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Power consumption: solved

A project log for OpenRPNCalc

Open-source and open-hardware custom scientific RPN calculator

Anton PoluektovAnton Poluektov 06/19/2021 at 18:100 Comments

I think I've solved all my problems with power consumption being somewhat higher than I hoped. Previously the calculator consumed 40-55 uA of current from the 3V battery, which was to my knowledge a bit on a higher side of what one should expect from STM32L476 in STOP mode. 

I discovered that in fact the STM32 was put into STOP1 mode, while there is a more economic one, STOP2, which, however, takes longer startup time (does not matter in my case). STOP2 is activated with the extended HAL function 

 HAL_PWREx_EnterSTOP2Mode(PWR_STOPENTRY_WFI);

That solves my problem. With STOP2, the STM32 itself draws only 2-3 uA, and that increases to 10-12 uA with LCD on. This must be sufficient to run the calculator from a single CR2032 for several years. 

Another small fix I had to do is to add an electrolytic capacitor of 47 uF in parallel to the battery. This fixes occasional resets of the calculator while waking up. As far as I understand, this could happen when the 3V->5V DC converter is powered up and starts to charge the capacitor C2. With the battery being not fresh enough, this could deplete the voltage for a short time below the minimum needed for STM32 to operate, causing it to reset when the voltage is back to normal. Adding extra capacitor with the value a few times higher than C2 fixed this. 

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