ARM CMSIS IIR Q15 filter problem

Hi,

I am trying to implement IIR filter using CMSIS library and "arm_biquad_cascade_df1_q15" function.

First of all I already made a design with "arm_biquad_cascade_df2T_f32" function but for my new project I don't have a MCU with FPU so I need to use Q15 format(I am using LPC4074).

C code:

first I init my filter

arm_biquad_cascade_df1_init_q15(S, numStages, pCoeffs, pState, postShift);

in my first test I tried a very simple filter
Using Matlab "filterbuilder" tool -
Highpass
FS = 64000Hz
Fstop = 60Hz
Fpass = 500Hz
attenuation = 10dB
ripple = 1dB

filter type = IIR Elliptic Direct Form I SOS
Data type = Fixed point

With this design Matlab coef are :
b0: 0.9881
b1: -0.9881
b2: 0
a0: 1.0000
a1: -0.9762
a2: 0

Transforming them into Q15 format (* 32768):
b0: 32378
b1: -32378
b2: 0
a0: 32768
a1: -31988
a2: 0

numStages = 1
Postshift = 0

pState is q15_t array with numstages*4 dimension

pCoef is my coefiscient array:

q15_t pCoeffs[numStages*6] =
{
        32378, //b0
        0, //facilities use of 16-bit SIMD instructions on the Cortex-M4
        -32378, //b1
        0,         //b2
        31988,   //a1
        0      //a2
};

As mentioned in CMSIS documentation, depending on the tool equation for filter coefiscients "a" coef must be negated (I noticed with Matlab I have to negate them)

Then I call my filter:

arm_biquad_cascade_df1_q15 (S, sample, result, blocksize);

where result and sample are q15_t array (sample are my ADC samples .. ^^') and blocksize is set to 8

with THIS filter everything works fine ..
I put a sin wave with magnitude of 200 and an offset of 2048
my RMS result after filtering is 141 (as expected) and If I put a constant, the RMS result is 0

Then I tried to change my ripple, I set 0.1 instead of 1

Matlab coef: (b0 to a2)
0.97552490234375 -1.95086669921875 0.97552490234375 1 -1.971923828125 0.97265625

Q15 coef (* 32768 / 2 to scale my coef between 1 and -1 as mentionned in CMSIS documentation)
15983 -31963 15983 16384 -32308 15936

My coef array:

q15_t pCoeffs[numStages*6] =
{
        15983,
        0,
        -31963 ,
        15983,
        32308,
        -15936
};

and postshift = 1

With these coef. that doesn't work anymore
For example, if I put a constant in my samples (all set to 200) I got a final RMS value of 1277 (that means that my filter results are not null, with a high pass filter, all my filter result should be null and my RMS should be null too)

I noticed that each time b2 and a2 are not equal to 0, my result is wrong
When they are equal to 0 (even for higher order filter) that works.

What did I miss ?

Regards