TM4C123GH6PM: TM4C RTC Data recover on power reset using Vbat.

The RTC continues to update the seconds (and sub-seconds) counters when running off of VBAT.  The 32 bit seconds counter will overflow after 138 years.  Here is a simple program that prints the number of elapsed seconds using UART0. The RTC counter continues to increment if the Hibernate module is supplied with a 3.3V battery.

//*****************************************************************************
//
// Copyright (c) 2012-2018 Texas Instruments Incorporated.  All rights reserved.
// Software License Agreement
// 
// Texas Instruments (TI) is supplying this software for use solely and
// exclusively on TI's microcontroller products. The software is owned by
// TI and/or its suppliers, and is protected under applicable copyright
// laws. You may not combine this software with "viral" open-source
// software in order to form a larger program.
// 
// THIS SOFTWARE IS PROVIDED "AS IS" AND WITH ALL FAULTS.
// NO WARRANTIES, WHETHER EXPRESS, IMPLIED OR STATUTORY, INCLUDING, BUT
// NOT LIMITED TO, IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
// A PARTICULAR PURPOSE APPLY TO THIS SOFTWARE. TI SHALL NOT, UNDER ANY
// CIRCUMSTANCES, BE LIABLE FOR SPECIAL, INCIDENTAL, OR CONSEQUENTIAL
// DAMAGES, FOR ANY REASON WHATSOEVER.
// 
//*****************************************************************************

#include <stdint.h>
#include <stdbool.h>
#include <time.h>
#include "inc/hw_types.h"
#include "inc/hw_memmap.h"
#include "driverlib/pin_map.h"
#include "driverlib/rom.h"
#include "driverlib/rom_map.h"
#include "driverlib/sysctl.h"
#include "driverlib/gpio.h"
#include "driverlib/uart.h"
#include "driverlib/hibernate.h"
#include "utils/uartstdio.h"

uint32_t g_ui32SysClock;


//*****************************************************************************
//
// Configure the UART and its pins.  This must be called before UARTprintf().
//
//*****************************************************************************
void
ConfigureUART(void)
{
    //
    // Enable the GPIO Peripheral used by the UART.
    //
    ROM_SysCtlPeripheralEnable(SYSCTL_PERIPH_GPIOA);

    //
    // Enable UART0
    //
    ROM_SysCtlPeripheralEnable(SYSCTL_PERIPH_UART0);

    //
    // Configure GPIO Pins for UART mode.
    //
    ROM_GPIOPinConfigure(GPIO_PA0_U0RX);
    ROM_GPIOPinConfigure(GPIO_PA1_U0TX);
    ROM_GPIOPinTypeUART(GPIO_PORTA_BASE, GPIO_PIN_0 | GPIO_PIN_1);

    //
    // Use the internal 16MHz oscillator as the UART clock source.
    //
    UARTClockSourceSet(UART0_BASE, UART_CLOCK_PIOSC);

    //
    // Initialize the UART for console I/O.
    //
    UARTStdioConfig(0, 115200, 16000000);
}
void ConfigureRTC(void)
{
    //
    // Need to enable the hibernation peripheral after wake/reset, before using
    // it.
    //
    SysCtlPeripheralEnable(SYSCTL_PERIPH_HIBERNATE);
    //
    // Wait for the Hibernate module to be ready.
    //
    while(!SysCtlPeripheralReady(SYSCTL_PERIPH_HIBERNATE))
    {
    }
    //
    // Enable clocking to the Hibernation module.
    //
    HibernateEnableExpClk(g_ui32SysClock);
    //
    // User-implemented delay here to allow crystal to power up and stabilize.
    //
    //
    // Configure the clock source for Hibernation module and enable the RTC
    // feature.
    //
    HibernateClockConfig(HIBERNATE_OSC_LOWDRIVE);
    HibernateRTCEnable();
}

//*****************************************************************************
//
// Main 'C' Language entry point.  
//
//*****************************************************************************
int
main(void)
{

    uint32_t seconds;
    //
    // Setup the system clock to run at 80 MHz from PLL with crystal reference
    //
    SysCtlClockSet(SYSCTL_SYSDIV_4|SYSCTL_USE_PLL|SYSCTL_XTAL_16MHZ|
                    SYSCTL_OSC_MAIN);
    g_ui32SysClock = SysCtlClockGet();

    ConfigureUART();
    UARTprintf("Elapsed Seconds!\n");
    UARTprintf("Press 'ESC' to reset the timer\n");
    ConfigureRTC();

    //
    // Loop Forever
    //
    while(1)
    {
        //
        // Delay for a bit
        //
        SysCtlDelay(g_ui32SysClock/8);
        seconds = HibernateRTCGet();
        UARTprintf("\r%d            ", seconds);
        if(UARTCharGetNonBlocking(UART0_BASE) == 27)
        {
            HibernateRTCSet(0);
        }

    }
}