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elphi
Joined: 17 Apr 2007
Posts: 20
Location: tijuana,bajacalifornia,mexico
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| ADC inconsistencies |
 Posted: Mon Jul 02, 2007 12:33 pm |
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I have a little problem, I connected an LCD with a 18f4420 , I just tried to emulate a digital voltmeter.
It works, but with some inconsistencies, I mean, it shows a measure, but it changes in a range of +-70mV, I don't know why, but this behavior is not adequate to my project.
I also measured the same voltage at same time with my digital Multimeter (in parallel), and the voltage never changes in the multimeter but in the LCD it changes.
I tried to put a capacitor in parallel with the ADC pin, but it still varies.
Any help will be well came.
this is may code:
| Code: |
#include <18F4420.h>
#device adc=10
#FUSES NOWDT //No Watch Dog Timer
#FUSES WDT128 //Watch Dog Timer uses 1:128 Postscale
//#FUSES XT //Crystal osc <= 4mhzUT
#FUSES INTRC_IO //Internal RC Osc, no CLKOUT
#FUSES NOPROTECT //Code not protected from reading
#FUSES BROWNOUT //Reset when brownout detected
#FUSES BORV25 //Brownout reset at 2.5V
#FUSES NOPUT //No Power Up Timer
#FUSES NOCPD //No EE protection
#FUSES STVREN //Stack full/underflow will cause reset
#FUSES NODEBUG //No Debug mode for ICD
#FUSES LVP //Low Voltage Programming on B3(PIC16) or B5(PIC18)
#FUSES NOWRT //Program memory not write protected
#FUSES NOWRTD //Data EEPROM not write protected
#FUSES IESO //Internal External Switch Over mode enabled
#FUSES FCMEN //Fail-safe clock monitor enabled
#FUSES PBADEN //PORTB pins are configured as analog input channels on RESET
#FUSES NOWRTC //configuration not registers write protected
#FUSES NOWRTB //Boot block not write protected
#FUSES NOEBTR //Memory not protected from table reads
#FUSES NOEBTRB //Boot block not protected from table reads
#FUSES NOCPB //No Boot Block code protection
#FUSES LPT1OSC //Timer1 configured for low-power operation
#FUSES MCLR //Master Clear pin enabled
#FUSES NOXINST //Extended set extension and Indexed Addressing mode disabled (Legacy mode)
#use delay(clock=4000000)
#include <hplcd.C>
#include <stdio.h>
#include <stdlib.h>
#use fast_io(d)
#use standard_io(c)
int16 vi=0;
float va;
#int_EXT
EXT_isr()
{
}
#int_EXT1
EXT1_isr()
{
}
void main()
{
set_tris_d(0);
set_tris_c(1);
output_c(0xff);
delay_ms(1000);
output_c(0x0);
setup_adc_ports(AN0|VSS_VDD);
// setup_adc_ports(AN0|VSS_VREF);
setup_adc(ADC_CLOCK_DIV_2|ADC_TAD_MUL_2);
setup_psp(PSP_DISABLED);
setup_spi(FALSE);
setup_wdt(WDT_OFF);
setup_timer_0(RTCC_INTERNAL);
setup_timer_1(T1_DISABLED);
setup_timer_2(T2_DISABLED,0,1);
setup_timer_3(T3_DISABLED|T3_DIV_BY_1);
setup_comparator(NC_NC_NC_NC);
setup_vref(FALSE);
enable_interrupts(INT_EXT);
enable_interrupts(INT_EXT1);
enable_interrupts(GLOBAL);
setup_low_volt_detect(FALSE);
// setup_oscillator(FALSE);
setup_oscillator(OSC_8MHZ|OSC_INTRC|OSC_PLL_OFF);
//my code
lcd_init();
printf(lcd_putc,"\f");
printf(lcd_putc,"VOLTAGE:");
while (true)
{
lcd_gotoxy(1,2);
vi=read_adc();
va=(vi*5/1023.0);
printf(lcd_putc,"%.6f",va);
}
}
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_________________
Every little thing gonna be alright |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Mon Jul 02, 2007 12:47 pm |
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| Quote: |
#FUSES LVP
#use delay(clock=4000000)
setup_oscillator(OSC_8MHZ|OSC_INTRC|OSC_PLL_OFF);
setup_adc(ADC_CLOCK_DIV_2|ADC_TAD_MUL_2);
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You need to fix the problems shown above.
Do you have a low voltage programmer ? 99% of all people have
a high-voltage programmer. In that case, the fuse must be NOLVP.
You have the #use delay() set for one frequency, but you have
the oscillator set to a different frequency.
Look in the A/D section of the 18F4420 data sheet. What clock divisor
does it say to use for 4 MHz operation ?
Where's the line of code to set the channel ? Example:
| Code: | | set_adc_channel(0); |
At least two or three of the problems above could be the cause of
your problem. |
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elphi
Joined: 17 Apr 2007
Posts: 20
Location: tijuana,bajacalifornia,mexico
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| sorry |
| Posted: Mon Jul 02, 2007 6:44 pm |
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OK, I post a bad version of may software, sorry , the #use delay is 8000000 as in the setting of the internal oscillator.
The datasheet says that I must use an 8TOSC, but I don’t understand that. Does it means some like :
| Quote: | | setup_adc(ADC_CLOCK_DIV_8|ADC_TAD_MUL_8); |
and sincerely I don’t know what is the channel in the adc.
It’s embarrassing but, i will need more support
_________________
Every little thing gonna be alright |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Mon Jul 02, 2007 7:27 pm |
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| Quote: | | I don’t know what is the channel in the adc. |
Here, in your code, you setup the A/D to use channel 0:
| Quote: | | setup_adc_ports(AN0|VSS_VDD); |
That's why I said you must add this line of code:
| Code: | | set_adc_channel(0); |
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elphi
Joined: 17 Apr 2007
Posts: 20
Location: tijuana,bajacalifornia,mexico
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| ok |
| Posted: Mon Jul 02, 2007 10:23 pm |
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Ok, i get it, i made the change, but regarding the other seccion of your answer:
| Quote: |
Look in the A/D section of the 18F4420 data sheet. What clock divisor
does it say to use for 4 MHz operation ?
|
The datasheet says that I must use an 8TOSC, but I don’t understand that. Does it means some like :
| Quote: | | setup_adc(ADC_CLOCK_DIV_8|ADC_TAD_MUL_8); |
but remember that i am using the 8mhz internal clock.
tnks it advance for your help.
_________________
Every little thing gonna be alright |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Tue Jul 03, 2007 1:39 am |
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That would work. According to the data sheet, the minimum Tad is 0.7 us.
Using the 8 MHz internal oscillator and dividing it by 8 gives you a 1 MHz
Tad clock, which gives a period of 1.0 us. That satisfies the minimum
requirement for Tad.
According to the data sheet, the minimum acquisition time is 2.4 us.
That means you could use ADC_TAD_MUL_4 to set the acquisition time,
because with a Tad of 1.0 us, this would give 4.0 us and it would satisfy
the Tacq requirement. But the ADC_TAD_MUL_8 value would still work.
It's longer than you need, but I doubt if it would make much difference
in the result. |
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