FFT Runtime Configuration and Status - 2021.2 English

Vitis High-Level Synthesis User Guide (UG1399)

Document ID
UG1399
ft:locale
English (United States)
Release Date
2021-12-15
Version
2021.2 English

The FFT supports runtime configuration and runtime status monitoring through the configuration and status ports. These ports are defined as arguments to the FFT function, shown here as variables fft_status1 and fft_config1:

hls::fft<param1> (xn1, xk1, &fft_status1, &fft_config1);

The runtime configuration and status can be accessed using the predefined structs from the FFT C library:

  • hls::ip_fft::config_t<param1>
  • hls::ip_fft::status_t<param1>
Note: In both cases, the struct requires the name of the static parameterization struct, shown in these examples as param1. Refer to the previous section for details on defining the static parameterization struct.

The runtime configuration struct allows the following actions to be performed in the C code:

  • Set the FFT length, if runtime configuration is enabled
  • Set the FFT direction as forward or inverse
  • Set the scaling schedule

The FFT length can be set as follows:

typedef hls::ip_fft::config_t<param1> config_t;
config_t fft_config1;
// Set FFT length to 512 => log2(512) =>9
fft_config1-> setNfft(9);
Important: The length specified during runtime cannot exceed the size defined by max_nfft in the static configuration.

The FFT direction can be set as follows:

typedef hls::ip_fft::config_t<param1> config_t;
config_t fft_config1;
// Forward FFT
fft_config1->setDir(1);
// Inverse FFT 
fft_config1->setDir(0);

The FFT scaling schedule can be set as follows:

typedef hls::ip_fft::config_t<param1> config_t;
config_t fft_config1;
fft_config1->setSch(0x2AB);

The output status port can be accessed using the pre-defined struct to determine:

  • If any overflow occurred during the FFT
  • The value of the block exponent

The FFT overflow mode can be checked as follows:

typedef hls::ip_fft::status_t<param1> status_t;
status_t fft_status1;
// Check the overflow flag
bool *ovflo = fft_status1->getOvflo();
Important: After each transaction completes, check the overflow status to confirm the correct operation of the FFT.

And the block exponent value can be obtained using:

typedef hls::ip_fft::status_t<param1> status_t;
status_t fft_status1;
// Obtain the block exponent
unsigned int *blk_exp = fft_status1-> getBlkExp();