delay_basic.h

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/* Copyright (c) 2002, Marek Michalkiewicz
   Copyright (c) 2007 Joerg Wunsch
   All rights reserved.
 
   Redistribution and use in source and binary forms, with or without
   modification, are permitted provided that the following conditions are met:
 
   * Redistributions of source code must retain the above copyright
     notice, this list of conditions and the following disclaimer.
 
   * Redistributions in binary form must reproduce the above copyright
     notice, this list of conditions and the following disclaimer in
     the documentation and/or other materials provided with the
     distribution.
 
   * Neither the name of the copyright holders nor the names of
     contributors may be used to endorse or promote products derived
     from this software without specific prior written permission.
 
  THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
  AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
  IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
  ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE
  LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR
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  CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE)
  ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
  POSSIBILITY OF SUCH DAMAGE. */
 
#ifndef _UTIL_DELAY_BASIC_H_
#define _UTIL_DELAY_BASIC_H_ 1
 
#include <inttypes.h>
#include <bits/attribs.h>
 
#if !defined(__DOXYGEN__)
static __ATTR_ALWAYS_INLINE__ void _delay_loop_1(uint8_t __count);
static __ATTR_ALWAYS_INLINE__ void _delay_loop_2(uint16_t __count);
#endif
 
/** \file */
/** \defgroup util_delay_basic <util/delay_basic.h>: Basic busy-wait delay loops
    \code
    #include <util/delay_basic.h>
    \endcode
 
    The functions in this header file implement simple delay loops
    that perform a busy-waiting.  They are typically used to
    facilitate short delays in the program execution.  They are
    implemented as count-down loops with a well-known CPU cycle
    count per loop iteration.  As such, no other processing can
    occur simultaneously.  It should be kept in mind that the
    functions described here do not disable interrupts.
 
    In general, for long delays, the use of hardware timers is
    much preferable, as they free the CPU, and allow for
    concurrent processing of other events while the timer is
    running.  However, in particular for very short delays, the
    overhead of setting up a hardware timer is too much compared
    to the overall delay time.
 
    Two inline functions are provided for the actual delay algorithms.
*/
 
/** \ingroup util_delay_basic
 
    Delay loop using an 8-bit counter \c __count, so up to 256
    iterations are possible.  (The value 256 would have to be passed
    as 0.)  The loop executes three CPU cycles per iteration, not
    including the overhead the compiler needs to setup the counter
    register.
 
    Thus, delays of up to 768 / f<sub>CPU</sub> microseconds can be achieved,
    where f<sub>CPU</sub> denotes the CPU speed in units of 1 MHz.
 
    As an alternative, consider <a
    href="https://gcc.gnu.org/onlinedocs/gcc/AVR-Built-in-Functions.html#index-_005f_005fbuiltin_005favr_005fdelay_005fcycles"
    ><tt>__builtin_avr_delay_cycles</tt></a> which allows to specify the cycle
    count and can implement delays of up to 71.5 / f<sub>CPU</sub> seconds.
*/
void
_delay_loop_1(uint8_t __count)
{
    __asm__ volatile (
        "1: dec %0" "\n\t"
        "brne 1b ; [[len=nl]]"
        : "=r" (__count)
        : "0" (__count)
    );
}
 
/** \ingroup util_delay_basic
 
    Delay loop using a 16-bit counter \c __count, so up to 65536
    iterations are possible.  (The value 65536 would have to be
    passed as 0.)  The loop executes four CPU cycles per iteration,
    not including the overhead the compiler requires to setup the
    counter register pair.
 
    Thus, delays of up to 262.1 / f<sub>CPU</sub> milliseconds can be achieved,
    where f<sub>CPU</sub> denotes the CPU speed in units of 1 MHz.
 
    As an alternative, consider <a
    href="https://gcc.gnu.org/onlinedocs/gcc/AVR-Built-in-Functions.html#index-_005f_005fbuiltin_005favr_005fdelay_005fcycles"
    ><tt>__builtin_avr_delay_cycles</tt></a> which allows to specify the cycle
    count and can implement delays of up to 71.5 / f<sub>CPU</sub> seconds.
 */
void
_delay_loop_2(uint16_t __count)
{
#if defined (__AVR_TINY__)
    __asm__ volatile (
        "1: subi %A0,1" "\n\t"
        "   sbci %B0,0" "\n\t"
        "brne 1b ; [[len=nl]]"
        : "+d" (__count)
    );
#else
    __asm__ volatile (
        "1: sbiw %0,1" "\n\t"
        "brne 1b ; [[len=nl]]"
        : "+w" (__count)
    );
#endif /* AVR_TINY */
}
 
#endif /* _UTIL_DELAY_BASIC_H_ */