fft latency problem in nrf52

RE: fft latency problem in nrf52

Susheel Nuguru — Fri, 11 Nov 2016 10:04:43 GMT

quote

9.1 Floating point interrupt
The floating point unit (FPU) may generate exceptions when used due to e.g. overflow or underflow. These exceptions will trigger the FPU interrupt (see Instantiation on page 25). To clear the IRQ line when an exception has occurred, the relevant exception bit within the FPSCR register needs to be cleared. For more information about the FPSCR or other FPU registers, see Cortex-M4 Devices Generic User Guide

Are you checking for these bits in FPSCR and clearing them ? I do not know the latency for arm_rfft_fast_f32 on nrf52.

RE: fft latency problem in nrf52

bigzero — Fri, 11 Nov 2016 09:40:46 GMT

I just inserted for a mcu sleep mode . I want to advance a fft operation time. thank you.

RE: fft latency problem in nrf52

Susheel Nuguru — Fri, 11 Nov 2016 09:31:11 GMT

What are you doing inside FPU ISR? can you paste that code here.

RE: fft latency problem in nrf52

bigzero — Fri, 11 Nov 2016 07:25:38 GMT

sorry, i don't understand why need to find a value on the stack. the guide is how to decrease a latency?

i think that because FPU Handler is called, the FPU enabled. and in nrf52.h, the following preprocessor was predefiend #define __FPU_PRESENT 1 /*!< FPU present or not

but, the Handler so many is called. so, the fft performance is slow , compare to disable a IRQ.

thank you so much.

RE: fft latency problem in nrf52

RK — Fri, 11 Nov 2016 02:33:31 GMT

FPSCR is stacked as part of the interrupt handler, so masking it doesn't help you as it's restored again. You need to find the value on the stack and adjust that so it's popped back into the FPSCR.

RE: fft latency problem in nrf52

bigzero — Thu, 10 Nov 2016 13:53:13 GMT

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.

RE: fft latency problem in nrf52

Susheel Nuguru — Tue, 08 Nov 2016 06:48:52 GMT

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