Help with Arduino sleep and watchdog timer interrupt

Question

I want to use Arduino's sleep and wakeup functionality but have hit a roadblock and can't seem to get past it. As I am just prototyping for a final system, I cannot use the LowPower library to implement the functionality so I am doing it barebones. I am able to put the Arduino to sleep just fine, the main problem is with the watchdog timer. I have or I think I have configured it for a 8 sec interrupt to wake the Arduino back up from sleep but Arduino comes out of the sleep just as it enters it without any delay. The code is

#include <Arduino.h>
#include <avr/sleep.h>
void goToSleep();
void setupWDT();
void stopWDT();
ISR(WDT_vect) {
 sleep_disable(); // wake up the Arduino
}
void setup() {
 set_sleep_mode(SLEEP_MODE_PWR_DOWN);
 stopWDT();
 pinMode(LED_BUILTIN, OUTPUT);
 Serial.begin(9600);
 Serial.println("Uno is in setup funtion.");
 bool ledState = 0;
 while(millis()<3000){
 digitalWrite(LED_BUILTIN, ledState);
 ledState = !ledState;
 delay(500);
 }
}
void loop() {
 Serial.println("Uno is awake.");
 digitalWrite(LED_BUILTIN, HIGH);
 delay(2000);
 Serial.println("Going to sleep...");
 digitalWrite(LED_BUILTIN, LOW);
 delay(100);
 setupWDT();
 goToSleep();
 stopWDT();
 delay(100);
 digitalWrite(LED_BUILTIN, HIGH);
}
void goToSleep() {
 ADCSRA &= ~(1<<ADEN); // disable ADC
 sleep_mode(); // put the Arduino to sleep
}
void setupWDT() {
 cli();
 WDTCSR |= (1 << WDCE);
 WDTCSR &= ~(1 << WDE);
 WDTCSR |= (1 << WDCE) | (1 << WDIE) | (1 << WDP3) | (1 << WDP0); // configured for 8 sec delay interrupt
 sei();
}
void stopWDT() {
 cli();
 MCUSR &= ~(1 << WDRF); // clear reset flag
 WDTCSR |= (1 << WDCE);
 WDTCSR &= ~(1<<WDE);
 WDTCSR = 0x00; // stop WDT
 sei();
}

Answer 1

Try changing this:

void setupWDT() {
 cli();
 WDTCSR |= (1 << WDCE);
 WDTCSR &= ~(1 << WDE);
 WDTCSR |= (1 << WDCE) | (1 << WDIE) | (1 << WDP3) | (1 << WDP0); // configured for 8 sec delay interrupt
 sei();
}

to:

void setupWDT() {
 cli();
 WDTCSR = (1 << WDCE) | (1 << WDE) ;
 WDTCSR = (1 << WDCE) | (1 << WDIE) | (1 << WDP3) | (1 << WDP0); // configured for 8 sec delay interrupt 
 sei();
}

It appears that your construct violates the 4 clock cycle window that the WDCE Watchdog Change Enable bit allows for updates to the WDE bit.