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199 lines
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4 years ago
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<h1><util/delay.h>: Convenience functions for busy-wait delay loops</h1><table border="0" cellpadding="0" cellspacing="0">
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<tr><td colspan="2"><h2>Defines</h2></td></tr>
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<tr><td class="memItemLeft" align="right" valign="top">#define </td><td class="memItemRight" valign="bottom"><a class="el" href="group__util__delay.html#ga43bafb28b29491ec7f871319b5a3b2f8">F_CPU</a> 1000000UL</td></tr>
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<tr><td colspan="2"><h2>Functions</h2></td></tr>
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<tr><td class="memItemLeft" align="right" valign="top">void </td><td class="memItemRight" valign="bottom"><a class="el" href="group__util__delay.html#gad22e7a36b80e2f917324dc43a425e9d3">_delay_ms</a> (double __ms)</td></tr>
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<tr><td class="memItemLeft" align="right" valign="top">void </td><td class="memItemRight" valign="bottom"><a class="el" href="group__util__delay.html#gab20bfffeacc678cb960944f5519c0c4f">_delay_us</a> (double __us)</td></tr>
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</table>
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<hr/><a name="_details"></a><h2>Detailed Description</h2>
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<div class="fragment"><pre class="fragment"><span class="preprocessor"> #define F_CPU 1000000UL // 1 MHz</span>
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<span class="preprocessor"></span> <span class="comment">//#define F_CPU 14.7456E6</span>
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<span class="preprocessor"> #include <<a class="code" href="delay_8h.html">util/delay.h</a>></span>
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</pre></div><dl class="note"><dt><b>Note:</b></dt><dd>As an alternative method, it is possible to pass the F_CPU macro down to the compiler from the Makefile. Obviously, in that case, no <code>#define</code> statement should be used.</dd></dl>
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<p>The functions in this header file are wrappers around the basic busy-wait functions from <<a class="el" href="delay__basic_8h.html">util/delay_basic.h</a>>. They are meant as convenience functions where actual time values can be specified rather than a number of cycles to wait for. The idea behind is that compile-time constant expressions will be eliminated by compiler optimization so floating-point expressions can be used to calculate the number of delay cycles needed based on the CPU frequency passed by the macro F_CPU.</p>
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<dl class="note"><dt><b>Note:</b></dt><dd>In order for these functions to work as intended, compiler optimizations <em>must</em> be enabled, and the delay time <em>must</em> be an expression that is a known constant at compile-time. If these requirements are not met, the resulting delay will be much longer (and basically unpredictable), and applications that otherwise do not use floating-point calculations will experience severe code bloat by the floating-point library routines linked into the application.</dd></dl>
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<p>The functions available allow the specification of microsecond, and millisecond delays directly, using the application-supplied macro F_CPU as the CPU clock frequency (in Hertz). </p>
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<hr/><h2>Define Documentation</h2>
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<a class="anchor" id="ga43bafb28b29491ec7f871319b5a3b2f8"></a><!-- doxytag: member="delay.h::F_CPU" ref="ga43bafb28b29491ec7f871319b5a3b2f8" args="" -->
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<td class="memname">#define F_CPU 1000000UL</td>
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<p>CPU frequency in Hz. </p>
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<p>The macro F_CPU specifies the CPU frequency to be considered by the delay macros. This macro is normally supplied by the environment (e.g. from within a project header, or the project's Makefile). The value 1 MHz here is only provided as a "vanilla" fallback if no such user-provided definition could be found.</p>
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<p>In terms of the delay functions, the CPU frequency can be given as a floating-point constant (e.g. 3.6864E6 for 3.6864 MHz). However, the macros in <<a class="el" href="setbaud_8h.html">util/setbaud.h</a>> require it to be an integer value. </p>
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<hr/><h2>Function Documentation</h2>
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<a class="anchor" id="gad22e7a36b80e2f917324dc43a425e9d3"></a><!-- doxytag: member="delay.h::_delay_ms" ref="gad22e7a36b80e2f917324dc43a425e9d3" args="(double __ms)" -->
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<td class="memname">void _delay_ms </td>
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<td>(</td>
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<td class="paramtype">double </td>
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<td class="paramname"> <em>__ms</em></td>
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<td> ) </td>
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<td></td>
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<div class="memdoc">
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<p>Perform a delay of <code>__ms</code> milliseconds, using <a class="el" href="group__util__delay__basic.html#ga74a94fec42bac9f1ff31fd443d419a6a">_delay_loop_2()</a>.</p>
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<p>The macro F_CPU is supposed to be defined to a constant defining the CPU clock frequency (in Hertz).</p>
