AD Conversion not functioning as expected on 16C773

[Greg Kuric]

<font face="Courier New" size=-1>I'm in the beginning stages of building a handheld, recording PRESSURE gauge, battery powered, with IR download link. I've built a prototype board with the PIC16C773 as the brain, an 8x2 LCD for display, and a 10K pot for the A/D input. When I run the following code, my LCD scales from 0 to 65535 (approx.). This is not what I was expecting. If the A/D is 12 bit, the scale should be from 0 to 4095.

#include <16c773.h>
// Note: Modified the following line in 16c773.h to add the 'ADC=12'. Without mod. the A/D scale was 0 to 255
// #device PIC16C773 ADC=12
#include "Lcd.c"
#use delay (clock=3686400)

void main(void){

long value;

lcd_init();
delay_ms(500);

setup_adc_ports( AN0_ANALOG | VSS_VDD );
setup_adc( ADC_CLOCK_INTERNAL );
set_adc_channel( 0 );

while (TRUE) {
value = read_adc( ADC_START_AND_READ );
printf( lcd_putc, "\n\%8lu", value );
delay_ms(1000);
}
}

The second question I have is with changing the voltage reference. When I change the following line to activate RA3 to be the high reference, the pin reads 0.00v and not the 4.096 that I was expecting.

.................... setup_adc_ports( AN0_ANALOG | VSS_VRH );
0162: MOVLW 4E
0163: BSF 03.5
0164: MOVWF 1F


Here is the enviroment I'm working with:
> CCS PCM C Compiler, Version 3.147, 17877
> Using MPLAB IDE 6.10
> Programming Chip on PICSTART Plus, w/ version3.10


Any help that could be given is greatly appreciated.</font>
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This message was ported from CCS's old forum
Original Post ID: 12571

[PCM programmer]

:=<font face="Courier New" size=-1>I'm in the beginning stages of building a handheld, recording PRESSURE gauge, battery powered, with IR download link. I've built a prototype board with the PIC16C773 as the brain, an 8x2 LCD for display, and a 10K pot for the A/D input. When I run the following code, my LCD scales from 0 to 65535 (approx.). This is not what I was expecting. If the A/D is 12 bit, the scale should be from 0 to 4095.
</font>
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The most likely reason is that the ADFM bit is set to 0.
This is the default power-up state, and it causes the A/D
result to be left-justified in a 16-bit word. That's what
you're seeing.

Fix this by setting the ADFM bit to 1.

Example:

#bit ADFM_BIT = 0x9F.7

main()
{
// Put A/D setup code here.

// Put this line after the other A/D init code.
ADFM_BIT = 1; // Right justify the A/D result

// Put other code here.


while(1);
}
___________________________
This message was ported from CCS's old forum
Original Post ID: 12580