IIR filter help

Zorjak · 2007-08-10T21:20:16+00:00

Hi,I have one question regarding implementation of IIR filter. I try towrite C code of IIR filter for ARM microcontroler for real timesignalprocessing. Filter should be lowpass and second order. I try to dothis in interupt function. This should be part of the code.volatile int count, count_on;char On_ind; volatile float Sample[3],Filtered[3], Max, Min, Power_curent,Power_wanted, Scal, FB;; /* Timer0 interupt funkcija*/void tc0 (void) __irq { IODIR0 = 0x00000001; Sample[2]=Sample[1]; Sample[1]=Sample[0]; Sample[0]=(float)ADC1_read(); Filtered[2]=Filtered[1]; Filtered[1]=Filtered[0]; Filtered[0]=0.0002478584065*Sample[0]+0.000495716813*Sample[1]+0.0002478584065*Sample[2]+ 1.964326262*Filtered[1]-0.9654386044*Filtered[2]; //ffilteringBut there is a problem this code doesnt work. I try to implement IIRfilter order 1I replaceFiltered[0]=0.0002478584065*Sample[0]+0.000495716813*Sample[1]+0.0002478584065*Sample[2]+ 1.964326262*Filtered[1]-0.9654386044*Filtered[2]; //ffilteringwithFiltered[0]=0.044263*Sample[0]+0.044263*Sample[1]+0.91147*Filtered[1];and this filter works fine. I am not shure but I don't know what is aproblem when i am trying to implement second order IIR filter.Could anyone help me, please.I designed this filters using Matlab "filters design and analystool" (fdatool). I tested bouth filters in matlab and as test signal Iused signal that is almost the same the one I am using with lpc2148.When I am testing filter order 1(with mc) the results is expectedbut when I use filter of second order I am getting wrong results (I amconstantly getting '0').I apreciated any kind of help.ThanksZoran