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<p>The maximal possible delay is 262.14 ms / F_CPU in MHz.</p>
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<p>When the user request delay which exceed the maximum possible one, <a class="el" href="group__util__delay.html#gad22e7a36b80e2f917324dc43a425e9d3">_delay_ms()</a> provides a decreased resolution functionality. In this mode <a class="el" href="group__util__delay.html#gad22e7a36b80e2f917324dc43a425e9d3">_delay_ms()</a> will work with a resolution of 1/10 ms, providing delays up to 6.5535 seconds (independent from CPU frequency). The user will not be informed about decreased resolution.</p>
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<p>If the avr-gcc toolchain has __builtin_avr_delay_cycles() support, maximal possible delay is 4294967.295 ms/ F_CPU in MHz. For values greater than the maximal possible delay, overflows results in no delay i.e., 0ms.</p>
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<p>Conversion of <code>__ms</code> into clock cycles may not always result in integer. By default, the clock cycles rounded up to next integer. This ensures that the user gets at least <code>__ms</code> microseconds of delay.</p>
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<p>Alternatively, by defining the macro <code>__DELAY_ROUND_DOWN__</code>, or <code>__DELAY_ROUND_CLOSEST__</code>, before including this header file, the algorithm can be made to round down, or round to closest integer, respectively.</p>
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<dl class="note"><dt><b>Note:</b></dt><dd></dd></dl>
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<p>The implementation of <a class="el" href="group__util__delay.html#gad22e7a36b80e2f917324dc43a425e9d3">_delay_ms()</a> based on __builtin_avr_delay_cycles() is not backward compatible with older implementations. In order to get functionality backward compatible with previous versions, the macro <code>"__DELAY_BACKWARD_COMPATIBLE__"</code> must be defined before including this header file. Also, the backward compatible algorithm will be chosen if the code is compiled in a <em>freestanding environment</em> (GCC option <code>-ffreestanding</code>), as the math functions required for rounding are not available to the compiler then. </p>
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</div>
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</div>
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<a class="anchor" id="gab20bfffeacc678cb960944f5519c0c4f"></a><!-- doxytag: member="delay.h::_delay_us" ref="gab20bfffeacc678cb960944f5519c0c4f" args="(double __us)" -->
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<div class="memitem">
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<td class="memname">void _delay_us </td>
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<td>(</td>
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<td class="paramtype">double </td>
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<td class="paramname"> <em>__us</em></td>
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<td> ) </td>
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<td></td>
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</table>
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</div>
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<div class="memdoc">
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<p>Perform a delay of <code>__us</code> microseconds, using <a class="el" href="group__util__delay__basic.html#ga4e3957917c4c447d0f9166dac881b4e3">_delay_loop_1()</a>.</p>
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<p>The macro F_CPU is supposed to be defined to a constant defining the CPU clock frequency (in Hertz).</p>
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<p>The maximal possible delay is 768 us / F_CPU in MHz.</p>
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<p>If the user requests a delay greater than the maximal possible one, <a class="el" href="group__util__delay.html#gab20bfffeacc678cb960944f5519c0c4f">_delay_us()</a> will automatically call <a class="el" href="group__util__delay.html#gad22e7a36b80e2f917324dc43a425e9d3">_delay_ms()</a> instead. The user will not be informed about this case.</p>
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<p>If the avr-gcc toolchain has __builtin_avr_delay_cycles() support, maximal possible delay is 4294967.295 us/ F_CPU in MHz. For values greater than the maximal possible delay, overflow results in no delay i.e., 0us.</p>
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<p>Conversion of <code>__us</code> into clock cycles may not always result in integer. By default, the clock cycles rounded up to next integer. This ensures that the user gets at least <code>__us</code> microseconds of delay.</p>
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<p>Alternatively, by defining the macro <code>__DELAY_ROUND_DOWN__</code>, or <code>__DELAY_ROUND_CLOSEST__</code>, before including this header file, the algorithm can be made to round down, or round to closest integer, respectively.</p>
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<dl class="note"><dt><b>Note:</b></dt><dd></dd></dl>
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<p>The implementation of <a class="el" href="group__util__delay.html#gad22e7a36b80e2f917324dc43a425e9d3">_delay_ms()</a> based on __builtin_avr_delay_cycles() is not backward compatible with older implementations. In order to get functionality backward compatible with previous versions, the macro <code>__DELAY_BACKWARD_COMPATIBLE__</code> must be defined before including this header file. Also, the backward compatible algorithm will be chosen if the code is compiled in a <em>freestanding environment</em> (GCC option <code>-ffreestanding</code>), as the math functions required for rounding are not available to the compiler then. </p>
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