samd/modmachine: Implement machine.lightsleep().

Which just sets the CPU clock to 200kHz and switches the peripheral clock
off.  There are two modes:

    machine.lightsleep(duration_ms)

and

    machine.lightsleep()

In any mode any configured pin.irq() event will terminate the sleep.

Current consumption in lightsleep for some boards:
- 1.5 - 2.5 mA when supplied trough an active USB
  (Seeed XIAO w/o power LED, Adafruit ItsyBitsy)
- 0.8 - 2 mA when supplied through Gnd/+5V (Vusb)
  (Seeed XIAO w/o power LED, Adafruit ItsyBitsy)
- < 1 mA for SAMD51 when supplied trough a battery connector
  (Sparkfun Thing SAMD51 plus)

Related change: move the calls to SysTick_Config() into set_cpu_freq().  It
is required after each CPU freq change to have ticks_ms run at the proper
rate.

ports/samd/machine_pin.c

@@ -57,6 +57,8 @@ typedef struct _machine_pin_irq_obj_t {
 
 STATIC const mp_irq_methods_t machine_pin_irq_methods;
 
+bool EIC_occured;
+
 uint32_t machine_pin_open_drain_mask[4];
 
 // Open drain behaviour is simulated.
@@ -410,6 +412,7 @@ void EIC_Handler() {
     for (int eic_id = 0; eic_id < 16; eic_id++, mask <<= 1) {
         // Did the ISR fire?
         if (isr & mask) {
+            EIC_occured = true;
             EIC->INTFLAG.reg |= mask; // clear the ISR flag
             machine_pin_irq_obj_t *irq = MP_STATE_PORT(machine_pin_irq_objects[eic_id]);
             if (irq != NULL) {

ports/samd/mcu/samd21/clock_config.c

@@ -81,6 +81,7 @@ void set_cpu_freq(uint32_t cpu_freq_arg) {
     }
     // Set 0 waitstates for slower CPU clock
     NVMCTRL->CTRLB.reg = NVMCTRL_CTRLB_MANW | NVMCTRL_CTRLB_RWS(cpu_freq > 24000000 ? 1 : 0);
+    SysTick_Config(cpu_freq / 1000);
 }
 
 void check_usb_recovery_mode(void) {

ports/samd/mcu/samd51/clock_config.c

@@ -112,6 +112,7 @@ void set_cpu_freq(uint32_t cpu_freq_arg) {
         while (GCLK->SYNCBUSY.bit.GENCTRL5) {
         }
     }
+    SysTick_Config(cpu_freq / 1000);
 }
 
 void check_usb_recovery_mode(void) {

ports/samd/modmachine.c

@@ -50,6 +50,10 @@
 #define DBL_TAP_MAGIC_LOADER 0xf01669ef
 #define DBL_TAP_MAGIC_RESET 0xf02669ef
 
+#define LIGHTSLEEP_CPU_FREQ 200000
+
+extern bool EIC_occured;
+
 STATIC mp_obj_t machine_reset(void) {
     *DBL_TAP_ADDR = DBL_TAP_MAGIC_RESET;
     NVIC_SystemReset();
@@ -71,7 +75,6 @@ STATIC mp_obj_t machine_freq(size_t n_args, const mp_obj_t *args) {
         uint32_t freq = mp_obj_get_int(args[0]);
         if (freq >= 1000000 && freq <= MAX_CPU_FREQ) {
             set_cpu_freq(freq);
-            SysTick_Config(get_cpu_freq() / 1000);
         }
         return mp_const_none;
     }
@@ -151,6 +154,60 @@ STATIC mp_obj_t machine_reset_cause(void) {
 }
 MP_DEFINE_CONST_FUN_OBJ_0(machine_reset_cause_obj, machine_reset_cause);
 
