TARGET_NRF5: changed 'sleep()' implementation to the one used in mbedOS 3.

Author: anangl

hal/targets/hal/TARGET_NORDIC/TARGET_NRF5/sleep.c

@@ -16,13 +16,55 @@
 #include "sleep_api.h"
 #include "cmsis.h"
 #include "mbed_interface.h"
+#include "nrf_soc.h"
+#include "nrf_sdm.h"
+
+// Mask of reserved bits of the register ICSR in the System Control Block peripheral
+// In this case, bits which are equal to 0 are the bits reserved in this register
+#define SCB_ICSR_RESERVED_BITS_MASK     0x9E43F03F
 
 void sleep(void)
 {
     // ensure debug is disconnected if semihost is enabled....
     NRF_POWER->TASKS_LOWPWR = 1;
-    // wait for interrupt
-    __WFE();
+
+    SCB->SCR |= SCB_SCR_SEVONPEND_Msk; /* send an event when an interrupt is pending.
+                                        * This helps with the wakeup from the following app_evt_wait(). */
+
+    uint8_t sd_enabled;
+
+    // look if exceptions are enabled or not, if they are, it is possible to make an SVC call
+    // and check if the soft device is running
+    if ((__get_PRIMASK() == 0) && (sd_softdevice_is_enabled(&sd_enabled) == NRF_SUCCESS) && (sd_enabled == 1)) {
+        // soft device is enabled, use the primitives from the soft device to go to sleep
+        sd_app_evt_wait();
+    } else {
+        // Note: it is not possible to just use WFE at this stage because WFE
+        // will read the event register (not accessible) and if an event occured,
+        // in the past, it will just clear the event register and continue execution.
+        // SVC call like sd_softdevice_is_enabled set the event register to 1.
+        // This means that a making an SVC call followed by WFE will never put the
+        // CPU to sleep.
+        // Our startegy here is to clear the event register then, if there is any
+        // interrupt, return from here. If no interrupts are pending, just call
+        // WFE.
+
+        // Set an event and wake up whatsoever, this will clear the event
+        // register from all previous events set (SVC call included)
+        __SEV();
+        __WFE();
+
+        // Test if there is an interrupt pending (mask reserved regions)
+        if (SCB->ICSR & (SCB_ICSR_RESERVED_BITS_MASK)) {
+            // Ok, there is an interrut pending, no need to go to sleep
+            return;
+        } else {
+            // next event will wakeup the CPU
+            // If an interrupt occured between the test of SCB->ICSR and this
+            // instruction, WFE will just not put the CPU to sleep
+            __WFE();
+        }
+    }
 }
 
 void deepsleep(void)