fpu-fft demo on nrf52840

RE: fpu-fft demo on nrf52840

Arthur — Fri, 19 Jun 2020 08:49:48 GMT

I got it, thanks very mush

RE: fpu-fft demo on nrf52840

ovrebekk — Mon, 24 Feb 2020 10:41:18 GMT

Using the fpu_fft example in nRF5 SDK v16.0.0 as a starting point I am getting a quite different result.

I changed the FFT type to len256 and the FFT_TEST_COMP_SAMPLES_LEN define to 512, and measure about 670us running time.

Which compiler and IDE are you using to test this?

You will find my test example attached:
devzone.nordicsemi.com/.../244796_5F00_fpu_5F00_fft.zip

Best regards
Torbjørn

RE: fpu-fft demo on nrf52840

Arthur — Fri, 21 Feb 2020 08:32:09 GMT

Hi, ovrebekk:

I measured the fft_process(...) function, I modified the Demo to compute the complex number FFT-256

, fft_process(...) function takes about 2.5ms.

demo_project: nRF5_SDK_14.2.0_17b948a\examples\peripheral\fpu_fft\ fpu_fft_example_blank_pca10056_irq.uvprojx

modify point:

#define FFT_TEST_COMP_SAMPLES_LEN 512 //128 --> 512

nrf_gpio_pin_write(ARDUINO_13_PIN, 1);

fft_process(m_fft_input_f32,
&arm_cfft_sR_f32_len256, //arm_cfft_sR_f32_len64 --> arm_cfft_sR_f32_len256
m_fft_output_f32,
FFT_TEST_OUT_SAMPLES_LEN);

nrf_gpio_pin_write(ARDUINO_13_PIN, 0);

measured method:

I use an oscilloscope to check the IO pin status, The duration of pin state high level is about 2.5ms，I think the function takes 2.5ms to execute.

Is there something I haven't modify corrected, what should I do, Can you give me a correct example project, thanks very much!

RE: fpu-fft demo on nrf52840

ovrebekk — Fri, 21 Feb 2020 07:44:45 GMT

What are you measuring? The time spend by the fft_process(..) function, or the time spent by the fft_generate_samples(..) function?

I did a test myself and measured the fft_process(..) function to run in about 128us.

The fft_generate_samples(..) function spends closer to 2ms, but you can optimize this by replacing the call to the sin() function by sinf(). Most likely the developers made a small mistake and used the double version instead of the float version of the sin function.

Increasing the output length to 256 points increases the fft_process(..) runtime to 170us in my test.

Best regards
Torbjørn