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Implications of not having VBAT connected?

Devon St. Pierre
Prodigy 250 points
Other Parts Discussed in Thread: MSP430F5636

Greetings... I'm working with a MSP430F5636 and I'm using the Battery Backup system.  The 5xx/6xx Family User's Guide clearly states that if you're not using the Battery Backup system that VBAT should be connect to VCC.  In my case VBAT is connected to a 3 volt chip style battery holder.  Things work fine as long as I have a battery installed.  But if I don't have the battery installed my system hangs.  Where does it hang? How does it hang you ask?  We'll I"m not quite sure.  If I'm running with a debugger things run fine even without the backup battery installed.  If I run stand a lone I'm not able to communicate with the UCA0 serial interface like I can otherwise.  I'm using the usual start up code to get XT1 and RTC going:

UCSCTL6 &= ~(XT1OFF);                     // XT1 On
RTCCTL01 &= ~(RTCHOLD);                   // Start RTC calendar mode
while(BAKCTL & LOCKBAK) // Unlock XT1 pins for operation 
    BAKCTL &= ~(LOCKBAK);  

// Loop until XT1 & DCO stabilizes - In this case loop until XT1 and DCO settle  
do  {  
    UCSCTL7 &= ~(XT2OFFG + XT1LFOFFG + DCOFFG);  // Clear XT2,XT1,DCO fault flags  
    SFRIFG1 &= ~OFIFG; // Clear fault flags  
}  while (SFRIFG1&OFIFG); // Test oscillator fault flag

So thus my question:  What are the implications of not having VBAT connected?  I don't expect the RTC to maintain the time if there is no battery installed but I would expect my system to come up and run (albeit with a bad time).
  • 0
    Guru 25210 points

    Devon St. Pierre said:
    What are the implications of not having VBAT connected?

    The implications are that your device will not work correctly, and that TI does not guarantee correct operation. As you said yourself:

    Devon St. Pierre said:
    The 5xx/6xx Family User's Guide clearly states that if you're not using the Battery Backup system that VBAT should be connect to VCC.

    You should probably look at battery switchover ICs (a number of manufacturer's make them) that will power VBAT from VCC and then if VCC fails switches to supplying VBAT from the battery.

    See this product catalog search: http://www.ti.com/lsds/ti/power-management/supervisor-reset-ic-products.page#p2192=Battery%20Backup%20Switchover

  • 0
    Prodigy 250 points

    Greetings... what I've found is that program execution gets stuck in this while loop that is used in all the TI example code when modifying clock configuration:

    // Loop until XT1 & DCO stabilizes - In this case loop until XT1 and DCo settle
    do
    {
      UCSCTL7 &= ~(XT2OFFG + XT1LFOFFG + DCOFFG);
                                              // Clear XT2,XT1,DCO fault flags
      SFRIFG1 &= ~OFIFG;                      // Clear fault flags
    }
    while (SFRIFG1&OFIFG);                   // Test oscillator fault flag
     
    I've found that if I comment out this while loop that things will come up and run.  My testing shows that the DCO and XT1LF fault flags remain set in UCSCTL7 and that the RTC doesn't increment, which is not really a problem since you won't expect the RTC to work if you don't have a backup battery installed.  What's interesting is that despite the fault flag being set for XT1LF, ACLK is working so that I can use it to successfully generate the baud rate on UCA0 to be able to communicate with my board. For now I'll leave the while loop commented out and watch for other problems that may results from the DCO and XT1LF fault flags remaining set.
  • 0 in reply to Brian Boorman
    Prodigy 250 points

    Brian,

    Thanks for your reply... As you've probably already seen in my other post, I found a work around for the problem I was experiencing.  

    I did take a look at one of the battery switchover ICs you referenced.  I must be missing something.  What good does it do to have a battery backup system built into the MSP430 if you have to use an additional external chip to be able to use it when there is no backup battery installed?

    As I mentioned in my original post: 

    I don't expect the RTC to maintain the time if there is no battery installed but I would expect my system to come up and run (albeit with a bad time).

    Thanks...
  • 0
    Guru 227245 points

    Take a look at the schematics in the users guide. Depending on configuration, the backup subsystem (including XT1) is powered by VBAT. If it is left unconnected, XT1 won’t run and your software loop will wait eternally for it to launch.
    Besides using one of the ‘ideal diode’ supply switches, you can simply use two diodes to connect VBAT with VCC and your battery mount. So VBAT will be powered by either one, depending on which one has the higher voltage. You will, however, have a lower supply voltage then (diode forward voltage drop), but then, it’s cheap. :)

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