Interrupt Service Routine (continued)

• //------------------------------------------------
• // Digital IIR Filter "Temp" divided by 2^8
• // MS Byte must be all FF's or all 00's
• // (max value <2^24 which is 16,777,216)
• // nominal value 8,192*425 (larger coef) = 3,481,600
• //------------------------------------------------
• p_Temp = (BYTE*)&Temp;
• p_Temp[0] = p_Temp[1];
• p_Temp[1] = p_Temp[2];
• p_Temp[2] = p_Temp[3];
• //------------------------------------------------
• // Input summation
• //------------------------------------------------
• Intermediate = Input - Temp;
• //------------------------------------------------
• // Digital IIR Filter delay
• //------------------------------------------------
• Intermediate2 = Intermediate1;
• Intermediate1 = Intermediate;
• //------------------------------------------------
• // Digital FIR Filter (IIR Filter output summation)
• //------------------------------------------------
• Output = Intermediate-Intermediate2;

• Because “Temp” is a partial product sum of intermediate values inflated by 256, after the multiply we immediately divide “Temp” by 256. Since this is 8 bits, we do a byte shift rather than performing a divide. “p_Temp” is merely used as a pointer to the 4 byte “Temp” variable.
• No attempt is made to bit shift the MS bit of the MS byte of “Temp”. As long as the max value is <16,777,216, the MS byte would be all 00’s or all FF’s anyway.

• Completing the digital filter is trivial …
• Compute a new intermediate
• Transfer (delay) prior intermediate outputs
• Compute output