User guide

Neuron C Programmer’s Guide 149
become TRUE. It is thus important that these when
clauses be evaluated in
their given order after a network variable update. Using scheduler_reset, the
nv_update_occurs event for nviSwitch1 is always checked first whenever a new
network variable update is at the head of the queue.
Updates to nviSwitch3 or nviSwitch4 trigger only the third event.
Bypass Mode
All scheduling of Neuron C programs, as described above, is event-driven and
handled by the scheduler. Within a program, however, you can choose when to
return control to the scheduler. The term
bypass mode refers to a method of
programming in which one when clause always evaluates to TRUE and never
returns. In this case, a single task must handle all event processing.
You should use bypass mode rarely, and only in cases where you need a different
scheduling algorithm than that provided by the Neuron firmware scheduler.
While in bypass mode, your program is responsible for all event processing. You
define critical sections through the post_events( ) function (see the following
section), and then check for predefined events in if, while, and for expressions.
If your application runs on a Series 5000 device, consider using interrupts for
specific kinds of events, so that you do not need to use bypass mode for the
scheduler. See
Interrupts on page 153 for more information.
The post_events( ) Function
Use the post_events( ) function to define a boundary of a critical section at which
network variable updates are sent and incoming network variable updates are
processed.
Note: The post_events( ) function is automatically called at the top of the
scheduling loop.
When the post_events( ) function is called, a number of things happen:
• Any outgoing network variable updates are sent. In the case of
synchronous network variables, all the updates are sent. For
nonsynchronous network variables, as many updates are sent as
application output buffers are available. Any unsent updates are sent the
next time post_events( ) is called.
• Incoming network variable update events are received.
• New incoming messages are examined.
• Timers are examined to see if they have expired.
• The watchdog timer is reset (to keep it from timing out). See the
following section about the watchdog timer.
You can use the post_events( ) function to improve network performance by
calling it immediately after modifying network variables. Call the post_events( )
function to signal the network processor to start execution of the formatting of
the outgoing packet before the when task completes, thus increasing parallel
processing and utilizing the Neuron Chip and Smart Transceiver multi-processor
architecture to its fullest.