fft latency problem in nrf52

[bigzero]

i am working using NRF52, s132, sdk11, freertos.

for fft , i used "arm_rfft_fast_f32(...)" in cmsis-dsp library.

and i inserted a fpu interrupt.

NVIC_SetPriority(FPU_IRQn, APP_IRQ_PRIORITY_LOW);
NVIC_ClearPendingIRQ(FPU_IRQn);
NVIC_EnableIRQ(FPU_IRQn);

void FPU_IRQHandler(void)
{
     uint32_t *fpscr = (uint32_t *)(FPU->FPCAR+0x40);
     (void)__get_FPSCR();
      NVIC_ClearPendingIRQ(FPU_IRQn);

     *fpscr = *fpscr & ~(FPU_EXCEPTION_MASK);
}

if i enable a FPU_IRQ, the fft latency is very long.

if i disalbe a FPU_IRQ, the fft latency is very fast. but I can't enter to sleep.

and system is very unstable.

i would like to know the best setting for a fft performance in freertos+s132+nrf52.

thank you

[Susheel Nuguru]

I think you have enable FPU in ARM core

Either enable __FPU_USED then the system_nrf52.c file OR

do it manually where you want to enable it like below

SCB->CPACR |= (3UL << 20) | (3UL << 22);
__DSB();
__ISB();

and disable it when you want to save power when you know its being not used by

SCB->CPACR = 0; 
__DSB();
__ISB();

In assembly you can do like this

[bigzero]

thank you for reply.

I tried your guides. but my result is same to previous status.

currently, when i used "NVIC_EnableIRQ(FPU_IRQn)", the fft latency(arm_rfft_fast_f32) is 10~12ms in FFT size 1024. by the way, in NVIC_DisbleIRQ(FPU_IRQn), it is 2ms .

i think that the FPU_IRQHandler is so many called. so the latency is long. right?

i would like to know the fft(arm_rfft_fast_f32) latency in fft size 1024, NRF52.

i hope that the latency is about 2ms in a FPU_IRQ enable.

thank you.