Merge pull request #2948 from d00616/cpu_nrf51_poweropt_random

cpu/nrf51822/periph/random Power optimization

cpu/nrf51822/periph/random.c

@@ -14,6 +14,7 @@
  * @brief       Driver for the NRF51822 random number generator
  *
  * @author      Hauke Petersen <hauke.petersen@fu-berlin.de>
+ * @author      Frank Holtz <frank-riot2015@holtznet.de>
  *
  * @}
  */
@@ -27,31 +28,56 @@
 
 void random_init(void)
 {
-    NRF_RNG->POWER = 1;
-    NRF_RNG->TASKS_START = 1;
+    /* The RNG is initiated every time when RNG read is called
+     * This reduces power consumption when RNG is not needed
+     */
 }
 
 int random_read(char *buf, unsigned int num)
 {
     unsigned int count = 0;
 
+    /* power on RNG */
+    NRF_RNG->POWER = 1;
+    /* Generate events when RNG is finished */
+    NRF_RNG->INTENSET = RNG_INTENSET_VALRDY_Msk;
+    /* Start Task */
+    NRF_RNG->TASKS_START = 1;
+    /* NRF51 PAN #21 */
+    NRF_RNG->EVENTS_VALRDY = 0;
+
     while (count < num) {
-        while (NRF_RNG->EVENTS_VALRDY == 0);
-        NRF_RNG->EVENTS_VALRDY = 0;
+        /* sleep until number is generated */
+        while (NRF_RNG->EVENTS_VALRDY == 0) {
+            /* enable wake up on events for __WFE CPU sleep */
+            SCB->SCR |= SCB_SCR_SEVONPEND_Msk;
+            /* sleep until next event */
+            __SEV();
+            __WFE();
+            __WFE();
+        }
+
         buf[count++] = (char)NRF_RNG->VALUE;
+        /* NRF51 PAN #21 */
+        NRF_RNG->EVENTS_VALRDY = 0;
+        /* clear interrupt state */
+        NVIC_ClearPendingIRQ(RNG_IRQn);
     }
 
+    /* power off RNG */
+    NRF_RNG->POWER = 0;
+
     return count;
 }
 
 void random_poweron(void)
 {
-    NRF_RNG->POWER = 1;
+    /* RNG is powered on when needed */
 }
 
 void random_poweroff(void)
 {
-    NRF_RNG->POWER = 0;
+    /* RNG is powered off automaticly */
 }
 
 #endif /* RANDOM_NUMOF */