Floating point error not working.

[Tsemr.1]

Dear ST Hello,

I have a big problem with floating point calculations,

I am trying to implement the IIR filter to filter a current but the result is Zero.

Sometimes the watch expression window shows me 0 but the input filter is not zero.

 /* Get the HImax */
  HimaxCurrentCons = HAL_ADC_GetValue(&hadc2);
 
  /* Get LSM current */
  l_u16AdcCurrent = HAL_ADC_GetValue(&hadc1);
 
  /* Filter the current LSM */
  Input[0] = (float_t)1E6;//l_u16AdcCurrent;
  filter_out = FilterCoefInput[0] * Input[0];
 
  for (int i = 6; i > 0; i--)
  {
	  /* adding the ai * xi i=7..1 */
	  filter_out   = filter_out + (FilterCoefInput[i] * Input[i]);
 
	  /* Shifting the register input */
	  Input[i]      = Input[i - 1];
  }
 
  for (int j = 6; j > 0; j--)
  {
	/* Adding the bj * yj j = 6..1 */
	filter_out   = filter_out + (FilterCoefOutput[j] * Output[j-1]);
 
	/* Shifting the register output */
	Output[j]    = Output[j - 1];
  }
 
  Output[0] = filter_out;
 
  /* Current compute */
  LsmCurrent = (300.0/2559) * filter_out;

Thank in advance,

S.tarik

[KnarfB]

What MCU? And: you are not showing all types.

[Tsemr.1]

I am using the STM32F407 discovery board,

const float_t FilterCoefInput [7] = {
#if IMPULS
		8.576557E-06f, 5.145934E-05f,
		1.286483E-04f, 1.715311E-04f,
		1.286483E-04f, 5.145934E-05f,
		8.576557E-06f
#else
		8.576557, 51.45934,
		128.6483, 171.5311,
		128.6483, 51.45934,
		8.576557
#endif
 
};
const float_t FilterCoefOutput [7] = {
#if IMPULS
		  1.0f,			+4.787135f,
		 -9.6495177f,   +10.46907f,
		 -6.44111188f,  +2.129038f,
		 -0.2951724f
#else
		  1.0f,			+4.787135f,
		 -9.6495177f,   +10.46907f,
		 -6.44111188f,  +2.129038f,
		 -0.2951724f
#endif
 
};
float_t Input[7]  = {0.0};
float_t Output[6] = {0.0};
float_t filter_out = 0.0f;

I am trying different configurations to solve this problem,

thank you

[waclawek.jan]

You define:

> Output[6]

but in the loop you write to Output[j] when j == 6.

JW

[KnarfB]

If Output array has size 6, the first write to Output[j] with j=6 is illegal, potentially overwriting filter_out. Make the array larger.

[Tsemr.1]

Thank you,

that was the problem

[Tsemr.1]

yes JW, thank you for help.