Strange output from ADC

[vellamike]

I am trying to get the ADC module of the PIC16F870 to work. Unfortunately I am using somebody else's code as I have little experience with microcontrollers. The ADC works but seems to be susceptible to jumping from one "plateau" to another. At the bottom of this post I have included two images with examples of the voltage traces I'm getting when I input a sine wave or a triangle signal. I've also included the relevant code. Has anyone ever experienced this kind of thing? Please let me know if any more information is needed.

adcstart    clrf    temph        clrf    templ    
        clrf    store
        bcf    portc,3        ;ADC CLK low
        bcf    portc,4        ;ADC CS LOW
        bsf    portc,5        ;ADC DIN HIGH START
        bsf    portc,3        ;CLKH1
        bcf    portc,3        ;CLKL1
        bsf    portc,5        ;DIN LOW=DIFFERENTAL MODE/single=High    
        bsf    portc,3        ;CLKH2
        bcf    portc,3        ;CLKL2
        return        


adcfinish    bcf    portc,5        ;ADC DIN low
        movlw    d'4'        ;CLK7-10
        movwf    adcclk2

;PROGRAM STARTSmain        clrf    sendreg
        clrf    highmanh
        clrf    highmanl
        clrf    templ
        clrf    temph
        clrf    store
        bsf    portc,4        ;ADC CS high
        call    longdly        ;debounce        
        bcf    portb,7        ;led off
        call    longdly        ;debounce


div5        bsf    portb,7        ;sample time
spaced        movlw    0xf0        ;test data high
        movwf    store
        call    manconvert
        call    uart_out
        nop
        movlw    0xf0        ;test data high
        movwf    store
        call    manconvert
        call    uart_out
        nop
        call    adcstart
        call    cha
        call    adcfinish
        call    adcstart
        call    chb
        call    adcfinish
        call    adcstart
        call    chc
        call    adcfinish
        call    adcstart
        call    chd
        call    adcfinish
        call    adcstart
        call    che
        call    adcfinish
        bcf    portb,7
        goto    div5
        end