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manual: Slightly untangle rtio input paragraph [nfc] 

This is just an attempt at making the explanation slightly
easier to skim, based on user feedback.
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David Nadlinger 2019-01-12 12:11:03 +00:00
parent f3323a35d5
commit cd725a8352
1 changed files with 3 additions and 4 deletions
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doc/manual/rtio.rst
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@ -158,15 +158,14 @@ Input channels and events
Input channels detect events, timestamp them, and place them in a buffer for the experiment to read out. Input channels detect events, timestamp them, and place them in a buffer for the experiment to read out.
The following example counts the rising edges occurring during a precisely timed 500 ns interval. The following example counts the rising edges occurring during a precisely timed 500 ns interval.
If more than 20 rising edges were received it outputs a pulse:: If more than 20 rising edges are received, it outputs a pulse::
if input.count(input.gate_rising(500*ns)) > 20: if input.count(input.gate_rising(500*ns)) > 20:
delay(2*us) delay(2*us)
output.pulse(500*ns) output.pulse(500*ns)
The :meth:`artiq.coredevice.ttl.TTLInOut.count` method of an input channel can lead to a situation of negative slack (timeline cursor ``now`` smaller than the current wall clock ``rtio_counter``): The :meth:`artiq.coredevice.ttl.TTLInOut.count` method of an input channel will often lead to a situation of negative slack (timeline cursor ``now`` smaller than the current wall clock ``rtio_counter``):
The :meth:`artiq.coredevice.ttl.TTLInOut.gate_rising` method leaves the timeline cursor at the closure time of the gate and ``count()`` must necessarily wait until the gate closing event has actually been executed which is sometime with ``rtio_counter > now``. The :meth:`artiq.coredevice.ttl.TTLInOut.gate_rising` method leaves the timeline cursor at the closing time of the gate. ``count()`` must necessarily wait until the gate closing event has actually been executed, at which point ``rtio_counter > now``: ``count()`` synchronizes timeline cursor (``now``) and wall clock (``rtio_counter``). In these situations, a ``delay()`` is necessary to re-establish positive slack so that further output events can be placed.
In these situations where ``count()`` leads to a synchronization of timeline cursor and wall clock, a ``delay()`` is necessary to reestablish positive slack (i.e. set the timeline cursor ``now`` greater than the current wall clock ``rtio_counter``) so that output events can be placed.
Similar situations arise with methods such as :meth:`artiq.coredevice.ttl.TTLInOut.sample_get` and :meth:`artiq.coredevice.ttl.TTLInOut.watch_done`. Similar situations arise with methods such as :meth:`artiq.coredevice.ttl.TTLInOut.sample_get` and :meth:`artiq.coredevice.ttl.TTLInOut.watch_done`.