The example msp430fr5994x_lpm4-5_02.c is supposed to show how little current is used in this mode.
In the file it says:
// MSP430FR5x9x Demo - Entering and waking up from LPM4.5 via P1.3 interrupt
// with SVS disabled
// Description: Download and run the program. When entered LPM4.5, no LEDs
// should be on. Use a multimeter to measure current on JP1 and
// compare to the datasheet. When a positive voltage is applied
// to P1.3 the device should wake up from LPM4.5. This will enable
// the LFXT oscillator and blink the LED (on P1.0).
Even for a high-end multimeter this current is too low to be accurately measured.
So I helped myself this way:
- power the processor from the supercap
- a 10k resistor with two antiparallel diodes act as a shunt,
- connect the volt meter across the supercap, not across the processor
0.43mV over a 10k resistor gives 43 Nanoamps. (!) Yes, the datasheet (page 32) is right, typical value at 25°C is 45nA.
A CR2032 (200mAh) cell would allow the processor to wait for an interrupt for 530 years.