is used to perform conditional operations depending on their payload. This operation can be timed in two ways. The recommended one is to specify the time of execution with respect to the last start trigger as a second parameter of getFeedback or executeTableEntry. Such time can be deduced by looking at the RTLogger, or by using the timing model provided by the package zhinst-utils. This is called time-based feedback. Another method is to wait for the trigger associated with the feedback data and only then perform the conditional action. This is called trigger-based feedback. This method, while simpler on the surface, doesn’t give the user explicit information about the timing, nor does it guarantee a stable feedback time across experiments. Moreover, it is prone to errors when multiple feedback events are generated.

Example

Time-based feedback sequence

A sample sequence using time-based feedback. The constant FEEDBACK_DELAY holds the feedback delay in sequencer clock cycles. It has to be provided by the user using the timing model.

while(true) {
  //Wait for a start trigger
  waitZSyncTrigger();
  //Play a conditional pulse, only when feedback is received
  executeTableEntry(ZSYNC_DATA_PQSC_REGISTER, FEEDBACK_DELAY);
}

Trigger-based feedback sequence

When trigger-based feedback is used, a trigger is used to detect the arrival of the feedback message

while(true) {
  //Wait for a start trigger
  waitZSyncTrigger();
  //Wait for the feedback trigger
  waitZSyncTrigger();
  //Play a conditional pulse
  executeTableEntry(ZSYNC_DATA_PQSC_REGISTER);
}

New behavior since L1 23.06

The start triggers and the feedback data are now two semantically different messages sent over ZSync. The command waitZSyncTrigger will now only wait for start triggers. If a feedback message is received while the sequencer is waiting for a start trigger, it will continue to wait. This avoids the issue of the sequencer getting confused and starting a new portion of the sequence at the wrong point in time. Similarly, a new start trigger doesn’t clear the last message received.

As a side effect, it is no longer possible to use the trigger-based feedback. All the sequences using such a method must be migrated to time-based feedback to keep working.

The instructions getZSyncData and playWaveZSync are deprecated. The more generic instructions getFeedback and executeTableEntry should be used instead with the same arguments respectively.