FPU Power Consumption

[Lorenzo]

Hi everybody,

I'm using nRF52832 on a custom board mounting a BME680 sensor.

I'm running a third party library for the usage of the sensor (namely BSEC Library), and it uses floating point calculations.

I'm experiencing a ∼6 mA power consumption only when dealing with floating point calculations. I've read and tried some solutions on FPU handling on this forum but they're quite old (3 years ago) and with some broken links, long story short: none of them worked for me.

Any suggestion/workaround to suggest?

Thank you and best regards

Gino

[awneil]

That's Bosch, isn't it?

I've not used that one, but for some others, their low-level driver doesn't require FP.
Or they might have some non-FP code in app notes?

But I am also interested in the FPU question ...

[Simonr]

Hi Gino

This is an erratum in our nRF52832 chips where the CPU is awoken from System ON Idle after FPU has been used. To prevent unexpected wake-ups from System ON Idle, you have to add this code snippet before entering sleep: 

#if (__FPU_USED == 1)
 _set_FPSCR(_get_FPSCR() & ~(0x0000009F)); 
 (void) __get_FPSCR();
 NVIC_ClearPendingIRQ(FPU_IRQn);
#endif
 __WFE();

Best regards,

Simon

[Lorenzo]

Hi awneil, yeah that's from Bosch, however it needs to be used together with their BSEC Library (coming as a binary blob where I cannot do much) to get some special functions such as IAQ calculation.

[Lorenzo]

Hi Simon,

thanks for your answer. I've tried your solution (note that __set_FPSCR() and __get_FPSCR() require two initial underscores):

void idle_state_handle(void)
{   
    if (NRF_LOG_PROCESS() == false)
    {  
       #if (__FPU_USED == 1)
        __set_FPSCR(__get_FPSCR() & ~(0x0000009F)); 
        (void) __get_FPSCR();
        NVIC_ClearPendingIRQ(FPU_IRQn);
      #endif
      __WFE();
      nrf_pwr_mgmt_run();
    }
}

I had another problem, I was using nrf_delay_ms() to delay readings, now I've used a timer and everything is low power. Thanks again for your help.