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Embedded Software and Motor Control Libraries for PXS20xx

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4.8.3 Return<br />

The function returns a 32-bit value in fractional <strong>for</strong>mat 1.31, representing the filtered<br />

value of the input signal in step (k).<br />

4.8.4 Description<br />

This function calculates the first order infinite impulse (IIR) filter. The IIR filters are also<br />

called recursive filters because both the input <strong>and</strong> the previously calculated output values<br />

are used <strong>for</strong> calculation of the filter equation in each step. This <strong>for</strong>m of feedback enables<br />

transfer of the energy from the output to the input, which theoretically leads to an<br />

infinitely long impulse response (IIR).<br />

A general <strong>for</strong>m of the IIR filter expressed as a transfer function in the Z-domain is<br />

described as follows:<br />

Equation GDFLIB_FilterIIR1_Eq1<br />

where N denotes the filter order. The first order IIR filter in the Z-domain is there<strong>for</strong>e<br />

given from equation GDFLIB_FilterIIR1_Eq1 as:<br />

Equation GDFLIB_FilterIIR1_Eq2<br />

In order to implement the first order IIR filter on a microcontroller, the discrete time<br />

domain representation of the filter, described by equation GDFLIB_FilterIIR1_Eq2, must<br />

be trans<strong>for</strong>med into a time difference equation as follows:<br />

Equation GDFLIB_FilterIIR1_Eq3<br />

Chapter 4 API References<br />

Equation GDFLIB_FilterIIR1_Eq3 represents a Direct Form I implementation of a first<br />

order IIR filter. It is well known that Direct Form I (DF-I) <strong>and</strong> Direct Form II (DF-II)<br />

implementations of an IIR filter are generally sensitive to parameter quantization if a<br />

finite precision arithmetic is considered. This, however, can be neglected when the filter<br />

transfer function is broken down into low order sections, i.e. first or second order. The<br />

<strong>Embedded</strong> <strong>Software</strong> <strong>and</strong> <strong>Motor</strong> <strong>Control</strong> <strong>Libraries</strong> <strong>for</strong> <strong>PXS20xx</strong>, Rev. 1.0<br />

Freescale Semiconductor, Inc. 169

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