problem in ADC

[khor]

i have a problem in comparing the two analog input signals, i am using Analog to digital function in pic16f877, if input_1 in higher than input_2 the pin_b7 is high, else is low.
when i comparing the input, the output is sometime showing high and low. At the thershold voltage is showing low but when i increase to 0.5V the output is high instead of low.

my coding is on below

#include <16f877.h>
#device ADC=10
#fuses XT, NOWDT, NOLVP,PUT,NOPROTECT
#use delay(clock=4000000)
#use rs232(baud=9600, xmit=PIN_C6, rcv=PIN_C7)

main()
{
int val0, val1;
setup_adc_ports( ALL_ANALOG);
setup_adc( ADC_CLOCK_INTERNAL);


while(true)
{
set_adc_channel(0);
delay_us(80);
val0=read_adc();

set_adc_channel(7);
delay_ms(80);
val1=read_adc();

if(val0 > val1)
output_high(pin_b7);

else
output_low(pin_b7);

}
}
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This message was ported from CCS's old forum
Original Post ID: 12859

[R.J.Hamlett]

:=i have a problem in comparing the two analog input signals, i am using Analog to digital function in pic16f877, if input_1 in higher than input_2 the pin_b7 is high, else is low.
:=when i comparing the input, the output is sometime showing high and low. At the thershold voltage is showing low but when i increase to 0.5V the output is high instead of low.
:=
:=my coding is on below
:=
:=#include <16f877.h>
:=#device ADC=10
:=#fuses XT, NOWDT, NOLVP,PUT,NOPROTECT
:=#use delay(clock=4000000)
:=#use rs232(baud=9600, xmit=PIN_C6, rcv=PIN_C7)
:=
:=main()
:={
:=int val0, val1;
:=setup_adc_ports( ALL_ANALOG);
:=setup_adc( ADC_CLOCK_INTERNAL);
:=
:=
:=while(true)
:={
:= set_adc_channel(0);
:= delay_us(80);
:= val0=read_adc();
:=
:= set_adc_channel(7);
:= delay_ms(80);
:= val1=read_adc();
:=
:=if(val0 > val1)
:=output_high(pin_b7);
:=
:=else
:=output_low(pin_b7);
:=
:= }
:=}
Very simple.
Remember an int, is 8bits, not 16. You are reading a 10bit value, and putting it into an 8bit store. You will then only have the low 8 bits, and the results of the comparison will be odd...
Make your storage variables into int16 types, and the comparison should be fine.

Best Wishes
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This message was ported from CCS's old forum
Original Post ID: 12862