 |
 |
| View previous topic :: View next topic |
| Author |
Message |
POPE19
Joined: 27 Jun 2017
Posts: 71
|
| Incorrect ADC values for PIC16F1939 |
 Posted: Thu Sep 07, 2017 7:43 am |
 |
|
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;
}
}
}
|
|
|
 |
newguy
Joined: 24 Jun 2004
Posts: 1909
|
| Re: INCORRECT ADC VALUES FOR PIC16F1939 |
| Posted: Thu Sep 07, 2017 7:47 am |
|
|
| 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. |
|
|
alan
Joined: 12 Nov 2012
Posts: 357
Location: South Africa
|
|
| Posted: Thu Sep 07, 2017 7:58 am |
|
|
Is this correct?
| Code: | y = read_adc();
printf("Y = %LU \t ",x); |
Shouldn't x be y. Same hold for z.[/b] |
|
|
POPE19
Joined: 27 Jun 2017
Posts: 71
|
|
| Posted: Thu Sep 07, 2017 8:13 am |
|
|
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);
}
|
|
|
|
Ttelmah
Joined: 11 Mar 2010
Posts: 19545
|
|
| Posted: Thu Sep 07, 2017 8:21 am |
|
|
Now it makes sense...... 
#device ADC-=10
Without this it is defaulting to returning the 8bit ADC value. |
|
|
POPE19
Joined: 27 Jun 2017
Posts: 71
|
|
| Posted: Thu Sep 07, 2017 9:08 am |
|
|
| It works !! Thank you. |
|
|
|
|
You cannot post new topics in this forum
You cannot reply to topics in this forum
You cannot edit your posts in this forum
You cannot delete your posts in this forum
You cannot vote in polls in this forum
|
Powered by phpBB © 2001, 2005 phpBB Group
|
FAQ 
Search
Register
Profile 
Log in to check your private messages 
Log in
