What happens on a single device, The scheduler, When clauses – Echelon Neuron C User Manual
Page 28
16
Focusing on a Single Device
What Happens on a Single Device?
In this chapter, you begin to learn about programming a Neuron Chip or Smart
Transceiver by focusing first on a single device. Each Neuron Chip and each
Smart Transceiver has standard firmware, called the
Neuron firmware
, and
hardware support that implement a scheduler, timers, and I/O device drivers and
interfaces. Series 5000 chips also provide hardware support for interrupts; see
on page 153 for more information.
The Neuron C language includes predefined objects that provide access to these
firmware features. These objects are described briefly here, and in more detail
later in this chapter:
• The Neuron firmware's
event scheduler
handles task scheduling for the
application program. This chapter explains how to use the Neuron C
language to define events and tasks, how the scheduler evaluates
nonpriority events, and how you can define priority events.
• The Neuron C language offers two types of
timer
objects: millisecond and
second timers. These timers can be used to affect the scheduling of tasks,
as described in
on page 25.
• A number of
I/O objects
can be declared using Neuron C extensions to
ANSI C. These I/O objects, as well as related I/O functions and events,
are described in
on page 27.
The Scheduler
The scheduling of application program tasks is event driven: when a given
condition becomes TRUE, a body of code (called a
task
) associated with that
condition is executed. The scheduler allows you to define tasks that run as the
result of certain events, such as a change in the state of an input pin, receiving a
new value for a network variable, or the expiration of a timer. You can also
specify certain tasks as priority
tasks, so that they receive preferential service
(see
on page 23). Series 5000 chips also allow you to
specify interrupt tasks that are serviced independently of the scheduler; see
on page 153 for more information.
When Clauses
Events are defined through when clauses. A when clause contains an expression
that, if evaluated as TRUE, causes the body of code (the
task
) following the
expression to be executed to completion. Multiple when clauses can be associated
with a single task. A simple when clause and its associated task are shown
below. The when clause or clauses and the associated task are frequently
referred to as one entity known as a
when task
or a
when statement
.
when (timer_expires(led timer))
{
// Turn off the LED
io_out(io_led, OFF);
}
when clause
task