nrf/modules/machine: Implement deepsleep().

Replace the previous dummy implementation of machine.deepsleep() with one
that invokes the nRF's deep sleep state "System OFF". The argument is
ignored, nRF microcontrollers are incapable of waking on a timer as all
clocks are off in the deep sleep state.

There is no way yet to wake from deep sleep except using the reset pin, a
way to configure GPIO pins as wake sources is to be added in a subsequent
commit.

machine.reset_cause() returns the newly added DEEPSLEEP_RESET, like on
other ports – the previous mapping to PWRON_RESET seemed wrong.

Signed-off-by: Christian Walther <cwalther@gmx.ch>

docs/library/machine.rst

@@ -160,6 +160,12 @@ Power related functions
      return `machine.DEEPSLEEP` and this can be used to distinguish a deepsleep wake
      from other resets.
 
+   *nrf port:*
+   
+   * The *time_ms* argument to `deepsleep` is ignored, the sleep is always indefinite.
+     nRF microcontrollers are incapable of waking on a timer as all clocks are off in
+     the deep sleep state.
+
 .. function:: wake_reason()
 
    Get the wake reason. See :ref:`constants <machine_constants>` for the possible return values.

ports/nrf/modules/machine/modmachine.c

@@ -49,12 +49,20 @@
 #if MICROPY_PY_MACHINE_RTCOUNTER
 #include "rtcounter.h"
 #endif
+#if defined(POWER_SYSTEMOFF_SYSTEMOFF_Enter)
+// nRF51/52
+#include "nrf_power.h"
+#elif defined(REGULATORS_SYSTEMOFF_SYSTEMOFF_Enable)
+// nRF91
+#include "nrf_regulators.h"
+#endif
 
+#define PYB_RESET_POWER_ON  (32)
 #define PYB_RESET_HARD      (0)
 #define PYB_RESET_WDT       (1)
 #define PYB_RESET_SOFT      (2)
 #define PYB_RESET_LOCKUP    (3)
-#define PYB_RESET_POWER_ON  (16)
+#define PYB_RESET_DEEPSLEEP (16)
 #define PYB_RESET_LPCOMP    (17)
 #define PYB_RESET_DIF       (18)
 #define PYB_RESET_NFC       (19)
@@ -93,11 +101,12 @@
     MICROPY_PY_MACHINE_RTCOUNTER_ENTRY \
     MICROPY_PY_MACHINE_TIMER_ENTRY \
     MICROPY_PY_MACHINE_TEMP_ENTRY \
+    { MP_ROM_QSTR(MP_QSTR_PWRON_RESET),        MP_ROM_INT(PYB_RESET_POWER_ON) }, \
     { MP_ROM_QSTR(MP_QSTR_HARD_RESET),         MP_ROM_INT(PYB_RESET_HARD) }, \
     { MP_ROM_QSTR(MP_QSTR_WDT_RESET),          MP_ROM_INT(PYB_RESET_WDT) }, \
     { MP_ROM_QSTR(MP_QSTR_SOFT_RESET),         MP_ROM_INT(PYB_RESET_SOFT) }, \
     { MP_ROM_QSTR(MP_QSTR_LOCKUP_RESET),       MP_ROM_INT(PYB_RESET_LOCKUP) }, \
-    { MP_ROM_QSTR(MP_QSTR_PWRON_RESET),        MP_ROM_INT(PYB_RESET_POWER_ON) }, \
+    { MP_ROM_QSTR(MP_QSTR_DEEPSLEEP_RESET),    MP_ROM_INT(PYB_RESET_DEEPSLEEP) }, \
     { MP_ROM_QSTR(MP_QSTR_LPCOMP_RESET),       MP_ROM_INT(PYB_RESET_LPCOMP) }, \
     { MP_ROM_QSTR(MP_QSTR_DEBUG_IF_RESET),     MP_ROM_INT(PYB_RESET_DIF) }, \
     MICROPY_PY_MACHINE_NFC_RESET_ENTRY \
@@ -115,7 +124,7 @@ void machine_init(void) {
     } else if (state & POWER_RESETREAS_LOCKUP_Msk) {
         reset_cause = PYB_RESET_LOCKUP;
     } else if (state & POWER_RESETREAS_OFF_Msk) {
-        reset_cause = PYB_RESET_POWER_ON;
+        reset_cause = PYB_RESET_DEEPSLEEP;
     #if !defined(NRF9160_XXAA)
     } else if (state & POWER_RESETREAS_LPCOMP_Msk) {
         reset_cause = PYB_RESET_LPCOMP;
@@ -126,10 +135,12 @@ void machine_init(void) {
     } else if (state & POWER_RESETREAS_NFC_Msk) {
         reset_cause = PYB_RESET_NFC;
     #endif
+    } else {
+        reset_cause = PYB_RESET_POWER_ON;
     }
 
     // clear reset reason
-    NRF_POWER->RESETREAS = (1 << reset_cause);
+    NRF_POWER->RESETREAS = 0xFFFFFFFF;
 }
 
 // machine.info([dump_alloc_table])
@@ -200,7 +211,18 @@ static void mp_machine_lightsleep(size_t n_args, const mp_obj_t *args) {
 }
 
 NORETURN static void mp_machine_deepsleep(size_t n_args, const mp_obj_t *args) {
-    mp_machine_reset();
+    #if defined(POWER_SYSTEMOFF_SYSTEMOFF_Enter)
+    // nRF51/52
+    nrf_power_system_off(NRF_POWER);
+    #elif defined(REGULATORS_SYSTEMOFF_SYSTEMOFF_Enable)
+    // nRF91
+    nrf_regulators_system_off(NRF_REGULATORS);
+    #else
+    #error figure out how to enter System OFF mode on this chip
+    #endif
+    // never reached, just to convince the compiler of NORETURN
+    for (;;) {
+    }
 }
 
 static mp_int_t mp_machine_reset_cause(void) {