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Guest
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| PIC18F8723 Analog to Digital Setup |
 Posted: Fri Mar 27, 2009 11:45 am |
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CCS version 4.068
Hello!! I am trying to setup my A2D conversion on my pic but the voltage I read with a meter and the voltage I read with the pic are off by about 120mv. My chip uses a 12 bit A2D. here is my code.
This is the function I call:
| Code: |
void Get_Altitude()
{
int i;
atemp=0;
for(i=0;i<10;++i)
{
set_adc_channel(1);
delay_us(20);
value=read_adc();
avoltage=((value*(5.0367/(4095))));
pressure=((avoltage)/(5.0367*.009)+.095/.009)*0.296133971;
atemp=atemp+pressure;
}
atemp=atemp/10;
height=((1-pwr((pressure/29.785),.19026))*288.15)/.00198122;
// printf("height %ld pressure %2.3f voltage %5.3f avgpressure %5.3f \r\n",height, pressure,avoltage, temp);
//delay_ms(500);
} |
Here is my setup in the main:
| Code: |
void main()
{
//Basic Chip Setup
setup_adc_ports(AN0_TO_AN1|VREF_VREF );
setup_adc(ADC_CLOCK_DIV_8);
setup_psp(PSP_DISABLED);
//setup_spi2(SPI_MASTER|SPI_MODE_3|SPI_CLK_DIV_16);
setup_wdt(WDT_OFF);
setup_timer_0(RTCC_INTERNAL);
setup_timer_1(T1_DISABLED);
setup_timer_2(T2_DISABLED,0,1);
setup_timer_4(T4_DISABLED,0,1);
setup_comparator(NC_NC_NC_NC);
setup_vref(False);
setup_oscillator(OSC_4MHZ|OSC_INTRC|OSC_31250|OSC_PLL_OFF); |
And:
| Code: |
#include <18F8723.h>
//#device ICD=TRUE
#device adc=12
#FUSES NOWDT //No Watch Dog Timer
#FUSES WDT128 //Watch Dog Timer uses 1:128 Postscale
#FUSES INTRC_IO //Internal Osc
#FUSES NOPROTECT //Code not protected from reading
#FUSES IESO //Internal External Switch Over mode enabled
#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 NOLVP //No low voltage prgming, B3(PIC16) or B5(PIC18) used for I/O
#FUSES NOWRT //Program memory not write protected
#FUSES NOCPB //No Boot Block code protection
#FUSES NOEBTRB //Boot block not protected from table reads
#FUSES NOEBTR //Memory not protected from table reads
#FUSES NOWRTD //Data EEPROM not write protected
#FUSES NOWRTC //configuration not registers write protected
#FUSES NOWRTB //Boot block not write protected
#FUSES NOFCMEN //Fail-safe clock monitor disabled
#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)
#FUSES MCU //Microcontroller Mode
#FUSES NOWAIT //Wait selections unavailable for Table Reads or Table Writes
#FUSES BW16 //16-bit external bus mode
#FUSES ABW16 //16-bit Address bus
#FUSES ECCPE //Enhanced CCP PWM outpts multiplexed with RE6 thorugh RE3
#use delay(clock=4000000) |
I had everything working on a 4520 nicely but I needed more resolution. Thanks in advance. |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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Guest
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| Posted: Fri Mar 27, 2009 12:45 pm |
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| My "Value" varies between 3418 and 3421. This along with the fact that that when I multiply that by 5/4095 I am still off 120mv which makes a huge difference for my application. But only varying a few steps I feel is pretty good. I don't know if there is a better way to setup the chip. |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Fri Mar 27, 2009 12:57 pm |
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| Quote: | setup_adc_ports(AN0_TO_AN1|VREF_VREF );
setup_adc(ADC_CLOCK_DIV_8);
I don't know if there is a better way to setup the chip.
|
See this section in the 18F8723 data sheet:
| Quote: | | TABLE 2-1: TAD vs. DEVICE OPERATING FREQUENCIES |
This clock divisor value is the correct one for your 4 MHz oscillator freq:
18F8723 data sheet:
http://ww1.microchip.com/downloads/en/DeviceDoc/18F8723%20Data%20Sheet_39894A.pdf |
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Guest
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| Posted: Fri Mar 27, 2009 1:28 pm |
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| Yeah I went through and tried setting that differently. It had no effect on my output at all. Can I go and just subtract the difference in the voltage, it would be like a manual offset? |
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