SEQ - Sequencer

The sequencer block performs automatic execution of sequenced lines to produce timing signals. Each line optionally waits for an external trigger condition and runs for an optional phase1, then a mandatory phase2 before moving to the next line. Each line sets the block outputs during phase1 and phase2 as defined by user-configured mask. Individual lines can be repeated, and the whole table can be repeated, with a value of 0 meaning repeat forever.

Fields¶

Sequencer Table Line Composition¶

Generating fixed pulse trains¶

The basic use case is for generating fixed pulse trains when enabled. For example we can ask for 3x 50% duty cycle pulses by writing a single line table that is repeated 3 times. When enabled it will become active and immediately start producing pulses, remaining active until the pulses have been produced:

We can also use it to generate irregular streams of pulses on different outputs by adding more lines to the table. Note that OUTB which was high at the end of Phase2 of the first line remains high in Phase1 of the second line:

And we can set repeats on the entire table too. Note that in the second line of this table we have suppressed phase1 by setting its time to 0:

There are 6 outputs which allow for complex patterns to be generated:

Statemachine¶

There is an internal statemachine that controls which phase is currently being output. It has a number of transitions that allow it to skip PHASE1 if there is none, or skip WAIT_TRIGGER if there is no trigger condition.

digraph pcomp_sm {
     WAIT_ENABLE [label="State 0\nWAIT_ENABLE"]
     UNREADY [label="State 1\nUNREADY"]
     WAIT_TRIGGER [label="State 2\nWAIT_TRIGGER"]
     PHASE1 [label="State 3\nPHASE1"]
     PHASE2 [label="State 4\nPHASE2"]

     WAIT_ENABLE -> UNREADY [label=" TABLE load started "]
     WAIT_ENABLE -> WAIT_TRIGGER [label=" rising ENABLE and trigger not met "]
     WAIT_ENABLE -> PHASE1 [label=" rising ENABLE and trigger met "]
     WAIT_ENABLE -> PHASE2 [label=" rising ENABLE and trigger met and no phase1 "]

     UNREADY -> WAIT_ENABLE [label=" TABLE load complete "]

     WAIT_TRIGGER -> UNREADY [label=" TABLE load started "]
     WAIT_TRIGGER -> PHASE1 [label=" trigger met "]
     WAIT_TRIGGER -> PHASE2 [label=" trigger met and no phase1 "]

     PHASE1 -> UNREADY [label=" TABLE load started "]
     PHASE1 -> PHASE2 [label=" time1 elapsed "]

     PHASE2 -> UNREADY [label=" TABLE load started "]
     PHASE2 -> WAIT_TRIGGER [label=" next trigger not met "]
     PHASE2 -> PHASE1 [label=" next trigger met "]
     PHASE2 -> PHASE2 [label=" next trigger met and no phase1 "]
}

External trigger sources¶

The trigger column in the table allows an optional trigger condition to be waited on before the phased times are started. The trigger condition is checked on each repeat of the line, but not checked during phase1 and phase2. You can see when the Block is waiting for a trigger signal as it will enter the WAIT_TRIGGER(2) state:

You can also use a position field as a trigger condition in the same way, this is useful to do a table based position compare:

Prescaler¶

Each row of the table gives a time value for the phases. This value can be scaled with a block wide prescaler to allow a frame to be longer than 2**32 * 8e-9 = about 34 seconds. For example:

Interrupting a sequence¶

Setting the repeats on a table row to 0 will cause it to iterate until interrupted by a falling ENABLE signal:

In a similar way, REPEATS=0 on a table will cause the whole table to be iterated until interrupted by a falling ENABLE signal:

And a rising edge of the ENABLE will re-run the same table from the start:

Streaming tables¶

When TABLE_LENGTH has the most significant bit set, it indicates there will be one more table pushed. The condition in which a new table can be pushed is signaled via an interrupt. When the last table is pushed (indicated by TABLE_LENGTH’s most significant bit cleared), the sequencer will repeat that table according to the value of the REPEATS register.