stm32 - CMSIS real-FFT on 8192 samples in Q15 -


i need perform fft on block of 8192 samples on stm32f446 microcontroller. wanted use cmsis dsp library it's available , optimised stm32f4.

my 8192 samples of input values internal 12-bit adc (left aligned , converted q15 flipping sign bit)., testing purpose i'm feeding fft test-buffers.

with cmsis's fft functions, q15 version supports lengths of 8192. using arm_rfft_q15().

because fft functions of cmsis libraries include default 32k of luts - adapt many fft lengths, have "rewritten" them remove tables corresponding other length 1 i'm interested in. haven't touched except removing useless code.

my samples stored on external sdram access via dma.

when using fft, have several problems :

  • both source buffer , destination buffer modified ;
  • the result not @ expected

to make sure had wrong results did ifft right after fft confirmed code wasn't working.

here code :

status_codes fsm::fft_state(void) {   // flush sdram section   si_ovf_buf_clr_u16((uint16_t *)0xc0000000, 8192);   q15_t* buf = (q15_t*)(0xc0000000);   for(int = 0; i<50; i++)     buf[i] = 0x0fff; // fill buffer test vector (50 sp gate)    // initialise fft   // ---> forward, 8192 samples, bitreversed   arm_rfft_instance_q15 s;   if(arm_rfft_init_q15(&s, 8192, 0, 1) != arm_math_success)     return state_error;    // perform fft   arm_rfft_q15(&s, (q15_t*)0xc0000000, (q15_t*)0xc0400000);    // post-shift 12, in place (see doc)   arm_shift_q15((q15_t*)0xc0400000, 12, (q15_t*)0xc0400000, 16384);    // init inverse fft   if(arm_rfft_init_q15(&s, 8192, 1, 1) != arm_math_success)     return state_error;    // perform ifft   arm_rfft_q15(&s, (q15_t*)0xc0400000, (q15_t*)0xc0800000);    // post shift   arm_shift_q15((q15_t*)0xc0800000, 12, (q15_t*)0xc0800000, 8192);    return state_success; }

and here result (from gdb)

gdb result

ps : i'm using chibios - not sure if relevant.


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