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TL
Joined: 15 Sep 2003
Posts: 75
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| AD7705 - reading problem |
 Posted: Wed Feb 07, 2007 9:50 am |
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I have the AD7705 connected to PIC18F2620, so that the MCU reads the 16-bit ADC values. The following are configured:
AD7705:
- 4MHz resonator used but divided by 2 to give 2MHz internally. However, in AD7705 datasheet example uses 2.4576MHz.
- Precision 2.5V across Ref+ (pin 9) and Ref- (pin 10) inputs.
- Both AIN1+ (pin 7) and AIN1- (pin 8) connected to precision 2.5V. Therefore, no differential voltage across these 2 input pins.
- Vdd=5V, Vss=0V.
- Master clock enable on, clock div on, 50Hz update rate, self cal mode,
gain=1, bipolar inputs, buffer on, fync=start.
PIC18F2620:
- 4MHz resonator (separate resonator from AD7705).
Pin connections:
AD7705 PIC18F2620
===============
DRDY A0
DOUT A1
DIN A2
SCLK A3
RESET A4
CS A5
CCS PCWH Compiler V3.249
I have checked the following:
- Serial clock, data in, and CS pulses on a scope, and they appear to be correct.
- All the connections and voltages and they appear to be correct.
- Already tried 2 new samples of AD7705 and the same results obtained.
I have configured the AD7705 to accept bipolar inputs. Therefore, for the above AD7705 conditions, I would expect a data (ADC value) output of 0 because its differential input is 0V (i.e. 2.5V � 2.5V). However, I received �32768!
I would appreciate any comments.
After reading some threads on this Forum, I made some changes to the CCS AD7705.c driver. Please find attached code below:
| Code: |
// AD7705.c
//// Driver routines for the AD7705 chip
//Assuming a 2.4576 crystal ocsillator is used between MCLK IN and MCLK OUT
//connection pins to the PIC
#ifndef ADC_DRDY
#define ADC_DRDY PIN_A0
#define ADC_DO PIN_A1
#define ADC_DI PIN_A2
#define ADC_CLK PIN_A3
#define ADC_RESET PIN_A4
#define ADC_CS PIN_A5
#endif
//Operation modes (MD1, MD0 bits in Setup Register)
#define ADC_NORMAL 0x00
#define ADC_SELF 0x40
#define ADC_ZERO_SCALE 0x80
#define ADC_FULL_SCALE 0xC0
//Gain settings (G2, G1, G0 bits in Setup Register)
#define ADC_GAIN_1 0x00
#define ADC_GAIN_2 0x08
#define ADC_GAIN_4 0x10
#define ADC_GAIN_8 0x18
#define ADC_GAIN_16 0x20
#define ADC_GAIN_32 0x28
#define ADC_GAIN_64 0x30
#define ADC_GAIN_128 0x38
//Polar operations (B/U bit in Setup Register)
#define ADC_BIPOLAR 0x00
#define ADC_UNIPOLAR 0x04
//Buffer control (BUF bit in Setup Register)
#define ADC_BUFFER_ON 0x02
#define ADC_BUFFER_OFF 0x00
//Fsync (FSYNC bit in Setup Register)
#define ADC_FSYNC_RESET 0x01
#define ADC_FSYNC_START 0x00
//Master clock (CLKDIS bit in Clock Register)
#define ADC_MASTERCLK_ENABLE 0x00
#define ADC_MASTERCLK_DISABLE 0x10
//Clock divider (CLKDIV bit in Clock Register)
#define ADC_CLKDIV_ON 0x08
#define ADC_CLKDIV_OFF 0x00
//Update rates (CLK, FS1, FS0 bits in Clock Register)
#define ADC_20 0x00
#define ADC_25 0x01
#define ADC_100 0x02
#define ADC_200 0x03
#define ADC_50 0x04
#define ADC_60 0x05
#define ADC_250 0x06
#define ADC_500 0x07
// Function prototypes
void adc_init();
void setup_adc_device(int masterclk, int clkdiv, int rate,
int mode, int gain, int polar, int buffer, int fsync);
void write_adc_byte(BYTE data);
long read_adc_word();
long read_adc_value(int1 ch);
//initialization routine
void adc_init()
{
output_low(ADC_RESET); // Reset all ADC registers to their default state.
