====== SLEEP Mode ======

In SLEEP mode only the cpu clock is disabled, everything else stays as previously configured. \\ \\

=== Enter SLEEP mode ===

  * WFI (wait for interrupt) instuction enters SLEEP mode.
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=== Exit SLEEP mode ===

  * Clear the EXTI (and, if not EXTI, the corresponding NVIC) pending bit.
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===== Programming Example =====

The code snippet bellow shows how to enter (and exit) SLEEP mode.

<code c>
#include "reg_stm32f4xx.h"

RCC->AHB1ENR |= (0x1 <<  0u);       /* Enable GPIOA clock */
RCC->APB2ENR |= (0x1 << 14u);       /* Enable SYSCFG clock */

/* Configure wake up pin (PA.0). */
GPIOA->MODER &= ~(0x3 << 0u);         /* Clear existing mode bits 10 and 11. */

GPIOA->PUPDR &= ~(0x3 << 0u);         /* Clear existing pull-up/-down bits 0 and 1. */
GPIOA->PUPDR |= (0x1 << 0u);          /* Set pin 0 to pull-up mode. */

/* Configure interrupt. */
SYSCFG->EXTICR1 |= (0u << 0u);      /* Set EXTI0 to GPIOA. */
EXTI->RTSR |= (0x1 << 0u);            /* Trigger on rising edge. */
EXTI->IMR |= (0x1 << 0u);             /* Unmask interrupt line. */
NVIC->ISER0 |= (0x1 << 6u);         /* Enable EXTI0 interrupt. */

/* Disable SLEEPDEEP (STOP or STANDBY mode). */
SCB->SCR &= ~(0x1 << 2u);


/* -------------------- Enter SLEEP mode -------------------- */

__asm volatile ("wfi");


/* -------------------- Exit SLEEP mode --------------------- */

/* Since all registers and the SRAM are untouched by the SLEEP mode,
 * the program continues after the WFI instruction.
 * No further steps required. 
 */

</code>
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