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<title>CMSIS-RTOS: Configuration for Low-Power Modes</title>
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<div class="title">Configuration for Low-Power Modes </div> </div>
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<div class="textblock"><p>The system thread <b>os_idle_demon</b> can be use to switch the system into a low-power mode. The easiest form to enter a low-power mode is the execution of the <code>__WFE</code> function that puts the processor into a sleep mode where it waits for an event.</p>
<p><b>Configuration Example:</b></p>
<div class="fragment"><div class="line"><span class="preprocessor">#include "device.h"</span> <span class="comment">/* Device definitions */</span></div>
<div class="line"> </div>
<div class="line"><span class="keywordtype">void</span> <a class="code" href="group__RTX__Global__Functions.html#gafb4dc4d3dff8343a393726d2860282e4">os_idle_demon</a> (<span class="keywordtype">void</span>) {</div>
<div class="line"> <span class="comment">/* The idle demon is a system thread, running when no other thread is */</span></div>
<div class="line"> <span class="comment">/* ready to run. */</span></div>
<div class="line"> </div>
<div class="line"> <span class="keywordflow">for</span> (;;) {</div>
<div class="line"> __WFE(); <span class="comment">/* Enter sleep mode */</span></div>
<div class="line"> }</div>
<div class="line">}</div>
</div><!-- fragment --><dl class="section note"><dt>Note</dt><dd><code>__WFE()</code> is not available at every Cortex-M implementation. Check device manuals for availability.</dd></dl>
<h1><a class="anchor" id="TickLess"></a>
Tick-less operation</h1>
<p>CMSIS-RTOS RTX provides extension for tick-less operation which is useful for applications that use extensively low-power modes where the SysTick timer is also disabled. To provide a time-tick in such power-saving modes a wake-up timer is used to derive timer intervals. The RTX functions <a class="el" href="group__RTX__Global__Functions.html#ga3e9c57746ccbdfe90464ad50513c569e">os_suspend</a> and <a class="el" href="group__RTX__Global__Functions.html#ga1b85a217d43e6b971ffcf24f8aae1c33">os_resume</a> control the tick-less operation.</p>
<p>Using this functions allows the RTX thread scheduler to stop the periodic kernel tick interrupt. When all active threads are suspended, the system enters power-down and calculates how long it can stay in this power-down mode. In the power-down mode the processor and potentially peripherals can be switched off. Only a wake-up timer must remain powered, because this timer is responsible to wake-up the system after the power-down period expires.</p>
<p>The tick-less operation is controlled from the <a class="el" href="group__RTX__Global__Functions.html#gafb4dc4d3dff8343a393726d2860282e4">os_idle_demon</a> thread. The wake-up timeout value is set before the system enters the power-down mode. The function <a class="el" href="group__RTX__Global__Functions.html#ga3e9c57746ccbdfe90464ad50513c569e">os_suspend</a> calculates the wake-up timeout measured in RTX Timer Ticks; this value is used to setup the wake-up timer that runs during the power-down mode of the system.</p>
<p>Once the system resumes operation (either by a wake-up time out or other interrupts) the RTX thread scheduler is started with the function <a class="el" href="group__RTX__Global__Functions.html#ga1b85a217d43e6b971ffcf24f8aae1c33">os_resume</a>. The parameter <em>sleep_time</em> specifies the time (in RTX Timer Ticks) that the system was in power-down mode.</p>
<p><b>Code</b> <b>Example</b> </p>
<div class="fragment"><div class="line"><span class="preprocessor">#include "LPC11Uxx.h"</span> <span class="comment">/* LPC11Uxx definitions */</span></div>
<div class="line"> </div>
<div class="line"><span class="keywordtype">void</span> <a class="code" href="group__RTX__Global__Functions.html#gafb4dc4d3dff8343a393726d2860282e4">os_idle_demon</a> (<span class="keywordtype">void</span>) {</div>
<div class="line"> <span class="comment">/* The idle demon is a system thread, running when no other thread is */</span></div>
<div class="line"> <span class="comment">/* ready to run. */</span></div>
