Add test_reset example demonstrating/testing reset cause determination

optiboot/examples/test_reset/test_reset.ino

@@ -0,0 +1,82 @@
+/*
+ * test_reset
+ * May 2015 by Bill Westfield (WestfW)
+ * Released to the public domain.
+ *
+ * This sketch demonstrates retrival of the Reset Cause register (MCUSR) of the AVR.
+ * Normally, MCUSR itself is destroyed by the use of a bootloader, but Optiboot v4.6
+ * and later save the contents in register r2, where it can be accessed by an
+ * application.
+ */
+#include <avr/wdt.h>
+
+/*
+ * First, we need a variable to hold the reset cause that can be written before
+ * early sketch initialization (that might change r2), and won't be reset by the
+ * various initialization code.
+ * avr-gcc provides for this via the ".noinit" section.
+ */
+uint8_t resetFlags __attribute__ ((section(".noinit")));
+
+/*
+ * Next, we need to put some code to save reset cause from the bootload (in r2)
+ * to the variable.  Again, avr-gcc provides special code sections for this.
+ */
+void resetFlagsInit(void) __attribute__ ((naked))
+                          __attribute__ ((section (".init0")));
+void resetFlagsInit(void)
+{
+  /*
+   * save the reset flags passed from the bootloader
+   * This is a "simple" matter of storing (STS) r2 in the special variable
+   * that we have created.  We use assembler to access the right variable.
+   */
+  __asm__ __volatile__ ("sts %0, r2\n" : "=m" (resetFlags) :);
+}
+
+void setup() {
+  Serial.begin(9600);  // Initialize serial port
+}
+
+void loop() {
+  Serial.println("Reset flag test");
+  while (Serial.read() < 0)
+    ;  // wait for some type-in
+
+  Serial.print("Have reset flag value 0x");
+  Serial.print(resetFlags, HEX);
+  /*
+   * check for the usual bits.  Note that the symnbols defined in wdt.h are
+   * bit numbers, so they have to be shifted before comparison.
+   */
+  switch (resetFlags) {
+    case 1<<WDRF:
+      Serial.println(" (Watchdog)\n");
+      break;
+    case 1<<BORF:
+      Serial.println(" (Brownout)\n");
+      break;
+    case 1<<EXTRF:
+      Serial.println(" (External Reset)\n");
+      break;
+    case  1<<PORF:
+    case ((1<<PORF) | (1<<BORF)):
+      // A poweron can frequently result in both PORF and BORF being set, as the
+      //  power switch bounces or the voltage rises slowly.
+      Serial.println(" (PowerOn)\n");
+      break;
+    default:
+      // Multiple bits can be set under various circumstances.  For example, the
+      //  "hold down reset while powering up" trick used to recover from certain
+      //  problems can leave both PORF and EXTRF set.
+      Serial.println(" (Unknown)\n");
+      break;
+  }
+  /*
+   * Now we'll let the Watchdog reset the chip, and see if that cause is reported
+   * correctly.  Older versions of optiboot could not easily distinguish between
+   * WDRF and EXTRF resets, since the WDT was used inside the bootloader.
+   */
+  wdt_enable(WDTO_1S);
+}
+