You may have noticed that some of the coefficients you have computed for the second-order sections are larger than 1.0 in magnitude. For any stable second-order IIR section, the magnitude of the "0" and "2" coefficients ( a 0 a 0 and a 2 a 2 , for example) will always be less than or equal to 1.0. However, the magnitude of the "1" coefficient can be as large as 2.0. To overcome this problem, you will have to divide the a 1 a 1 and b 1 b 1 coefficients by two prior to saving them for your DSP code. Then, in your implementation, you will have to compensate somehow for using half the coefficient value.

Rather than write separate code for each second-order section, you are encouraged first to write one section, then write code that cycles through the second-order section code twice using the repeat structure below. Because the IIR code will have to run inside the block I/O loop and this loop uses the block repeat counter (BRC), you must use another looping structure to avoid corrupting the BRC.

	  
	  stm     (num_stages-1),AR1

	  start_stage

	  ; IIR code goes here

	  banz    start_stage,*AR1-
	

It may be necessary to add gain to the output of the system. To do this, simply shift the output left (which can be done using the ld opcode with its optional shift parameter) before saving the output to memory.