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Mixed Signal PICtail? Demo
Board User ’s Guide
Appendix D. DAC_dtmf.asm Source Code
Software License Agreement
The software supplied herewith by Microchip Technology Incorporated (the “Company”) is intended and supplied to you, the
Company’s customer, for use solely and exclusively with products manufactured by the Company.
The software is owned by the Company and/or its supplier, and is protected under applicable copyright laws. All rights are reserved.
Any use in violation of the foregoing restrictions may subject the user to criminal sanctions under applicable laws, as well as to civil
liability for the breach of the terms and conditions of this license.
THIS SOFTWARE IS PROVIDED IN AN “AS IS” CONDITION. NO WARRANTIES, WHETHER EXPRESS, IMPLIED OR STATU-
TORY, INCLUDING, BUT NOT LIMITED TO, IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICU-
LAR PURPOSE APPLY TO THIS SOFTWARE. THE COMPANY SHALL NOT, IN ANY CIRCUMSTANCES, BE LIABLE FOR
SPECIAL, INCIDENTAL OR CONSEQUENTIAL DAMAGES, FOR ANY REASON WHATSOEVER.
TABLE D-1:
DAC_DTMF.ASM
#include “p16F767.inc”
;*************************************************************************
;
; DTMF SUBROUTINE using a Rolling Loop Timer
;
; Program memory = 149 words for 32 step tone subroutine
; = 133 words for 16 step tone subroutine
; Data memory = 12 registers for tone subroutine
;
;*************************************************************************
;
; THEORY OF OPERATION - ROLLING LOOP TIMER
; 2’s complement subtraction of a marked time and a continuous timer
; (overflows from 0FFh to 0h) provides an elapsed time which can then
; be compared to some threshold value (another subtraction). The
; result of the compare (<, > or =) can then be used to perform some
; periodic event. The fewest possible cycles per loop is desired to
; achieve the least amount of error from the “zero” cycle (when the
; result of the compare is “=”). One major advantages of this
; technique is the ability to use only one timer to keep track of
; several independent periodic events with relatively few instructions
; and instruction cycles.
;
; THEORY OF OPERATION - DISTORTION
; This routine works with relatively low distortion because the
; statistical distribution of the possible error cycles doesn’t peak
; at the “zero” cycle but at some repeatable value relative to the
; “zero” cycle. Error cycles will be +-X from the distribution’s
; peak instead of from the “zero” cycle.
;
; THEORY OF OPERATION - LOOP TIMING
; LOW - No update yet = 6 cycles
; - Update output w/o resetting Sine_Table = 18
; - Update output w/reset of Sine_Table = 19
; HIGH- No update yet = 6 cycles
; - Update output w/o resetting Sine_Table = 18
; - Update output w/reset of Sine_Table & ToneLength = 22
; (Note: end of Tone until Repeat_tone re-enters >11 cycles
; depending upon user’s code)
? 2004 Microchip Technology Inc.
DS51523A-page 31
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