Special Filter Instructions

theory requires). The second to last RAM location in the circular buffer is tagged using an STAG instruction. Below is an example of how to set up circu- lar buffering with FIR or COR.

When using the FIR or COR instruction with circular buffering, RAM needs to be allocated for the circular buffer and the filter coefficients. Therefore, the filter coefficient RAM locations must be loaded into RAM and the circular buffer must be cleared before the first FIR or COR instruction is executed.

;Set up for FIR filtering (N = 3)

;First clear circular buffer and set tag of second to last

;sample

zac

a0

 

 

mov

r0,circBuff

;point to circular buffer

rpt

N–2

;repeat N times

mov

*r0++,a0

;clear RAM locations in circular

 

 

;

buffer

mov

*r0,a0

;N+1 sample in buffer

mov

r5,2

;now step back one word and set tag

sub

r0,r5

;point r0 back to 2nd to last sample

 

 

;

in buffer

stag

*r0

;set tag

;Second initialize filter coeffs to proper values

;––––– NOTE: In this code, N must be less than 33 since

;––––– there are only 32 accumulator registers!

mov

STR,N–2

;set string length to N

zacs

a0

;zero out N accumulators

mov

a0,FIR_COEFFS ;point to filter coeffs

movs

a0,*a0

;get N filter coeffs

mov

r0,coeffs

;point to RAM locs. for filter coeffs

movs

*r0,a0

;put filter coeffs into RAM locs.

mov

a0,circBuff

;set up pointer to start of circular

 

 

; buffer

mov

*startOfBuff,a0

; Initialize filterSTAT_tag (THIS IS IMPORTANT!)

rovm

;This line is MANDATORY!

sxm

;Sample values are signed

mov

*filterSTAT_tag,STAT

Three more details in the above example merit an explanation. The first detail is the pointer to the start of the circular buffer (startOfBuff). This keeps track of the location of the newest or current sample in the circular buffer. It moves backwards by one location in the buffer each time the FIR or COR instruction is executed so that the oldest sample in the buffer is overwritten with the next sample. This backwards movement is also circular. For example, sup- pose that startOfBuff points to the first RAM location of the circular buffer.

Assembly Language Instructions

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Texas Instruments MSP50C6xx manual Special Filter Instructions