Texas Instruments MSP50C6xx manual Important Note About Setting the Stat Register

Special Filter Instructions

Important Note About Setting the STAT Register

It is very important to consider the initial value of the filterSTAT_tag variable. Failure to set up the filterSTAT_tag variable can cause incorrect results in FIR/ COR operations. Overflow mode must always be reset. The overflow bit of the STAT register may not be set.

For samples or filter coefficients that are signed, the sign extension mode bit must also be set. Use the following set up for the filterSTAT_tag variable,

rovm	;	Mandatory
; –– Any addition modes can be set hereafter ––
sxm	;	For signed samples, coefficients, filter output
mov	*filterSTAT_tag,STAT

The FIRK/CORK instructions are almost identical to the FIR/COR instructions. The main difference is that the filter coefficients are placed in ROM instead of RAM. In other words, the filter coefficients are in a look-up table. As a result, the R1 register is not used. Before a FIRK/CORK instruction executes, the data pointer register, DP, must be set by the following code,

rovm		;reset overflow mode
mov	R5, –2 * N	;circular buffer length (3 words)
mov	A0,FIRK_COEFFS	; Loads address of lookup table
mov	A0,*A0	; Loads first coefficient to A0 and
		sets DP
mov	MR,A0	; Load first coefficient in to MR
		register

In the sequence of code above, the DP register points to the first filter coeffi- cient (in program memory located at FIRK_COEFFS). This happens during the mov A0,*A0 instruction. In addition, the DP register automatically incre- ments to the next address. It should be pointing to the second filter coefficient in program memory. If the contents of the DP register are used somewhere else in the program, a context save and restore must be performed on the DP register for each FIRK/CORK instruction. See the chapter 4 section called, Lookup Instructions. During FIRK/CORK execution, the MR register is loaded with the contents of the DP register, the DP register increments, pointing to the next filter coefficient, and the multiply-accumulate is performed.

The remaining FIRK/CORK code is almost the same as the FIR/COR code.

mov	R0,*startOfBuff	;R0 points to start of circular
		buffer
mov	AP0,0	;set up room for the
mov	STR,0	;32 bit output sample (AC0 and
		AC1)
zacs	A0	; STR should be 1 for COR/CORK
		instructions

Assembly Language Instructions 4-65