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lpm 4.5 current measurement, fun with low current

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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.  :smile:

A  CR2032 (200mAh) cell would allow the processor to wait for an interrupt  for 530 years. 



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