delay_ms(10);
output_high(ADC_RESET);
// See Fig. 17 of AD7705 datsheet
output_high(ADC_CLK); // CLK line at high state
output_high(ADC_CS); // CS line at high state
setup_adc_device(ADC_MASTERCLK_ENABLE,ADC_CLKDIV_ON,ADC_50,
ADC_SELF,ADC_GAIN_1,ADC_BIPOLAR,ADC_BUFFER_ON,ADC_FSYNC_START);
delay_ms(10);
}
//setup the device paramaters
void setup_adc_device(int masterclk, int clkdiv, int rate,
int mode, int gain, int polar, int buffer, int fsync)
{
int8 temp1, temp2;
write_adc_byte( 0x20 ); //Communications Register set to write of Clock Register
temp1 = masterclk|clkdiv|rate;
write_adc_byte( temp1 ); //Clock Register info here
write_adc_byte( 0x10 ); //Communications Register set to write of Setup Register
temp2 = mode|gain|polar|buffer|fsync;
write_adc_byte( temp2 ); //Setup Register info here
}
void write_adc_byte(BYTE data)
{
int8 i;
output_low(ADC_CS);
for(i=0; i<8; i++)
{
output_low(ADC_CLK);
delay_us(50);
output_bit(ADC_DI, shift_left(&data,1,0));
output_high(ADC_CLK);
delay_us(50);
}
output_high(ADC_CS);
}
long read_adc_word()
{
int8 i;
long data;
output_low(ADC_CS);
for(i=0; i<16; i++)
{
output_low(ADC_CLK);
delay_us(50);
shift_left(&data,2,input(ADC_DO));
output_high(ADC_CLK);
delay_us(50);
}
output_high(ADC_CS);
return data;
}
// read an adc value from the specified channel
// ch = 0 or 1
long read_adc_value(int1 ch)
{
long value;
while ( input(ADC_DRDY) ); // Loop until data is ready
if(ch) // ch=1 (AIN2+, AIN2-)
write_adc_byte(0x39); //communications register set to read of data
else // ch=0 (AIN1+, AIN1-)
write_adc_byte(0x38); //communications register set to read of data
value=read_adc_word();
return value;
}
// ============================================
#include <18F2620.h>
#device ADC=10
#device ICD=TRUE
#FUSES NOWDT // Watchdog disabled
#FUSES XT
#FUSES NOLVP
// 4MHz clock (internal clock of MCU=1MHz)
#use delay(clock=4000000)
#define ADC_DRDY PIN_A0
#define ADC_DO PIN_A1
#define ADC_DI PIN_A2
#define ADC_CLK PIN_A3
#define ADC_RESET PIN_A4
#define ADC_CS PIN_A5
#use fast_io(a)
#use fast_io(b)
#use fast_io(c)
#byte port_a = 0xF80 // Give meaningful names for these ports
#byte port_b = 0xF81
#byte port_c = 0xF82
#include "lcd1.c"
#include "ad7705.c"
void main(void)
{
long value;
set_tris_a(0b00000011); // A0, A1=inputs, A2 to A5 = outputs
set_tris_b(0b11000000);
set_tris_c(0b10010000);
setup_adc(ADC_OFF);
delay_ms(1);
setup_spi(FALSE);
setup_comparator(NC_NC_NC_NC);
setup_vref(FALSE);
setup_low_volt_detect(FALSE);
setup_oscillator(FALSE);
lcd_init(); // Initialise LCD
delay_ms(6); // Allow LCD to settle
lcd_putc("\fHello!");
delay_ms(100);
adc_init();
while(TRUE)
{
value=read_adc_value(0); // Read (AIN1+, AIN1-) differential inputs
printf(lcd_putc,"\fValue = %6ld", value);
delay_ms(1000);
}
}
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Ttelmah
Guest
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| Posted: Wed Feb 07, 2007 10:30 am |
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The value you have, is 'spot on'.....
You are in bipolar mode. Hence the input ranges from -2.5v(0) to +2.5v(0xFFFF), via 0v (0x8000). What is wrong, is that you are treating the 16bit value, as a signed number. If you take the hex value 0x8000, and display it as a signed value, you get -32768 (use the Pconvert program with the compiler, to see how values display).
Best Wishes |
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TL
Joined: 15 Sep 2003
Posts: 75
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| Posted: Thu Feb 08, 2007 8:39 am |
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Hi Ttelmah,
Many thanks for your quick response. You are absolutely correct with your explanation. Yes, I have compared various signed and unsigned long values with the Pconvert program. As there was no formula stated in the AD7705 datasheet for the output code, I made the wrong assumption!
Since then, I have read an FAQ document from the Analog Devices website and the bipolar mode uses �Offset Binary Coding�. The output code for any analogue input differential voltage can be represented as follows:
Output code = [2 to the power of (n-1)] * [(Vdiff * Gain/Vref)+1]
Where n = 16, and Vdiff can be negative or positive. |
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