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ntgcp_08
Joined: 21 Feb 2008
Posts: 24
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| A/D conversion using 18F67J50 |
 Posted: Thu Nov 06, 2008 2:00 pm |
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I'm using compiler PCH version 4.066.
I'm using the 18F67J50.
The PIC is powered by 3.3V.
The A/D reference voltage is 2.048V.
The issue I am seeing is I when set the A/D to use Vdd and Vss as it's limits or when I set the A/D to use Vref+ and Vss as it's limits I read the same value.
I have 3.3V going to all Vdd pins. I have 3.3V going to AVdd, pin 19. I have 2.048 going to Vref+, pin 21. I have all Vss pins, AVss, and Vref- tied to GND.
Here is a short sample program
| Code: |
#include <18F67J50.h>
#device adc=10
#FUSES NOWDT //No Watch Dog Timer
#FUSES WDT128 //Watch Dog Timer uses 1:128 Postscale
#FUSES H4_SW //High speed osc with SW enabled 4x PLL
#FUSES NODEBUG //No Debug mode for ICD
#FUSES NOXINST //Extended set extension and Indexed Addressing mode disabled (Legacy mode)
#FUSES STVREN //Stack full/underflow will cause reset
#FUSES NOPROTECT //Code not protected from reading
#FUSES FCMEN //Fail-safe clock monitor enabled
#FUSES IESO //Internal External Switch Over mode enabled
#FUSES MSSPMSK7
#FUSES NOCPUDIV
#FUSES PLL3 //No PLL PreScaler
#use delay(clock=48000000)
#use rs232(baud=9600,parity=N,xmit=PIN_G1,rcv=PIN_G2,bits=8,stream=DEBUG)
#byte OSCTUNE = 0xf9b
#bit PLL_EN = OSCTUNE.6
#define LED1 PIN_E4
#define LED2 PIN_C1
#define LED3 PIN_E3
#define XMIT_PWR_ON PIN_D1
#define REF_V PIN_E1
void main()
{
int16 cnts=0;
setup_adc(ADC_CLOCK_INTERNAL|ADC_TAD_MUL_0);
setup_psp(PSP_DISABLED);
setup_spi(SPI_SS_DISABLED);
setup_spi2(SPI_SS_DISABLED);
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_timer_4(T4_DISABLED,0,1);
setup_comparator(NC_NC_NC_NC);// This device COMP currently not supported by the PICWizard
PLL_EN = 1; //enable the software PLL
output_bit(REF_V, 1);
output_bit(XMIT_PWR_ON,1);
fprintf(DEBUG, "\fStarting A/D conversions\r\n");
while(1){
output_bit(LED1, 1);
output_bit(LED2, 0);
setup_adc_ports(sAN0|sAN1|sAN4|VSS_VREF);
delay_ms(1000);
set_adc_channel(0);
delay_us(10);
cnts = read_adc();
fprintf(DEBUG, "0a - Counts=%lu\r\n", cnts);
delay_ms(3000);
output_bit(LED1, 0);
output_bit(LED2, 1);
setup_adc_ports(sAN0|sAN1|sAN4|VSS_VDD);
delay_ms(1000);
set_adc_channel(0);
delay_us(10);
cnts = read_adc();
fprintf(DEBUG, "0b - Counts=%lu\r\n", cnts);
delay_ms(3000);
}
}
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I am putting 0.507V into AN0 and reading 160 counts. That is correct if the A/D is reference to VDD(3.3V). When the A/D is referenced to Vref+(2.048V) I am also reading 160 counts. I would expect to read around 253 counts. |
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PCM programmer
Joined: 06 Sep 2003
Posts: 21708
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| Posted: Thu Nov 06, 2008 6:08 pm |
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| Quote: | The PIC is powered by 3.3V.
The A/D reference voltage is 2.048V. |
The problem is in the Vref+ value. Look at this table in the PIC data
sheet, on page 456 in the Acrobat reader:
| Quote: | | TABLE 28-28: A/D CONVERTER CHARACTERISTICS: PIC18F87J50 FAMILY (INDUSTRIAL) |
Look at spec A20, the Reference Voltage Range. If Vdd is > 3.0v,
the Vref delta must be at least 3 volts. So in your case, the lowest
that you can set it is 3.0v. |
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ntgcp_08
Joined: 21 Feb 2008
Posts: 24
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| Posted: Fri Nov 07, 2008 8:26 am |
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| Thanks! |
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