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Using CMSIS DSP Function: arm_fir_f32 - Correct Use?

Hi All;

I have some questions about correct use of the CMSIS DSP library call arm_fir_32. First, I'll provide some background about what I am doing and what the setup is.

I have a STM32F4 Discovery board, using IAR EWARM for programming. Just for testing purposes, I'm generating a low frequency test signal at 40Hz and feeding it into one of the ADC inputs. The signal is biased to swing from 0 to about 2.5Vpp. The signal has a low to moderate amount of broadband noise - but at this point I am not purposely mixing or introducing any other signals with it. There is a timer interrupt set to sample frequency of 2KHz, with a sampling buffer of 2048 samples.

I have already tested and am using the FFT function arm_cfft_f32, and can accurately determine (track) the frequency of the incoming signal when I change it at the source.  This seems to be working well.

Now, I would like to use the arm_fir_32 filter. To do this, I started out reading the documentation from CMSIS on the function. To implement a FIR low pass, to generate the tap coefficients, I am using this website's only tool to do so.

I generated a 4th order filter, and set the sampling rate the same as my software, with a cutoff of 60Hz. I forced generation to 24 taps to be even. So the BLOCK_SIZE is 32, and the number of blocks is 1024/32 = 32.

Following the example from CMSIS  on this function, I believe I've set up correctly. So the chain looks like this:

ADC --> FIR --> FFT

However, I'm not getting the result I would expect. The values returned from the FFT's output buffer are exponentially large (not this way if I comment out /circumvent the FIR calls). This leads me to believe I am missing a step. Do I need to normalize the values? I thought that because I input the rate into the FIR function setup, this wouldn't be required  - but maybe this is incorrect.

Can someone please provide some insight or assistance as to what I am missing or doing incorrectly to apply the FIR processing?

Thank you,


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