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@c  COPYRIGHT (c) 1988-2002.

@c  On-Line Applications Research Corporation (OAR).

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@c  timer.t,v 1.8 2002/01/17 21:47:44 joel Exp

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@chapter Timer Driver

The timer driver is primarily used by the RTEMS Timing Tests.

This driver provides as accurate a benchmark timer as possible.

It typically reports its time in microseconds, CPU cycles, or

bus cycles.  This information can be very useful for determining

precisely what pieces of code require optimization and to measure the

impact of specific minor changes.

The gen68340 BSP also uses the Timer Driver to support a high performance

mode of the on-CPU UART.

@section Benchmark Timer

The RTEMS Timing Test Suite requires a benchmark timer.  The

RTEMS Timing Test Suite is very helpful for determining

the performance of target hardware and comparing its performance

to that of other RTEMS targets.

This section describes the routines which are assumed to exist by

the RTEMS Timing Test Suite.  The names used are @b{EXACTLY} what

is used in the RTEMS Timing Test Suite so follow the naming convention.

@subsection Timer_initialize

Initialize the timer source.

@example

void Timer_initialize(void)

@{

  initialize the benchmark timer

@}

@end example

@subsection Read_timer

The @code{Read_timer} routine

returns the number of benchmark time units (typically microseconds)

that have elapsed since the last call to @code{Timer_initialize}.

@example

int Read_timer(void)

@{

  stop time = read the hardware timer

  if the subtract overhead feature is enabled

    subtract overhead from stop time

  return the stop time

@}

@end example

Many implementations of this routine subtract the overhead required

to initialize and read the benchmark timer.  This makes the times reported

more accurate.

Some implementations report 0 if the harware timer value change is

sufficiently small.  This is intended to indicate that the execution time

is below the resolution of the timer.

@subsection An Empty Function

This routine is invoked by the RTEMS Timing Test Suite to measure

the cost of invoking a subroutine.

@example

rtems_status_code Empty_function (void)

@{

  return RTEMS_SUCCESSFUL;

@}

@end example

@subsection Set_find_average_overhead

This routine is invoked by the "Check Timer" (@code{tmck}) test in the

RTEMS Timing Test Suite.  It makes the @code{Read_timer}

routine NOT subtract the overhead required

to initialize and read the benchmark timer.  This is used

by the @code{tmoverhd} test to determine the overhead

required to initialize and read the timer.

@example

void Set_find_average_overhead(rtems_boolean find_flag)

@{

    disable the subtract overhead feature

@}

@end example

The @code{Timer_driver_Find_average_overhead} variable is usually

used to indicate the state of the "subtract overhead feature".

@section gen68340 UART FIFO Full Mode

The gen68340 BSP is an example of the use of the timer to support the UART

input FIFO full mode (FIFO means First In First Out and roughly means

buffer). This mode consists in the UART raising an interrupt when n

characters have been received (@i{n} is the UART's FIFO length). It results

in a lower interrupt processing time, but the problem is that a scanf

primitive will block on a receipt of less than @i{n} characters. The solution

is to set a timer that will check whether there are some characters

waiting in the UART's input FIFO. The delay time has to be set carefully

otherwise high rates will be broken:

@itemize @bullet

@item if no character was received last time the interrupt subroutine was

entered, set a long delay,

@item otherwise set the delay to the delay needed for @i{n} characters

receipt.

@end itemize