Coma FW-C2800 User's Guide download pdf (Page 108)

Overview of Thread Scheduling

Thread Scheduling 4-3

❏ Software interrupts (SWI). Patterned after hardware interrupt (HWIs).

While HWIs are triggered by a hardware interrupt, software interrupts are

triggered by calling SWI functions from the program. Software interrupts

provide additional priority levels between hardware interrupts and TSKs.

SWIs handle threads subject to time constraints that preclude them from

being run as tasks, but whose deadlines are not as severe as those of

hardware ISRs. Like HWI’s, SWI’s threads always run to completion.

Software interrupts should be used to schedule events with deadlines of

100 microseconds or more. SWIs allow HWIs to defer less critical

processing to a lower-priority thread, minimizing the time the CPU spends

inside an interrupt service routine, where other HWIs can be disabled. See

section 4.3, Software Interrupts, page 4-26, for details about software

interrupts.

❏ Tasks (TSK). Tasks have higher priority than the background thread and

lower priority than software interrupts. Tasks differ from software

interrupts in that they can be suspended during execution until necessary

resources are available. DSP/BIOS provides a number of structures that

can be used for inter task communication and synchronization. These

structures include queues, semaphores, and mailboxes. See section 4.4,

Tasks, page 4-40, for details about tasks.

❏ Background thread. Executes the idle loop (IDL) at the lowest priority in

a DSP/BIOS application. After main returns, a DSP/BIOS application

calls the startup routine for each DSP/BIOS module and then falls into the

idle loop. The idle loop is a continuous loop that calls all functions for the

IDL objects. Each function must wait for all others to finish executing

before it is called again. The idle loop runs continuously except when it is

preempted by higher-priority threads. Only functions that do not have

hard deadlines should be executed in the idle loop. See section 4.5, The

Idle Loop, page 4-53, for details about the background thread.

There are several other kinds of functions that can be performed in a

DSP/BIOS program. These are performed within the context of one of the

thread types in the previous list.

❏ Clock (CLK) functions. Triggered at the rate of the on-device timer

interrupt. By default, these functions are triggered by a hardware interrupt

and are performed as HWI functions. See section 4.8, Timers, Interrupts,

and the System Clock, page 4-67, for details.

❏ Periodic (PRD) functions. Performed based on a multiple of either the

on-device timer interrupt or some other occurrence. Periodic functions

are a special type of software interrupt. See section 4.9, Periodic

Function Manager (PRD) and the System Clock, page 4-74, for details.

❏ Data notification functions. Performed when you use pipes (PIP) or

host channels (HST) to transfer data. The functions are triggered when a

frame of data is read or written to notify the writer or reader. These

functions are performed as part of the context of the function that called

PIP_alloc, PIP_get, PIP_free, or PIP_put.

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Coma FW-C2800 User's Guide Page 108

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Coma FW-C2800 User's Guide Page 108

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