Arduino Micro serial communication is not working after wake from power down

Question

I'm trying to use the power down state for a battery application, but I'm having issues with the serial communication.

I'm using the lowpower.h library to put the Arduino Micro into power down mode.

When it wakes I flash an LED, which is working, so I know it's is waking up. And I also re-establish serial communication and send a message. This is not working. I receive nothing on the serial monitor, however the TX LED does flash.

I think the Arduino may be working correctly, but for some reason the serial monitor can't handle it. Perhaps there is a better serial monitor application?

#include "LowPower.h"

void setup()
{
    //20sec delay added to make uploading easier (cant upload in power down state)
    Serial.begin(115200);
    Serial.println("20");
    delay(5000);
    Serial.println("15");
    delay(5000);
    Serial.println("10");
    delay(5000);
    Serial.println("5");
    delay(5000);
    Serial.println("START");
    pinMode(LED_BUILTIN, OUTPUT);
    delay(1000);
    Serial.end();
}

void loop() 
{
    // Enter power down state for 8 s with ADC and BOD module disabled
    LowPower.powerDown(SLEEP_8S, ADC_OFF, BOD_OFF);  

    Serial.begin(115200);
    delay(100);
    Serial.println("BLINK");
    digitalWrite(LED_BUILTIN, HIGH);   // turn the LED on (HIGH is the voltage level)
    delay(5000);                       // wait for a second
    Serial.println("BLINK_stop");
    digitalWrite(LED_BUILTIN, LOW);    // turn the LED off by making the voltage LOW
    delay(5000);                       // wait for a second

    Serial.end();

}

Serial monitor output:

20
15
10
5
START

and then nothing...

Answer 1

I found the answer on this related question: Starting usb on pro micro after deep sleep (ATmega32u4)

Which itself was a link to a sparkfun forum

Note: it requires you to close and open the serial monitor at proper timings.

// disable the USB prior going to sleep
USBCON |= _BV(FRZCLK);  //freeze USB clock
PLLCSR &= ~_BV(PLLE);   // turn off USB PLL
USBCON &= ~_BV(USBE);   // disable USB

// and wake it up after
//routine to properly wake up
sleep_disable();
delay(100);
USBDevice.attach(); // keep this
delay(100);
Serial.begin(9600);
delay(100);