;*****************************************************************
;* Filename: DeltaSig.asm
;*****************************************************************;* Author: Dan Butler
;* Company: Microchip Technology Inc.
;* Revision: 1.00
;* Date: 02 December 1998
;* Assembled using MPASM V2.20
;*****************************************************************
;* Include Files:
;* p16C622.inc V1.01
;*****************************************************************;* Provides two functions implementing the Delta Sigma A2D.
;* InitDeltaSigA2D sets up the voltage reference and comparator
;* in the "idle" state.
;* DeltaSigA2D runs the actual conversion. Results provided in
;* result_l and result_h.
;* See An700 figure 2 for external circuitry required.
;*****************************************************************;* What's changed
;*
;* Date Description of change
;*
;*****************************************************************
#include <p16C622.inc>
cblock
result_l
result_h
counter:2
endc
;
;
;
InitDeltaSigA2D
bsf STATUS,RP0
movlw 0xEC
movwf VRCON
bcf PORTA,3 ;set comparator pin to output
bcf STATUS,RP0
movlw 0x06 ;set up for 2 analog comparators with common reference
movwf CMCON
return
;
; Delta Sigma A2D
; The code below contains a lot of nops and goto next instruction. These
; are necessary to ensure that each pass through the loop takes the same
; amount of time, no matter the path through the code.
;
DeltaSigA2D
clrf counter
clrf counter+1
clrf result_l
clrf result_h
movlw 0x03 ; set up for 2 analog comparators with common reference
movwf CMCON
loop
btfsc CMCON,C1OUT ; Is comparator high or low?
goto complow ; Go the low route
comphigh
nop ; necessary to keep timing even
bcf PORTA,3 ; PORTA.3 = 0
incfsz result_l,f ; bump counter
goto eat2cycles ;
incf result_h,f ;
goto endloop ;
complow
bsf PORTA,3 ; Comparator is low
nop ; necessary to keep timing even
goto eat2cycles ; same here
eat2cycles
goto endloop ; eat 2 more cycles
endloop
incfsz counter,f ; Count this lap through the loop.
goto eat5cycles ;
incf counter+1,f ;
movf counter+1,w ;
andlw 0x04 ; Are we done? (We're done when bit2 of
btfsc STATUS,Z ; the high order byte overflows to 1).
goto loop ;
goto exit
eat5cycles
goto $+1 ; more wasted time to keep the loops even
nop ;
goto loop ;
exit
movlw 0x06 ; set up for 2 analog comparators with common reference
movwf CMCON
return
end