PIC: EXAMPLE 11

EXAMPLE 11

Using A/D converter

PIC16F887 A/D converter is used in this example. Everything is quite simple. A variable analog signal is applied on the AN2 pin while the result of conversion is shown on port B as a binary number. In order to simplify the program as much as possible, only 8 lower bits of the result of conversion are shown. GND is used as a negative voltage reference Vref-, while positive voltage reference is applied on the AN3 pin. It enables voltage measurement scale to "stretch and shrink".
To make this clear, the A/D converter always generates a 10-bit binary result, which means that it detects a total of 1024 voltage levels (210=1024). The difference between two voltage levels is not always the same. The less the difference between Vref+ and Vref, the less the difference will be between two of 1024 levels. Accordingly, the A/D converter is able to detect slight changes in voltage.

Source Code

;*********************** Header *********************************************
;************************ PROGRAM START *************************************

       org         0x0000          ; Address of the first program instruction

       banksel     TRISB           ; Selects bank containing register TRISB
       clrf        TRISB           ; All port B pins are configured as outputs
       movlw       B'00001100'
       movwf       TRISA           ; Pins RA2 and RA3 are configured as inputs
       
       banksel     ANSEL           ; Selects bank containing register ANSEL
       movlw       B'00001100'     ; Inputs AN2 and AN3 are analog while
       movwf       ANSEL           ; all other pins are digital
       clrf        ANSELH
       
       banksel     ADCON1          ; Selects bank including register ADCON1
       bsf         ADCON1,ADFM     ; Right justification of result
       bcf         ADCON1,VCFG1    ; Voltage Vss is used as Vref
       bsf         ADCON1,VCFG0    ; RA3 pin voltage is used as Vref+
       
       banksel     ADCON0          ; Selects bank containing register ADCON0
       movlw       B'00001001'     ; AD converter uses clock Fosc/2, AD channel
       movwf       ADCON0          ; on RA2 pin is used for conversion and
                                   ; AD converter is enabled
loop
       banksel     ADCON0
       btfsc       ADCON0,1        ; Tests bit GO/DONE
       goto        loop            ; Conversion in progress, remain in
                                   ; loop
       banksel     ADRESL
       movf        ADRESL,w        ; Lower byte of conversion result is
                                   ; copied to W
       banksel     PORTB
       movwf       PORTB           ; Byte is copied to PORTB
       bsf         ADCON0,1        ; Starts new conversion
       goto        loop            ; Jump to label "loop"
       end                         ; End of program