Texas Instruments MSP50C614 manual 0x0104

		Special Filter Instructions
mov	A0,*nextSample	;Replace last sample with newest
		sample
mov	*R0,A0	; and update the start of the
mov	*startOfBuff,R0	; circular buffer to here (R0)

First, the overflow mode must be reset. Next, R5 must be loaded with the wrap around value of the circular buffer. Wrap around happens automatically. This tells the processor how many words to step back when the end of the circular buffer is reached. This value must be negative and equal to N words even though the buffer is N+1 words long. For example, suppose a four word circular buffer starts at RAM location 0x0100 and ends at 0x0106 (N = 3). In order to wrap around from location 0x0106 back to location 0x0100, the value 0x006 must be subtracted from 0x0106, giving 0x0100.

0x0100

0x0102

0x0104

0x0106

TAGGED LOCATION

Go back N words to wrap around

R0 must point to the current starting point of the circular buffer. R1 must point to the filter coefficients. The MR register must contain the first filter coefficient, h[0]. R0 and R1 must be used this way. The filtering operation will not work if the Rx registers are reversed. The following are the only allowable register combinations,

R0 points to circular buffer and R1 points to filter coefficients R2 points to circular buffer and R3 points to filter coefficients

Assembly Language Instructions 4-63