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