CMSIS FIR bandpass filter - c

Question

I am trying to implement a 60kHz bandpass filter on the STM32F407 microcontroller and I'm having some issues. I have generated the filter with the help of MATLABs fdatool and then simulated it in MATLAB as well. The following MATLAB script simlates it.
% FIR Window Bandpass filter designed using the FIR1 function.
% All frequency values are in Hz.
Fs = 5250000; % Sampling Frequency
N = 1800; % Order
Fc1 = 59950; % First Cutoff Frequency
Fc2 = 60050; % Second Cutoff Frequency
flag = 'scale'; % Sampling Flag
% Create the window vector for the design algorithm.
win = hamming(N+1);
% Calculate the coefficients using the FIR1 function.
b = fir1(N, [Fc1 Fc2]/(Fs/2), 'bandpass', win, flag);
Hd = dfilt.dffir(b);
%----------------------------------------------------------
%----------------------------------------------------------
T = 1 / Fs; % sample time
L = 4500; % Length of signal
t = (0:L-1)*T; % Time vector
% Animate the passband frequency span
for f=55500:50:63500
signal = sin(2*pi*f*t);
plot(filter(Hd, signal));
axis([0 L -1 1]);
str=sprintf('Signal frequency (Hz) %d', f);
title(str);
drawnow;
end
pause;
close all;
signal = sin(2*pi*50000*t) + sin(2*pi*60000*t) + sin(2*pi*78000*t);
signal = signal / 3;
signal = signal(1:1:4500);
filterInput = signal;
filterOutput = filter(Hd,signal);
subplot(2,1,1);
plot(filterInput);
axis([0 4500 -1 1]);
subplot(2,1,2);
plot(filterOutput)
axis([0 4500 -1 1]);
pause;
close all;
From the fdatool I extract the filter co-efficents to 16-bit unsigned integers in q15 format, this because of the 12-bit ADC that I'm using. The filter co-efficents header that is generated by MATLAB is here and the resulting plot of the co-efficents can be seen in the following picture
Below is the code for the filter implementation which obviously isn't working and I don't really know what I can do differently, I've looked at some examples online Example 1 and Example 2
#include "fdacoefs.h"
#define FILTER_SAMPLES 4500
#define BLOCK_SIZE 900
static uint16_t firInput[FILTER_SAMPLES];
static uint16_t firOutput[FILTER_SAMPLES];
static uint16_t firState[NUM_TAPS + BLOCK_SIZE - 1];
uint16_t util_calculate_filter(uint16_t *buffer, uint32_t len)
{
uint16_t i;
uint16_t max;
uint16_t min;
uint32_t index;
// Create filter instance
arm_fir_instance_q15 instance;
// Ensure that the buffer length isn't longer than the sample size
if (len > FILTER_SAMPLES)
len = FILTER_SAMPLES;
for (i = 0; i < len ; i++)
{
firInput[i] = buffer[i];
}
// Call Initialization function for the filter
arm_fir_init_q15(&instance, NUM_TAPS, &firCoeffs, &firState, BLOCK_SIZE);
// Call the FIR process function, num of blocks to process = (FILTER_SAMPLES / BLOCK_SIZE)
for (i = 0; i < (FILTER_SAMPLES / BLOCK_SIZE); i++) //
{
// BLOCK_SIZE = samples to process per call
arm_fir_q15(&instance, &firInput[i * BLOCK_SIZE], &firOutput[i * BLOCK_SIZE], BLOCK_SIZE);
}
arm_max_q15(&firOutput, len, &max, &index);
arm_min_q15(&firOutput, len, &min, &index);
// Convert output back to uint16 for plotting
for (i = 0; i < (len); i++)
{
buffer[i] = (uint16_t)(firOutput[i] - 30967);
}
return (uint16_t)((max+min));
}
The ADC is sampling at 5.25 MSPS and it is sampling a 60kHz signal 4500 times and here you can see the Input to the filter and then the Output of the filter which is pretty weird..
Is there anything obvious that I've missed? Because I'm completely lost and any pointers and tips are helpful!

Answer 1

As Lundin pointed out I changed it to work with 32 bit integers instead and that actually solved my problem. Ofcourse I generated new filter co-efficents with MATLABS fdatool as signed 32 bit integers instead.
static signed int firInput[FILTER_SAMPLES];
static signed int firOutput[FILTER_SAMPLES];
static signed int firState[NUM_TAPS + BLOCK_SIZE -1];
uint16_t util_calculate_filter(uint16_t *buffer, uint32_t len)
{
uint16_t i;
int power;
uint32_t index;
// Create filter instance
arm_fir_instance_q31 instance;
// Ensure that the buffer length isn't longer than the sample size
if (len > FILTER_SAMPLES)
len = FILTER_SAMPLES;
for (i = 0; i < len ; i++)
{
firInput[i] = (int)buffer[i];
}
// Call Initialization function for the filter
arm_fir_init_q31(&instance, NUM_TAPS, &firCoeffs, &firState, BLOCK_SIZE);
// Call the FIR process function, num of blocks to process = (FILTER_SAMPLES / BLOCK_SIZE)
for (i = 0; i < (FILTER_SAMPLES / BLOCK_SIZE); i++) //
{
// BLOCK_SIZE = samples to process per call
//arm_fir_q31(&instance, &firInput[i * BLOCK_SIZE], &firOutput[i * BLOCK_SIZE], BLOCK_SIZE);
arm_fir_q31(&instance, &firInput[i * BLOCK_SIZE], &firOutput[i * BLOCK_SIZE], BLOCK_SIZE);
}
arm_power_q31(&firOutput, len, &power);
// Convert output back to uint16 for plotting
for (i = 0; i < (len); i++)
{
buffer[i] = (uint16_t)(firOutput[i] - 63500);
}
return (uint16_t)((power/10));
}