CompuLab SBC-X270 Manual do Utilizador descarregar pdf (Página 73)

SCIOPTA - Real-Time Kernel

User’s Manual Manual Version 4.1 8-1

SCIOPTA - Real-Time Kernel

8 SCIOPTA Trigger

8.1 Description

The trigger in SCIOPTA is a method which allows to synchronise processes even faster as it would be possible

with messages. With a trigger a process will be notified and woken-up by another process. Trigger are used only

for process coordination and synchronisation and cannot carry data. Triggers should only be used if the designer

has severe timing problems and are intended for these rare cases where message passing would be to slow.

Each process has one trigger available. A trigger is basically

a integer variable owned by the process. At process

creation the value of the trigger is initialized to one.

Figure 8-1: SCIOPTA Trigger

Trigger

sc_triggerWait()

sc_trigger()

Process waiting

on the trigger

Process issuing a

trigger event

8.2 Using SCIOPTA Trigger

There are four system calls available to work with triggers. The sc_triggerWait call decrements the value of the

trigger and the calling process will be blocked and swapped out if the value gets negative or equal zero. Only the

owner process of the trigger can wait for it. An interrupt process cannot wait on its trigger. The process waiting on

the trigger will become ready when another process triggers it by issuing a sc_trigger call which will make the

value of the trigger nonnegative.

The process which is waiting on a trigger can define a time-out value. If the time-out has elapsed it will be triggered

(becom

e nonnegative) by the operating system (actually: The previous state of the trigger is restored). If the now

ready process has a higher priority than the actual running process the operating system will preempt the running

process and execute the triggered process.

The s

c_triggerValueSet system calls allows to sets the value of a trigger. Only the owner of the trigger can set the

value. Processes can also read the values of trigger by the sc_triggerValueGet call.

Also interrupt processes have a trigger but they cannot wait on it

. If a process is triggering an interrupt process, the

interrupt process gets a software event. This is the same as if an interrupt occurs. The user can investigate a flag

which informs if the interrupt process was activated by a real interrupt or woken-up by such a trigger event.

1 2 ... 68 69 70 71 72 73 74 75 76 77 78 ... 225 226

Sem comentários

CompuLab SBC-X270 Manual do Utilizador Página 73