+STATIC mp_obj_t machine_lightsleep(size_t n_args, const mp_obj_t *args) {
+    int32_t duration = -1;
+    uint32_t freq = get_cpu_freq();
+    if (n_args > 0) {
+        duration = mp_obj_get_int(args[0]);
+    }
+    EIC_occured = false;
+    // Slow down
+    set_cpu_freq(LIGHTSLEEP_CPU_FREQ);
+    #if defined(MCU_SAMD21)
+    // Switch the peripheral clock off
+    GCLK->GENCTRL.reg = GCLK_GENCTRL_ID(2);
+    while (GCLK->STATUS.bit.SYNCBUSY) {
+    }
+    // Switch the EIC temporarily to GCLK3, since GCLK2 is off
+    GCLK->CLKCTRL.reg = GCLK_CLKCTRL_CLKEN | GCLK_CLKCTRL_GEN_GCLK3 | EIC_GCLK_ID;
+    if (duration > 0) {
+        uint32_t t0 = systick_ms;
+        while ((systick_ms - t0 < duration) && (EIC_occured == false)) {
+            __WFI();
+        }
+    } else {
+        while (EIC_occured == false) {
+            __WFI();
+        }
+    }
+    GCLK->CLKCTRL.reg = GCLK_CLKCTRL_CLKEN | GCLK_CLKCTRL_GEN_GCLK2 | EIC_GCLK_ID;
+
+    #elif defined(MCU_SAMD51)
+    // Switch the peripheral clock off
+    GCLK->GENCTRL[2].reg = 0;
+    while (GCLK->SYNCBUSY.bit.GENCTRL2) {
+    }
+    // Switch the EIC temporarily to GCLK3, since GCLK2 is off
+    GCLK->PCHCTRL[EIC_GCLK_ID].reg = GCLK_PCHCTRL_CHEN | GCLK_PCHCTRL_GEN_GCLK3;
+    if (duration > 0) {
+        uint32_t t0 = systick_ms;
+        while ((systick_ms - t0 < duration) && (EIC_occured == false)) {
+            __WFI();
+        }
+    } else {
+        while (EIC_occured == false) {
+            __WFI();
+        }
+    }
+    GCLK->PCHCTRL[EIC_GCLK_ID].reg = GCLK_PCHCTRL_CHEN | GCLK_PCHCTRL_GEN_GCLK2;
+
+    #endif
+    // Speed up again
+    set_cpu_freq(freq);
+    return mp_const_none;
+}
+STATIC MP_DEFINE_CONST_FUN_OBJ_VAR_BETWEEN(machine_lightsleep_obj, 0, 1, machine_lightsleep);
+
 STATIC const mp_rom_map_elem_t machine_module_globals_table[] = {
     { MP_ROM_QSTR(MP_QSTR___name__),            MP_ROM_QSTR(MP_QSTR_umachine) },
     { MP_ROM_QSTR(MP_QSTR_reset),               MP_ROM_PTR(&machine_reset_obj) },
@@ -179,6 +236,8 @@ STATIC const mp_rom_map_elem_t machine_module_globals_table[] = {
     { MP_ROM_QSTR(MP_QSTR_enable_irq),          MP_ROM_PTR(&machine_enable_irq_obj) },
     { MP_ROM_QSTR(MP_QSTR_reset_cause),         MP_ROM_PTR(&machine_reset_cause_obj) },
     { MP_ROM_QSTR(MP_QSTR_time_pulse_us),       MP_ROM_PTR(&machine_time_pulse_us_obj) },
+    { MP_ROM_QSTR(MP_QSTR_lightsleep),          MP_ROM_PTR(&machine_lightsleep_obj) },
+
     { MP_ROM_QSTR(MP_QSTR_bitstream),           MP_ROM_PTR(&machine_bitstream_obj) },
     #if MICROPY_PY_MACHINE_DHT_READINTO
     { MP_ROM_QSTR(MP_QSTR_dht_readinto),        MP_ROM_PTR(&dht_readinto_obj) },

ports/samd/samd_soc.c

@@ -108,7 +108,6 @@ void init_us_counter(void) {
 
 void samd_init(void) {
     init_clocks(get_cpu_freq());
-    SysTick_Config(get_cpu_freq() / 1000);
     init_us_counter();
     usb_init();
     check_usb_recovery_mode();