MSP430FR2433: XT1 Clock Operation and requirement for XT1OFFG Crystal Fault Interrupt

Hi Larry,

This is about the oscillator-fault fail-safe feature and the function of XT1OFFG bit.

Firstly, please read the section 3.2.13 Fail-Safe Operation of MSP430FR4xx and MSP430FR2xx Family User's Guide. Then let me walk you through the code below with detailed explanation. After that you should have no problems understanding this topic.

#include <msp430.h>

int main(void)
{
    WDTCTL = WDTPW | WDTHOLD;               // Stop watchdog timer

    P2SEL0 |= BIT0 | BIT1;                  // set XT1 pin as second function
    do
    {
        CSCTL7 &= ~(XT1OFFG | DCOFFG);      // Clear XT1 and DCO fault flag
        SFRIFG1 &= ~OFIFG;
    }while (SFRIFG1 & OFIFG);               // Test oscillator fault flag

    while(1){
        LPM4;
    }
}

At start-up,

• XT1CLK is selected as the FLL reference by default (CSCTL3 = SELREF__XT1CLK)
• XT1 pins are in general-purpose I/O (GPIO) mode rather than the XT1 mode
• XT1 is not operating, or in other words, XT1 is faulty
• XT1OFFG is set, and the FLL reference source is automatically switched to REFO

After the execution of line 7: P2SEL0 |= BIT0 | BIT1

• XT1 pins are in XT1 mode
• XT1 is operating, but the fault bits remain set until software resets them, even if the fault condition no longer exists
• FLL reference source is still REFO

After the execution of line 11: SFRIFG1 &= ~OFIFG

• The fault bits are cleared
• FLL reference source is now XT1

In conclusion, when XT1 is to be used, it is required to write the below code (for MSP430FR2433) at initialization.

    P2SEL0 |= BIT0 | BIT1;                  // set XT1 pin as second function
    do
    {
        CSCTL7 &= ~(XT1OFFG | DCOFFG);      // Clear XT1 and DCO fault flag
        SFRIFG1 &= ~OFIFG;
    }while (SFRIFG1 & OFIFG);               // Test oscillator fault flag

Otherwise, MSP430 could be working well but it is actually REFO rather than XT1 being used in your clock system.

By the way, you can also refer to section 5.2.7 Basic Clock Module+ Fail-Safe Operation of  MSP430x2xx Family User's Guide, and you can find it pretty different from the oscillator-fault fail-safe feature discussed above. That's where difference lies between MSP430FR2433 and MSP430G2553.

Nevertheless, the attached code at section 5.2.7.1 Sourcing MCLK from a Crystal of  MSP430x2xx Family User's Guide, even though written in assembly, is interestingly very similar to the do-while part of the code discussed above.