alif/machine_rtc: Implement RTC.datetime to get and set the RTC.

The LPRTC peripheral is a 32-bit counter with a 16-bit prescaler.  It's
configured here to count at 1Hz (to get maximum date range) and then the
prescaler value is used to get 30 microsecond resolution.  That's
essentially a 32+15=47-bit counter.

Signed-off-by: Damien George <damien@micropython.org>

Author: dpgeorge

ports/alif/machine_rtc.c

@@ -28,9 +28,20 @@
 #include "py/mphal.h"
 #include "py/mperrno.h"
 #include "extmod/modmachine.h"
+#include "shared/timeutils/timeutils.h"
 #include "rtc.h"
 #include "sys_ctrl_rtc.h"
 
+// The LPRTC (low-power real-time counter) is a 32-bit counter with a 16-bit prescaler,
+// and usually clocked by a 32768Hz clock source.  To get a large date range of around
+// 136 years, the prescaler is set to 32768 and so the counter clocks at 1Hz.  Then the
+// counter counts the number of seconds since the 1970 Epoch.  The prescaler is used to
+// get the subseconds which are then converted to microseconds.
+//
+// The combined counter+prescaler counts starting at 0 from the year 1970 up to the year
+// 2106, with a resolution of 30.52 microseconds.
+#define LPRTC_PRESCALER_SETTING (32768)
+
 typedef struct _machine_rtc_obj_t {
     mp_obj_base_t base;
     LPRTC_Type *rtc;
@@ -44,24 +55,97 @@ void LPRTC_IRQHandler(void) {
     lprtc_interrupt_disable(machine_rtc.rtc);
 }
 
+// Returns the number of seconds and microseconds since the Epoch.
+uint32_t mp_hal_time_get(uint32_t *microseconds) {
+    uint32_t atomic_state = MICROPY_BEGIN_ATOMIC_SECTION();
+    uint32_t count = lprtc_get_count(machine_rtc.rtc);
+    if (microseconds == NULL) {
+        MICROPY_END_ATOMIC_SECTION(atomic_state);
+        return count;
+    }
+    uint32_t prescaler = machine_rtc.rtc->LPRTC_CPCVR;
+    uint32_t count2 = lprtc_get_count(machine_rtc.rtc);
+    if (count != count2) {
+        // The counter incremented during sampling of the prescaler, so resample the prescaler.
+        prescaler = machine_rtc.rtc->LPRTC_CPCVR;
+    }
+    MICROPY_END_ATOMIC_SECTION(atomic_state);
+
+    // Compute the microseconds from the up-counting prescaler value.
+    MP_STATIC_ASSERT(LPRTC_PRESCALER_SETTING == 32768);
+    *microseconds = 15625UL * prescaler / 512UL;
+
+    return count2;
+}
+
 static mp_obj_t machine_rtc_make_new(const mp_obj_type_t *type, size_t n_args, size_t n_kw, const mp_obj_t *args) {
     const machine_rtc_obj_t *self = &machine_rtc;
 
     // Check arguments.
     mp_arg_check_num(n_args, n_kw, 0, 0, false);
 
-    enable_lprtc_clk();
-    lprtc_prescaler_disable(self->rtc);
-    lprtc_counter_wrap_disable(self->rtc);
     lprtc_interrupt_disable(self->rtc);
     lprtc_interrupt_unmask(self->rtc);
 
+    // Initialise the LPRTC if it's not already enabled.
+    if (!((VBAT->RTC_CLK_EN & RTC_CLK_ENABLE)
+          && (self->rtc->LPRTC_CCR & CCR_LPRTC_EN)
+          && (self->rtc->LPRTC_CPSR == LPRTC_PRESCALER_SETTING))) {
+        enable_lprtc_clk();
+        self->rtc->LPRTC_CCR = 0;
+        lprtc_load_prescaler(self->rtc, LPRTC_PRESCALER_SETTING);
+        lprtc_load_count(self->rtc, 0);
+        self->rtc->LPRTC_CCR = CCR_LPRTC_PSCLR_EN | CCR_LPRTC_EN;
+    }
+
     NVIC_SetPriority(LPRTC_IRQ_IRQn, IRQ_PRI_RTC);
     NVIC_ClearPendingIRQ(LPRTC_IRQ_IRQn);
     NVIC_EnableIRQ(LPRTC_IRQ_IRQn);
+
     return MP_OBJ_FROM_PTR(self);
 }
 
