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POPE19
Joined: 27 Jun 2017
Posts: 71
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| Incorrect ADC values for PIC16F1939 |
 Posted: Thu Sep 07, 2017 7:43 am |
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Hello,
I am using pic16f1939 for adxl335. The adc values read by pic is incorrect with 3.3 v reference.
It should read:
x - 1.611v (499)
y - 1.578v (489)
z - 1.985v (615)
However on serial monitor it is displayed as:
x = 126 y = 126 z = 126
| Code: |
#include <16F1939.h>
#fuses HS ,NOVCAP, MCLR, NOWDT, PUT, NOIESO, BROWNOUT, NOFCMEN,PLL_SW
#use delay(crystal=20MHz, clock=20MHz)
#use rs232(baud=9600, xmit=PIN_C6, rcv=PIN_C7)
#define INTS_PER_SECOND 76 // (20000000/(4*256*256))
int int_count=INTS_PER_SECOND;
long seconds=0;
long x,y,z;
//============================================================================//
#INT_TIMER0 //This function is called
void clock_isr()
{ //every time the RTCC (timer0)
//overflows (255->0)
//For this program this is apx
//31 times per second.
if(--int_count==0)
{
seconds++;
int_count=INTS_PER_SECOND;
}
}
//============================================================================//
void adc_cal()
{
SET_ADC_CHANNEL (0); // select AN0 channel
delay_us(30); // delay for sampling time Tad.
x = read_adc(); // read channel
printf("X = %LU \t ",x);
SET_ADC_CHANNEL (5); // select AN5 channel
delay_us(30);
y = read_adc();
printf("Y = %LU \t ",x);
SET_ADC_CHANNEL (12); // select AN12 channel
delay_us(30);
z= read_adc();
printf("Z = %LU \n \r ",x);
}
//============================================================================//
void main()
{
set_tris_a (0xC9); //set RA-0,3,6,7 as inputs
set_tris_b (0xC9); //set RB-0,3,6,7 as inputs
set_rtcc(0);
setup_timer_0(T0_INTERNAL | T0_DIV_256 | T0_8_BIT);
setup_vref (VREF_OFF | VREF_ADC_OFF|VREF_COMP_DAC_OFF|TEMPERATURE_INDICATOR_DISABLED);
setup_adc (ADC_CLOCK_DIV_32);
//select A0,E0,B0 as analog input & (Vref+ = 3.3v) as voltage reference
setup_adc_ports(sAN0 | sAN5 | sAN12,VSS_VREF);
setup_comparator (NC_NC_NC_NC);
enable_interrupts (INT_TIMER0);
enable_interrupts(GLOBAL);
while (TRUE)
{
if(seconds>1)
{
adc_cal();
seconds =0;
}
}
}
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newguy
Joined: 24 Jun 2004
Posts: 1907
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| Re: INCORRECT ADC VALUES FOR PIC16F1939 |
| Posted: Thu Sep 07, 2017 7:47 am |
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| Code: | | setup_adc_ports(sAN0 | sAN5 | sAN12,VSS_VREF); |
This line is telling the processor that YOU are supplying an external positive voltage reference for the A/D into the PIC's VREF+ pin. Is that what you are doing? If you instead want to set up the A/D to use the PIC's positive supply as the positive reference, then this line needs to change. Open the PIC16F1939.h file and look for the A/D section for the proper constants to use in this function. |
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alan
Joined: 12 Nov 2012
Posts: 357
Location: South Africa
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| Posted: Thu Sep 07, 2017 7:58 am |
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Is this correct?
| Code: | y = read_adc();
printf("Y = %LU \t ",x); |
Shouldn't x be y. Same hold for z.[/b] |
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POPE19
Joined: 27 Jun 2017
Posts: 71
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| Posted: Thu Sep 07, 2017 8:13 am |
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Yes i am supplying the positive voltage 3.3volts on pin 5 (RA3). i don't want to use pic internal voltage reference.
Yes i changed the printf commands but still it displays wrong values:
x = 126 y=126 z = 156
| Code: |
void adc_cal()
{
SET_ADC_CHANNEL (0); // select AN0 channel
delay_us(30); // delay for sampling time Tad.
x = read_adc(); // read channel
printf("X = %LU \t ",x);
SET_ADC_CHANNEL (5); // select AN5 channel
delay_us(30);
y = read_adc();
printf("Y = %LU \t ",y);
SET_ADC_CHANNEL (12); // select AN12 channel
delay_us(30);
z= read_adc();
printf("Z = %LU \n \r ",z);
}
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Ttelmah
Joined: 11 Mar 2010
Posts: 19505
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| Posted: Thu Sep 07, 2017 8:21 am |
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Now it makes sense...... 
#device ADC-=10
Without this it is defaulting to returning the 8bit ADC value. |
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POPE19
Joined: 27 Jun 2017
Posts: 71
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| Posted: Thu Sep 07, 2017 9:08 am |
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| It works !! Thank you. |
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