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FFT Evaluation Guide*

Overview*

fft_evaluation is a test App for benchmarking FFT/IFFT performance of the CSKY DSP library. It generates test data and measures the execution time of FFT and IFFT operations on the target platform, helping developers evaluate the chip's audio signal processing capability.

Two operation modes are supported:

  • Real FFT (float): uses csky_rfft_fast_f32, single-precision floating-point
  • Fixed-point FFT: uses csky_rfft_q15, Q15 fixed-point

APP Usage*

Build Configuration*

Reference config for fixed-point FFT evaluation:

cp ./configs/release/nationalchip/grus_gx8002b_dev_1v_use_fixed_fft.config .config

Alternatively, configure manually via menuconfig:

  1. LVP Application Settings → select FFT Evaluation App
  2. Select operation type (under real or fixed choice):
    • use real fft — float FFT evaluation
    • use fixed fft — fixed-point FFT evaluation

Firmware Build*

make menuconfig
make clean; make

Flash output/mcu_nor.bin to the board and check the serial output.

Output*

The App runs FFT+IFFT once during initialization and prints:

  • fft_input: constructed test data (512-point sawtooth wave)
  • fft time: X us: FFT duration in microseconds
  • fft_output: frequency-domain data after FFT
  • ifft time: X us: IFFT duration in microseconds
  • ifft_output: time-domain data after IFFT

App Working Principle*

Test Data Generation*

A 512-point sawtooth wave is generated in AppInit:

for (int i = 0; i < 512; i++) {
    fft_input[i] = (i * 32767) / 511;
}

FFT*

// float: 512-point real FFT
csky_rfft_fast_f32(&csky_rfft_sR_f32_len512, fft_input, fft_output, 0);

// fixed-point: 512-point real FFT
csky_rfft_q15(&csky_rfft_sR_q15_len512, fft_input, fft_output);

IFFT*

// float: IFFT reconstruction
csky_rfft_fast_f32(&csky_rfft_sR_f32_len512, fft_output, fft_input, 1);

// fixed-point: IFFT reconstruction
csky_rfft_q15(&csky_inv_rfft_sR_q15_len512, fft_output, fft_input);

Timing*

Timestamps are captured via gx_get_time_us() before and after the operation:

int start = gx_get_time_us();
// FFT operation
int end = gx_get_time_us();
printf("fft time: %d us\n", end - start);

Key API Reference*

API Description
csky_rfft_fast_f32(instance, input, output, inverse) Float real FFT/IFFT; inverse=0 for forward, =1 for inverse
csky_rfft_q15(instance, input, output) Q15 fixed-point real FFT
csky_inv_rfft_q15(instance, input, output) Q15 fixed-point real IFFT (via csky_rfft_q15 with inverse instance)
gx_get_time_us() Get current system time in microseconds, used for performance measurement

FFT Instance Reference*

Instance Type Description
csky_rfft_sR_f32_len512 float 512-point float real FFT instance
csky_rfft_sR_q15_len512 Q15 512-point fixed-point real FFT instance
csky_inv_rfft_sR_q15_len512 Q15 512-point fixed-point real IFFT instance

Notes*

  • FFT runs once in AppInit; re-initialize to trigger again
  • Input data is fixed to 512 points; modify array size and FFT instance for other sizes (256/1024)
  • Float mode demands more MCU compute; fixed-point is more efficient — choose based on use case
  • Results are affected by MCU frequency; use dynamic frequency scaling to evaluate performance at different speeds

Reference*