+static mp_obj_t machine_rtc_datetime(mp_uint_t n_args, const mp_obj_t *args) {
+    if (n_args == 1) {
+        // Get datetime.
+        uint32_t microseconds;
+        mp_timestamp_t s = mp_hal_time_get(&microseconds);
+        timeutils_struct_time_t tm;
+        timeutils_seconds_since_epoch_to_struct_time(s, &tm);
+        mp_obj_t tuple[8] = {
+            mp_obj_new_int(tm.tm_year),
+            mp_obj_new_int(tm.tm_mon),
+            mp_obj_new_int(tm.tm_mday),
+            mp_obj_new_int(tm.tm_wday),
+            mp_obj_new_int(tm.tm_hour),
+            mp_obj_new_int(tm.tm_min),
+            mp_obj_new_int(tm.tm_sec),
+            mp_obj_new_int(microseconds),
+        };
+        return mp_obj_new_tuple(8, tuple);
+    } else {
+        // Set datetime.
+        mp_obj_t *items;
+        mp_obj_get_array_fixed_n(args[1], 8, &items);
+        timeutils_struct_time_t tm = {
+            .tm_year = mp_obj_get_int(items[0]),
+            .tm_mon = mp_obj_get_int(items[1]),
+            .tm_mday = mp_obj_get_int(items[2]),
+            .tm_hour = mp_obj_get_int(items[4]),
+            .tm_min = mp_obj_get_int(items[5]),
+            .tm_sec = mp_obj_get_int(items[6]),
+        };
+        mp_timestamp_t s = timeutils_seconds_since_epoch(tm.tm_year, tm.tm_mon, tm.tm_mday, tm.tm_hour, tm.tm_min, tm.tm_sec);
+
+        // Disable then re-enable the LPRTC so that the prescaler counter resets to 0.
+        machine_rtc.rtc->LPRTC_CCR = 0;
+        lprtc_load_count(machine_rtc.rtc, s);
+        machine_rtc.rtc->LPRTC_CCR = CCR_LPRTC_PSCLR_EN | CCR_LPRTC_EN;
+    }
+    return mp_const_none;
+}
+static MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(machine_rtc_datetime_obj, 1, 2, machine_rtc_datetime);
+
 static mp_obj_t machine_rtc_alarm(size_t n_args, const mp_obj_t *pos_args, mp_map_t *kw_args) {
     enum { ARG_id, ARG_time, ARG_repeat };
 
@@ -80,13 +164,18 @@ static mp_obj_t machine_rtc_alarm(size_t n_args, const mp_obj_t *pos_args, mp_ma
     if (mp_obj_is_int(args[ARG_time].u_obj)) {
         uint32_t seconds = mp_obj_get_int(args[1].u_obj) / 1000;
 
-        lprtc_counter_disable(self->rtc);
-        lprtc_load_count(self->rtc, 1);
-        lprtc_load_counter_match_register(self->rtc, seconds * 32768);
+        // Make sure we are guaranteed an interrupt:
+        // - if seconds = 0 it won't fire
+        // - if seconds = 1 it may miss if the counter rolls over just after it's read
+        // - if seconds >= 2 then it will always fire (when read/written close enough)
+        seconds = MAX(2, seconds);
 
+        // Configure the counter match as atomically as possible.
+        uint32_t atomic_state = MICROPY_BEGIN_ATOMIC_SECTION();
         lprtc_interrupt_ack(self->rtc);
+        lprtc_load_counter_match_register(self->rtc, lprtc_get_count(self->rtc) + seconds);
         lprtc_interrupt_enable(self->rtc);
-        lprtc_counter_enable(self->rtc);
+        MICROPY_END_ATOMIC_SECTION(atomic_state);
     } else {
         mp_raise_ValueError(MP_ERROR_TEXT("invalid argument(s)"));
     }
@@ -96,6 +185,7 @@ static mp_obj_t machine_rtc_alarm(size_t n_args, const mp_obj_t *pos_args, mp_ma
 static MP_DEFINE_CONST_FUN_OBJ_KW(machine_rtc_alarm_obj, 1, machine_rtc_alarm);
 
 static const mp_rom_map_elem_t machine_rtc_locals_dict_table[] = {
+    { MP_ROM_QSTR(MP_QSTR_datetime), MP_ROM_PTR(&machine_rtc_datetime_obj) },
     { MP_ROM_QSTR(MP_QSTR_alarm), MP_ROM_PTR(&machine_rtc_alarm_obj) },
 };
 static MP_DEFINE_CONST_DICT(machine_rtc_locals_dict, machine_rtc_locals_dict_table);

ports/alif/mphalport.h

@@ -373,3 +373,5 @@ enum {
 void mp_hal_generate_laa_mac(int idx, uint8_t buf[6]);
 void mp_hal_get_mac(int idx, uint8_t buf[6]);
 void mp_hal_get_mac_ascii(int idx, size_t chr_off, size_t chr_len, char *dest);
+
+uint32_t mp_hal_time_get(uint32_t *microseconds);