Implement the other bandpass filters and frequency detectors to complete your DTMF detection system. Here is a possible method to make the programming easier. Rather than repeating the code from Part 1 seven more times, the eight filters and detectors could be implemented with one code block executed eight times with a do loop. This method will require the use of pointers. For example consider the implemenation of a second-order IIR filter where the subsript A repesents the first bandpass filter (the 697 Hz bandpass filter):

This will be followed by filter B, the 770 Hz filter, etc.

You need to set aside storage for the coefficients, and for the delayed xand y values for filters A, B, etc.

Before starting the do loop, one pointer (r0 in the example code) points to the coefficients of the first filter, and another pointer (r4) points to the states (delayed values) for the first filter. In the loop as the delayed values are used they are replaced with updated values (e.g., y(n-2) is replaced by y(n-1), which will be y(n-2) the next time through the loop). At the end of the loop for the first filter, r0 points to the coefficients of the second filter, and r4 points to the states of the second filter, and the second time through the do loop the second filter and detector are implemented.

After implementing all the stages, the program should check the outputs of the eight detectors. If a DTMF signal is present at the input, one of the detectors for the lower four frequencies should be asserted, and one of the detectors for the upper four frequencies should be asserted. If this is the case put out a four-bit number on bits 3-0 of Port B indicating which of the 16 keys was pressed, and put out a one on bit 4 of Port B to indicate a DTMF signal is present. If a signal other than a DTMF tone is present (at least one detector output asserted, but the detection outputs do not match any DTMF pattern) put out a one on bit 5 of Port B. If none of the detectors are asserted all bits of Port B should be low.

       org     x:$100
coefs  equ     *
; Coefficients for Filter A, 697 Hz bandpass
bA2    dc      xxxxxxxxxx/2.0
bA1    dc      xxxxxxxxxx/2.0
bA0    dc      xxxxxxxxxx/2.0
aA2    dc      xxxxxxxxxx/2.0
aA1    dc      xxxxxxxxxx/2.0
; Coefficients for Filter B, 770 Hz bandpass
bB2    dc      xxxxxxxxxx/2.0
bB1    dc      xxxxxxxxxx/2.0
bB0    dc      xxxxxxxxxx/2.0
aB2    dc      xxxxxxxxxx/2.0
aB1    dc      xxxxxxxxxx/2.0

       org     y:$0
; States for Filter A, 697 Hz bandpass
states equ     *
xAnm2  ds      1
xAnm1  ds      1
yAnm2  ds      1
yAnm1  ds      1
; States for detector for stage A go here

; States for Filter B, 770 Hz bandpass
xBnm2  ds      1
xBnm1  ds      1
yBnm2  ds      1
yBnm1  ds      1
; States for detector for stage B go here



;  Program loop

; Set up pointers
; At start of first loop, they point to coefs and states of 1st filter
       move    #coefs,r0
       move    #states,r4

       do      #2,end_filter
       move    x:RX_BUF_BASE,x1   ; Put x(n) into x1

; b_2 -> x0, r0 points to b_1
; x(n-2) -> y0, r4 points to x(n-1)
       move    x:(r0)+,x0 y:(r4)+,y0 

; b_2 x(n-2) -> a
; b_1 -> x0, r0 points to b_0
; x(n-1) -> y0, r4 points to x(n-2)
       mpy     x0,y0,a  x:(r0)+,x0 y:(r4)-,y0

; b_2 x(n-2) + b_1 x(n-1) -> a
; b_0 -> x0, r0 points to a_2
; x(n-1) -> x(n-2) for next time through filter, r4 points to x(n-1)
       mac     x0,y0,a   x:(r0)+,x0  y0,y:(r4)+

; b_2 x(n-2) + b_1 x(n-1) + b_0 x(n) -> a
; x(n) -> x(n-1) for next time through filter, r4 points to y(n-2)
       mac     x0,x1,a   x1,y:(r4)+

; a_2 -> x0, r0 points to a_1
; y(n-2) -> y0, r4 points to y(n-1)
       move    x:(r0)+,x0 y:(r4)+,y0

; b_2 x(n-2) + b_1 x(n-1) + b_0 x(n) - a_2 y(n-2) -> a
; a_1 -> x0, r0 points to b_2 of next filter
; y(n-1) -> y0, r4 points to y(n-2)
       mac     -x0,y0,a  x:(r0)+,x0 y:(r4)-,y0

; b_2 x(n-2) + b_1 x(n-1) + b_0 x(n) - a_2 y(n-2) - a_1 y(n-1) -> a
; y(n-1) -> y(n-2) for next time through filter, r4 points to y(n-1)
       mac     -x0,y0,a  y0,y:(r4)+

; multiply y(n)/2 by 2 to get y(n)
       asl     a

; y(n) -> y(n-1) for next time through; r4 points to x(n-2) of next filter
       move    a,y:(r4)+

; save y(n) in a temp reg x1 as the input to the detector
       move    a,x1

; Code for detector goes here


; At end of loop, pointers point to coefs and states of next filter

end_filter:

; Check all eight outputs of the detectors

; If one from a DTMF row and one from a DTMF column are present, put out a
; four-bit number of Bits 3-0 of Port B to indicate which one.  Also, output
; a one on Bit 4 of Port B to indicate that a DTMF tone is present.

; If at least one detector output is asserted but the pattern does not match
; a DTMF tone, output a one on Bit 5 of Port B.

; If no detector output is asserted, output zeros on Bits 5-0 of Port B.