Texas Instruments MSP50C6xx manual STR,0

Special Filter Instructions

After the FIR or COR instruction executes, the new startOfBuff will be the last location in the circular buffer. After another FIR/COR instruction, the new startOfBuff will be the second to last location in the circular buffer, and so on.

The second detail is the STAT register. The STAT register must be saved im- mediately after every FIR or COR instruction. Consequently, this saved value must be loaded before every FIR or COR instruction. If the tag bit in the STAT register is set before an FIR or COR instruction, this tells the processor two things. First, it knows that it must wrap around to the first RAM location of the circular buffer. Second, it knows that the startOfBuff (and R0) currently points to the last location in the circular buffer. Thus, R0 will increment by R5 after the first multiply. This will become more clear after examining the next ex- ample code.

The third detail is that the filter coefficients take up only N RAM locations, but the circular buffer takes up N+1 RAM locations.

Below is an example of the FIR or COR execution inside a DAC interrupt ser- vice routine.

;FIR Filtering routine (N = 3)

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rovm		;reset overflow mode
mov	R5, –2 * N	;circular buffer length (3 words)
mov	R1,coeffs	;R1 points to first of N filter
		coefficients
mov	MR,*R1++	;must increment R1
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
mov	STAT,*filterSTAT_tag		;load STAT with last filter
			tag status
rpt	N–2
fir	A0,*R0++	;Do one sample ––> 32 bit result
mov	*filterSTAT_tag,STAT		;save STAT with last filter
			tag status
		;R0 now points to the last/oldest
		sample
movs	*ySampleOut,A0	;FIR outputs bits 0–15 in AC0,
		16–32 in AC1

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