<div class="line"> <span class="keywordtype">unsigned</span> <span class="keywordtype">int</span> sleep;</div>
<div class="line"> <span class="keywordtype">unsigned</span> <span class="keywordtype">int</span> tc;</div>
<div class="line"> </div>
<div class="line"> LPC_SYSCON->SYSAHBCLKCTRL |= (1UL << 15) | <span class="comment">/* Enable clock for WWDT */</span></div>
<div class="line"> (1UL << 19); <span class="comment">/* Enable clock for Pin IRQ */</span></div>
<div class="line"> </div>
<div class="line"> LPC_SYSCON->PINTSEL[0] = 1; <span class="comment">/* P0.1 selected as INT0 IRQ */</span></div>
<div class="line"> LPC_SYSCON->STARTERP0 |= (1UL << 0); <span class="comment">/* Enable INT0 wake-up */</span></div>
<div class="line"> LPC_SYSCON->STARTERP1 |= (1UL << 12); <span class="comment">/* Enable WWDT wake-up */</span></div>
<div class="line"> </div>
<div class="line"> LPC_SYSCON->WDTOSCCTRL = (3 << 0) | <span class="comment">/* WDOSC DIVSEL=3 */</span></div>
<div class="line"> (2 << 5); <span class="comment">/* WDOSC FREQ=0.8MHz */</span></div>
<div class="line"> LPC_SYSCON->PDRUNCFG &= ~(1UL << 6); <span class="comment">/* Power-up WDT Oscillator */</span></div>
<div class="line"> LPC_SYSCON->PDSLEEPCFG &= ~(1UL << 6); <span class="comment">/* Power WDT Oscillator in PD */</span></div>
<div class="line"> </div>
<div class="line"> LPC_WWDT->CLKSEL = (1UL << 0) | <span class="comment">/* Select WDOSC as Clock */</span></div>
<div class="line"> (1UL << 31); <span class="comment">/* Lock selection */</span></div>
<div class="line"> LPC_WWDT->WARNINT = 1000; <span class="comment">/* WDT Warning IRQ value */</span></div>
<div class="line"> LPC_WWDT->MOD = (1UL << 0); <span class="comment">/* Enable WDT */</span></div>
<div class="line"> </div>
<div class="line"> NVIC_EnableIRQ(FLEX_INT0_IRQn); <span class="comment">/* Enable INT0 IRQ (wake-up) */</span></div>
<div class="line"> NVIC_EnableIRQ(WDT_IRQn); <span class="comment">/* Enable WWDT IRQ (wake-up) */</span></div>
<div class="line"> </div>
<div class="line"> <span class="keywordflow">for</span> (;;) {</div>
<div class="line"> <span class="comment">/* HERE: include optional user code to be executed when no task runs.*/</span></div>
<div class="line"> sleep = <a class="code" href="group__RTX__Global__Functions.html#ga3e9c57746ccbdfe90464ad50513c569e">os_suspend</a>(); <span class="comment">/* Suspend RTX thread scheduler */</span></div>
<div class="line"> </div>
<div class="line"> <span class="keywordflow">if</span> (sleep) { <span class="comment">/* How long can we sleep? */</span></div>
<div class="line"> <span class="comment">/* "sleep" is in RTX Timer Ticks which is 10ms in this configuration */</span></div>
<div class="line"> </div>
<div class="line"> <span class="comment">/* Setup WDT wake-up: WDT ticks @25kHz (FREQ/2/(DIVSEL+1)/4) */</span></div>
<div class="line"> tc = (sleep * 250) + 1000;</div>
<div class="line"> LPC_WWDT->TC = tc;</div>
<div class="line"> LPC_WWDT->FEED = 0xAA;</div>
<div class="line"> LPC_WWDT->FEED = 0x55;</div>
<div class="line"> </div>
<div class="line"> <span class="comment">/* Enter Power-down mode */</span></div>
<div class="line"> LPC_SYSCON->PDAWAKECFG = LPC_SYSCON->PDRUNCFG; <span class="comment">/* Power after wake-up */</span></div>
<div class="line"> LPC_PMU->PCON = 0x02; <span class="comment">/* Select Power-down mode */</span></div>
<div class="line"> SCB->SCR = (1UL << 2); <span class="comment">/* Set SLEEPDEEP */</span></div>
<div class="line"> __WFE(); <span class="comment">/* Enter Power-down mode */</span></div>
<div class="line"> </div>
<div class="line"> <span class="comment">/* After Wake-up */</span></div>
<div class="line"> sleep = (tc - LPC_WWDT->TV) / 250;</div>
<div class="line"> }</div>
<div class="line"> </div>
<div class="line"> <a class="code" href="group__RTX__Global__Functions.html#ga1b85a217d43e6b971ffcf24f8aae1c33">os_resume</a>(sleep); <span class="comment">/* Resume RTX thread scheduler */</span></div>
<div class="line"> }</div>
<div class="line">}</div>
</div><!-- fragment --><dl class="section note"><dt>Note</dt><dd><code>__WFE()</code> is not available at every Cortex-M implementation. Check device manuals for availability. </dd></dl>
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