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rrb011270
Joined: 07 Sep 2003
Posts: 51
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| Need help on ADC Voltage Reference? |
 Posted: Wed Feb 18, 2004 10:29 pm |
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Mabuhay!
I need your help with my code below... I use an external reference for my ADC at AN3 of the PIC18F452 (pin #5). I attach a 5V power supply to AN3 that will act as a reference voltage for my ADC and another 5V power supply for my main power of the entire circuits.
My concern is... is that my ADC doesn't capture any data but when I use only one(1) power supply with ADC reference to VDD, my ADC works.
I need your comments please.
Thanx
The Code:
| Code: |
/////////////////////////////////////
// Interrupt Service Routines //
/////////////////////////////////////
/*
The data sheet says there must be a delay of 2 x Tad
after a conversion. Since we're using a Tad of 3.2 usec,
this delay must be 2 x 3.2 = 6.4 usec. But we know that the
CCS interrupt handler takes at least that long to go in
and out of this interrupt routine (with our 20 MHz crystal),
so we don't need to put in a line of code to do the delay.
*/
#int_ad // ADC interrupt service routine
void adc_isr()
{
// read the 10-bit ADC result
gi16ADCresult = read_adc(ADC_READ_ONLY);
glADCdone = 1; // ADC conversion complete
}
/////////////////////////////////////
// End Interrupt Service Routines //
/////////////////////////////////////
///////////////////////////////////
// MCU Initialization Routines //
///////////////////////////////////
//************************************************************************
void init_chip() { // Initialize the MCU Chip
lcd_init(); // LCD should be initialized first
delay_ms(100);
lcd_init(); // Init LCD again, coz sometimes can't initialize LCD
delay_ms(100);
lcd_init(); // Once more, to be sure LCD gets initialized
setup_adc_ports(RA0_RA1_ANALOG_RA3_REF); // setup the ADC analog pins
// with a 20MHz clock, we divide the oscillator by 64
// to get an ADC clock of 312.5KHz, so Tad=3.2usec
setup_adc(ADC_CLOCK_DIV_64); // configure the ADC clock to 3.2usec
set_adc_channel(0); // specify ADC channel to use (PortA0)
// the default is to right justify
enable_interrupts(int_ad); // enable ADC interrupt
enable_interrupts(global); // enable global interrupt
}
///////////////////////////////////
//End MCU Initialization Routines//
///////////////////////////////////
//************************************************************************
// Main Program
//************************************************************************
main()
{
init_chip(); // Initialize the MCU Chip
lcd_putc("\fADC Demo");
gi16ADCresult = 0; // initialize the ADC result variable to zero
delay_us(20); // wait for ADC aquisition time
read_adc(ADC_START_ONLY); // start an ADC conversion
while (TRUE)
{
if (glADCdone) // ADC conversion completed
{
glADCdone = 0; // prevent multi-trigger of ADC conversion
printf("ADC Result = %lx ADC Steps = %lu\n\r",gi16ADCresult,gi16ADCresult); // print the result
// for demo purpose delay 1sec. for screen updating with
// the new ADC result
delay_ms(1000);
read_adc(ADC_START_ONLY); // start another ADC conversion
}
}
}
//************************************************************************
// End Program
//************************************************************************
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Ttelmah
Guest
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| Posted: Thu Feb 19, 2004 6:08 am |
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Obvious questions Have you checked with a voltmeter that you do have 5v on the reference pin when running with the seperate supply?. What is the source impedance of the second source?.
Best Wishes |
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ac9z
Joined: 03 Feb 2004
Posts: 2
Location: Wheeling, IL USA
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| Posted: Thu Feb 19, 2004 7:09 am |
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If the ADC works with one supply and not both, the problem would appear to involve the dual supply. Are both supplies connected to the same ground? The grounds need to be tied together at one point.
Regards
_________________
Marty L |
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ac9z
Joined: 03 Feb 2004
Posts: 2
Location: Wheeling, IL USA
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| Posted: Thu Feb 19, 2004 7:09 am |
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If the ADC works with one supply and not both, the problem would appear to involve the dual supply. Are both supplies connected to the same ground? The grounds need to be tied together at one point.
Regards
_________________
Marty L |
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