///////////////////////////////////////////////////////////////////////////////////////////////////////
//// AD7705.C ////
//// Driver for analog device AD7705 ////
//// adc_init() Call after power up ////
//// read_adc_value(channel)Read adc value from the specified channel ////
//// adc_disable() Disables the adc conversion ////
///////////////////////////////////////////////////////////////////////////////////////////////////////
//// Driver routines for the AD7705 chip
//Assuming a 2.4576 crystal ocsillator is used between MCLK IN and MCLK OUT

// ************************************************
// *************** AD7705 采样 ******************* **
// ************************************************
// connection pins to the PIC
#define ADC_DRDY PIN_B0 // 数据准备好
#define ADC_DO PIN_B2 // SPI数据输出
#define ADC_DI PIN_B1 // SPI数据输入
#define ADC_CLK PIN_B3 // SPI数据时钟

//Operation modes
#define ADC_NORMAL 0x00
#define ADC_SELF 0x40
#define ADC_ZERO_SCALE 0x80
#define ADC_FULL_SCALE 0xc0

//Gain settings
#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
#define ADC_BIPOLAR 0x00
#define ADC_UNIPOLAR 0x04

//update rates
#define ADC_50 0x04
#define ADC_60 0x05
#define ADC_250 0x06
#define ADC_500 0x07

void adc_init(void);
void write_adc_byte(BYTE data);
unsigned long read_adc_word();
void setup_adc_device(int calmode, int gainsetting, int operation, int rate);

unsigned long read_adc_value(int1 ch);
void adc_disable();
float convert_to_volts(long data);

//initailaization routine

void adc_init(void)
{
restart_wdt();
write_adc_byte( 0x04 );
setup_adc_device(ADC_SELF,ADC_GAIN_1,ADC_UNIPOLAR,ADC_50);
}

void write_adc_byte(BYTE data)
{
BYTE i;
for(i=1;i<=8;i++) {
output_bit(ADC_DI, shift_left(&data,1,0));
output_low(ADC_CLK);
output_high(ADC_CLK);
}
}

unsigned long read_adc_word()
{
BYTE i;
unsigned long data;

for(i=1;i<=16;++i) {
output_low(ADC_CLK);
output_high(ADC_CLK);
shift_left(&data,2,input(ADC_DO));
}
return data;
}

//setup the device paramaters(mode, gainsetting, polar operation and output rate)
void setup_adc_device(int calmode, int gainsetting, int operation, int rate)
{
restart_wdt();
write_adc_byte( 0x20 ); //Communications Register set to write of clock register
write_adc_byte( rate ); //Clock Register info here
write_adc_byte( 0x10 ); //Communications Register set to write of setup register
write_adc_byte( calmode|gainsetting|operation ); //calmode|gainsetting|operation);
//Setup Register info here
write_adc_byte( 0x21 ); //Communications Register set to write of clock register
write_adc_byte( rate ); //Clock Register info here
write_adc_byte( 0x11 );
write_adc_byte( calmode|gainsetting|operation );
}

//read an adc value from the specified channel
unsigned long read_adc_value(int1 ch)
{
unsigned long value;
while ( input(ADC_DRDY) );
if(!ch)
write_adc_byte(0x38); //communications register set to read of data register of channel 1
else
write_adc_byte(0x39); //communications register set to read of data register of channel 0
value=read_adc_word();
while ( !input(ADC_DRDY) );//
return value;
}

//disable the a/d conversion
void adc_disable()
{
write_adc_byte( 0x20 );//Communications Register set to write of clock register
write_adc_byte( 0x10 );//Clock Register info here
}

//Convert the value read to volts
float convert_to_volts(long data)
{
return ((float)data*2.498/0xffff);
}

請看

C:\Program Files\PICC\Drivers\AD7705.C

AD7705我只使用了SCLK、DIN、DOUT、DRDY与PIC16F877A相接，读不出数据？