nrf: Initial WDT-in-C implementation

Currently it is difficult to deploy uPy on form factor devices without
a reset pin. Any mistake in the boot code risks the system getting
stuck (at least until the battery goes flat). A watchdog and user
button can be combined to give a long-press reset that gives the user
a better shot at recovering the device before the battery runs down!

ports/nrf/Makefile

@@ -232,6 +232,7 @@ SRC_C += \
 	drivers/flash.c \
 	drivers/softpwm.c \
 	drivers/ticker.c \
+	drivers/wdt.c \
 	drivers/bluetooth/ble_drv.c \
 	drivers/bluetooth/ble_uart.c \
 

ports/nrf/drivers/bluetooth/ble_uart.c

@@ -31,6 +31,7 @@
 #include "ringbuffer.h"
 #include "mphalport.h"
 #include "lib/utils/interrupt_char.h"
+#include "wdt.h"
 
 #if MICROPY_PY_BLE_NUS
 
@@ -93,10 +94,14 @@ static ubluepy_advertise_data_t m_adv_data_eddystone_url;
 #endif // BLUETOOTH_WEBBLUETOOTH_REPL
 
 int mp_hal_stdin_rx_chr(void) {
+    wdt_feed();
+
     while (!ble_uart_enabled()) {
         // wait for connection
+	wdt_feed();
     }
     while (isBufferEmpty(mp_rx_ring_buffer)) {
+	wdt_feed();
         ;
     }
 

ports/nrf/drivers/wdt.c

@@ -0,0 +1,93 @@
+/*
+ * This file is part of the MicroPython project, http://micropython.org/
+ *
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2016 Daniel Thompson
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+#include "py/mphal.h"
+
+#if MICROPY_PY_MACHINE_WDT
+
+#include "nrf_wdt.h"
+
+#if MICROPY_HW_HAS_WDT_BUTTON
+static void button_init(void)
+{
+    nrf_gpio_cfg_sense_input(
+            MICROPY_HW_WDT_BUTTON,
+            MICROPY_HW_WDT_BUTTON_PULL < 0 ? NRF_GPIO_PIN_PULLDOWN :
+            MICROPY_HW_WDT_BUTTON_PULL > 0 ? NRF_GPIO_PIN_PULLUP :
+                                             NRF_GPIO_PIN_NOPULL,
+            NRF_GPIO_PIN_NOSENSE);
+
+#if MICROPY_HW_WDT_BUTTON_ENABLE
+    nrf_gpio_cfg_output(MICROPY_HW_WDT_BUTTON_ENABLE);
+    nrf_gpio_pin_write(MICROPY_HW_WDT_BUTTON_ENABLE,
+                       MICROPY_HW_WDT_BUTTON_ACTIVE);
+#endif
+}
+
+static bool button_pressed(void)
+{
+    return nrf_gpio_pin_read(MICROPY_HW_WDT_BUTTON) == MICROPY_HW_WDT_BUTTON_ACTIVE;
+}
+#endif
+
+void wdt_init(void)
+{
+    if (!nrf_wdt_started()) {
+        // 1 => keep running during a sleep, stop during SWD debug
+        nrf_wdt_behaviour_set(1);
+
+        // timeout after 5 seconds
+        nrf_wdt_reload_value_set(5 * 32768);
+
+        // enable the 0th channel
+        nrf_wdt_reload_request_enable(NRF_WDT_RR0);
+
+        // set it running
+        nrf_wdt_task_trigger(NRF_WDT_TASK_START);
+
+#if MICROPY_HW_HAS_WDT_BUTTON
+	// if there was no bootloader to configure the WDT then it
+	// probably didn't setup the pins for the button either
+	button_init();
+#endif
+    }
+}
+
+void wdt_feed(void)
+{
+    /*
+     * A WDT button allows us to feed the dog from somewhere that would
+     * normally be "silly", such as a periodic timer interrupt. By providing
+     * the user direct control over feeding the dog we are, in effect,
+     * implementing a (reasonably robust) long-press reset button.
+     */
+#if MICROPY_HW_HAS_WDT_BUTTON
+    if (!button_pressed())
+#endif
+        nrf_wdt_reload_request_set(0);
+}
+
+#endif // MICROPY_PY_MACHINE_WDT

ports/nrf/drivers/wdt.h

@@ -0,0 +1,31 @@
+/*
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2020 Daniel Thompson
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+
+#ifndef __MICROPY_INCLUDED_LIB_WDT_H__
+#define __MICROPY_INCLUDED_LIB_WDT_H__
+
+void wdt_init(void);
+void wdt_feed(void);
+
+#endif // __MICROPY_INCLUDED_LIB_WDT_H__

ports/nrf/main.c

@@ -56,6 +56,7 @@
 #include "pwm.h"
 #endif
 #include "timer.h"
+#include "wdt.h"
 
 #if BLUETOOTH_SD
 #include "nrf_sdm.h"
@@ -99,9 +100,12 @@ extern uint32_t _heap_end;
 
 int main(int argc, char **argv) {
 
-
 soft_reset:
 
+#if MICROPY_PY_MACHINE_WDT
+    wdt_init();
+#endif
+
     led_init();
 
     led_state(1, 1); // MICROPY_HW_LED_1 aka MICROPY_HW_LED_RED