Electronic – MSP430FR5994 SVSH Interrupt from LPMx.5

Background: I plan to use lpmx.5 under cap power but need a way to save the rtc registers to FRAM when the super-cap runs down.

Plan: Use SYSRSTIV SVSH interrupt to wake up and save the counter, or use P5.6 to wake up if the power is restored.

Quote from manual:

The following conditions will cause an exit from LPM4.5:

• A wakeup event on an I/O if configured and enabled. The interrupt flag of the corresponding port pin is set (PxIFG). The PMMLPM5IFG bit is set.

• A wakeup from the RST pin.

• A power-cycle. Either the SVSHIFG or none of the PMMIFGs is set.

Any exit from LPM4.5 causes a BOR. The program execution starts at the address the reset vector points
to. PMMLPM5IFG = 1 indicates a wakeup from LPM4.5, or the System Reset Vector Word register (SYSRSTIV) can be used to decode the reset condition (see the device data sheet).

and

Figure 2-2. Voltage Failure and Resulting PMM Actions not showing the power-down SVSH interrupt.

Revised questions:

1. Is the SVSH interrupt available at power-down between SVSH trigger
   and BOR reset?
2. How do I detect a SVSH wake-up?
3. If a need IO pin P5.6 wake up – Table 6-10 does not mention PMMLPM5 by name. Is this the LPMx.5 wake-up or $08 vector and is
   this the specific one triggered by, say P5.6, if correctly
   configured?
4. Can the SVSH trigger be used to save the RTC counters RTCCNT12 AND RTCCNT34? (see below)

I needed to split question 1 in two parts as I can confirm that SVSH can be used by sampling SYSRSTIV == 0x0E at startup. However, when I read the counters under SVSH they always return zero. I have also confirmed this strange behaviour by triggering SVSH from the active mode.

Is this a hardware bug? I am testing the code with a Rev B chip.