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doc: add reminder of what positive slack means. Closes #1084

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Sebastien Bourdeauducq 2018-07-02 16:23:12 +08:00
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doc/manual/rtio.rst
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@ -166,7 +166,7 @@ If more than 20 rising edges were received it outputs a pulse::
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.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``.
In these situations where ``count()`` leads to a synchronization of timeline cursor and wall clock, a ``delay()`` is necessary to reestablish positive slack so that 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`.