tep107.txt

tinyos-2.x.rar

supports this::

  /* This platform_bootstrap macro exists in accordance with TEP
     107. A platform may override this through a platform.h file. */
  #include <platform.h>
  #ifndef platform_bootstrap
  #define platform_bootstrap() {}
  #endif

The boot sequence has three separate initializations: Scheduler,
PlatformInit, and SoftwareInit. The boot configuration (MainC) wires
the first two automatically, to TinySchedulerC (discussed in TEP 106)
and to PlatformC::

  configuration MainC {
    provides interface Boot;
    uses interface Init as SoftwareInit;
  }
  implementation {
    components PlatformC, RealMainP, TinySchedulerC;  
  
    RealMainP.Scheduler -> TinySchedulerC;
    RealMainP.PlatformInit -> PlatformC;
  
    // Export the SoftwareInit and Booted for applications
    SoftwareInit = RealMainP.SoftwareInit;
    Boot = RealMainP;
  }


MainC exports the Boot and SoftwareInit interfaces for applications to
wire to. TinySchedulerC is the standard name for the TinyOS
scheduler. As the initialization sequence requires being able to run
tasks, the boot sequence initializes it first. The second step of
initialization is to call PlatformInit.init(), which MainC wires to a
component named PlatformC. PlatformInit is for initializations which
must follow a very specific order due to hidden dependencies, e.g., as
part of making the overall hardware platform operable. One example of
this sort of initialization is clock calibration. Because PlatformInit
calls the component PlatformC, each platform can specify the required
initialization order. As these hidden dependencies must be due to
hardware, the sequence is platform-specific. A port of TinyOS to a
new plaform MUST include a component PlatformC which provides
one and only one instance of the Init interface.

Initializations invoked
through PlatformC meet some or all of the following criteria:

1. The initialization requires configuring hardware resources. This implies that the code is platform-specific. 

2. The initialization should always be performed.

3. The initialization is a prerequisite for common services in the system.

Three example operations that often belong in PlatformInit are I/O pin
configuration, clock calibration, and LED configuration. I/O pin 
configuration meets the first two criteria. It should always be performed
(regardless of what components the OS uses) for low-power reasons:
incorrectly configured pins can draw current and prevent the MCU from
entering its lowest power sleep state [2]_. Clock calibration meets
all three criteria. LED configuration is a special case: while it
nominally meets all three criteria, the most important one is the third:
as LEDs are often needed during SoftwareInit initialization, they must
be set up before it is invoked.

Note that not all code which meets some of these criteria is wired through 
PlatformC. In particular, criterion 1 is typically necessary but not 
sufficient to require PlatformC. For example, a timer system that
configures overflow and capture settings or  a UART stack that sets the
baud rate and transmission options can often be wired to SoftwareInit.
They are encapsulated abstractions which will not be invoked or
started until the boot event, and only need to be configured if the 
system includes their functionality.

Components whose initialization does not directly depend on hardware
resources SHOULD wire to MainC.SoftwareInit. If a component requires a
specific initialization ordering, then it is responsible for
establishing that ordering. Due to the semantics of Init, this is
usually quite rare; a component SHOULD NOT introduce initialization
dependencies unless they are required.

One common approach is for a configuration to "auto-wire" the
initialization routines of its internal components. The configuration
does not provide an Init interface. Virtualized services
often take this approach, as the service, rather than the clients, is
what needs to be initialized. For example, the standard Timer
virtualization [3]_, TimerMilliC, wires to TimerMilliP, which is
a very simple configuration that takes the underlying implementation
(HilTimerMilliC) and wires it to MainC::

  configuration TimerMilliP {
    provides interface Timer<TMilli> as Timinitialization in ordererMilli[uint8_t id];
  }
  implementation {
    components HilTimerMilliC, MainC;
    MainC.SoftwareInit -> HilTimerMilliC;
    TimerMilli = HilTimerMilliC; 
  }

Rather than require an application to wire HilTimerMilliC to MainC,
TimerMilliP does it automatically. When a component instantiates a
TimerMilliC, that names TimerMilliP, which will automatically make
sure that the timer system is initialized when TinyOS boots.


4.2 Interrupts in Initialization
--------------------------------------------------------------------

Interrupts are not enabled until all calls to Init.init have returned.
If a component's initialization needs to handle interrupts, it can
do one of three things:

  1) If a status flag for the interrupt exists, the Init.init()
     implementations SHOULD use a spin loop to test for when
     an interrupt has been issued.
  2) If no such flag exists, the Init.init() implementation MAY
     temporarily enable interrupts, if doing so will not cause any other
     components to handle an interrupt. That is, if a component enables
     an interrupt, it MUST NOT enable interrupts whose handlers would
     invoke any other component. Furthermore, when Init.init() exits,
     the interrupts must be disabled.
  3) If no such flag exists and there is no way to isolate which
     interrupt handlers are called, then the component MUST rely
     on mechanisms outside the Init sequence, such as SplitControl.

The boot sequence assumes that 1) is by far the dominant case. There
are, however, possible situations where a component might need to
handle an interrupt because of, e.g., hardware limitations (no pending
flag) or to catch a brief edge transition. In these cases, a component
can handle an interrupt in the boot sequence only if doing so will not
cause any other component to handle an interrupt. As they have all
been written assuming that interrupts are not enabled until after Init
completes, making one of them handle an interrupt could cause it to 
fail.

Depending on what capabilities the hardware provides, there are
several ways to meet these requirements. The simplest is to push these
initialization edge cases out of the main boot sequence, e.g., into
SplitControl. A second possibility is to redirect the interrupt table,
if the MCU supports doing so. Whichever mechanism is chosen, extreme
care needs to be used in order to not disrupt the operation of other
components.

Unless part of a hardware abstraction architecture (HAA) [4]_, the
Init.init() command MUST NOT assume that other components have been
initialized unless it has initialized them, and MUST NOT call any
functional interfaces on any components that might be shared or
interact with shared resources.  Components MAY call functions
on other components that are completely internal to the subsystem.
For example, a networking layer can  call queue operations to 
initialize its queue, but a link layer must not send commands
over an SPI bus.  An HAA
component MAY make other calls to initialize hardware state. A
component that is not part of an HAA SHOULD NOT call Init.init() on
other components unless it needs to enforce a temporal ordering on
initialization.

If a component A depends on another component, B, 
which needs to be initialized, then A SHOULD wire B's Init directly 
to the boot sequence, unless there is a temporal ordering requirement to
the initialization. The purpose of this convention is to simplify
component initialization and the initialization sequence.

5. Implementation
====================================================================

The following files in ``tinyos-2.x/tos/system`` contain the reference
implementations of the TinyOS boot sequence:

  * ``RealMainP.nc`` is the module containing the function ``main``.
  * ``MainC.nc`` is the configuration that wires RealMainP to
    PlatformC and TinySchedulerC [1]_.


6. Author's Address
====================================================================

| Philip Levis
| 467 Soda Hall
| UC Berkeley
| Berkeley, CA 94720
|
| phone - [phone]
|
| email - [e-mail]

7. Citations
====================================================================

.. [1] TEP 106: Schedulers and Tasks.

.. [2] TEP 112: Microcontroller Power Management.

.. [3] TEP 102: Timers.

.. [4] TEP 2: Hardware Abstraction Architecture.