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SPI mode confusion

 
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geolover



Joined: 08 Feb 2011
Posts: 18

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SPI mode confusion
PostPosted: Fri Mar 18, 2011 5:12 am     Reply with quote


This is the timing diagram of an ADS1271 module outputting data in SPI format

If the above picture isn't showing up then use the link below:

http://www.microchip.com/forums/download.axd?file=0;562791

I'm interfacing this module with my PIC, I got confused by the SPI mode type it is as it wasn't mentioned specifically in page25 of the datasheet, http://focus.ti.com.cn/cn/lit/ds/symlink/ads1271.pdf

my programme is listed below:

Code:


#include <24FJ128GA010.h>
#include "stdio.h"
#include "string.h"

#FUSES NOWDT                    //No Watch Dog Timer
#FUSES NOJTAG                   //JTAG disabled
#FUSES NOPROTECT                //Code not protected from reading
#FUSES NOWRT                    //Program memory not write protected
#FUSES NODEBUG                  //No Debug mode for ICD
#FUSES NOCOE                    //Device will reset into operational mode
#FUSES NOIESO                   //Internal External Switch Over mode disabled
#FUSES PR                       //Primary Oscillator
#FUSES CKSNOFSM                 //Clock Switching is enabled, fail Safe clock monitor is disabled
#FUSES NOOSCIO                  //OSC2 is clock output
#FUSES XT
#use delay(clock=8000000)
#use rs232(UART1,baud=9600,parity=N,bits=8)
#define AD_SYNC PIN_A0
#define DRDY PIN_A4
#bit frmen1 = 0x0244.15
#bit spifsd1= 0x0244.14
#bit spifpol1 = 0x0244.13
#bit spifsd1 = 0x0244.2
#bit spife1 = 0x0244.1


#bit spisidl2 = 0x0260.13
#bit frmen2  = 0x0264.15
#bit spifsd2 = 0x0264.14
#bit spifpol2= 0x0264.13
#bit spife2  = 0x0264.2
#bit spiben  = 0x0264.1
#bit TRISF7  = 0x02DE.7


void main()
{
   delay_ms(1);
  unsigned char colon= ':';
   unsigned char h='h';
   unsigned char i='i';
   unsigned char t='t';
   
 //  int8 i,j;
 //  int8 ad_samples1[256],ad_samples2[256],ad_samples3[256];
   int8 ad_samples1,ad_samples2,ad_samples3;
   TRISF7 = 1;
//   spisidl2 =0;
//   frmen2=1;
//   spifsd2=1;
//   spifpol2=1;
//   spife2=0;
//   spiben=0;

   set_tris_a(0x10); //A0,A7 output while A4 is input(/DRDY)

   
   setup_spi2(spi_master | spi_l_to_h | spi_clk_div_4);
   delay_ms(1);
   setup_spi(spi_master | spi_l_to_h| spi_xmit_l_to_h | spi_clk_div_4);
   output_low(AD_SYNC);
   delay_us(2);
   output_high(AD_SYNC);
   while(1)
   {
output_low(PIN_A6);
        while((DRDY==1)); 
          output_high(PIN_A6);
 //   while(!spi_data_is_in());   
//        output_low(PIN_A6);
       
        ad_samples1=spi_read2(0x00);
        ad_samples2=spi_read2(0x00);
        ad_samples3=spi_read2(0X00);
//        output_high(PIN_A6);

     


         spi_write(h);
         delay_us(100);
         spi_write(i);
         delay_us(100);
         spi_write(t);
         delay_us(100);
         spi_write(colon);
//         spi_write(0x3a);
         delay_us(100);
         output_low(PIN_A7);
         spi_write(ad_samples1);//write 8-bit data into spi
         delay_us(100);
         spi_write(ad_samples2);
         delay_us(100);
         spi_write(ad_samples3);
         delay_us(100);
output_high(PIN_A7);
   //   }
   }
}

My problem is, without connecting anything to the input of the ADC, I should be able to get noise signal in small number, however, I'm receiving data as large as FFFFFA in the DOUT pin, which is essentially negative voltage reference, I began to suspect if my SPI mode is not correct, could anyone take a look for me?

Thank you very much!
FvM



Joined: 27 Aug 2008
Posts: 2337
Location: Germany

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PostPosted: Fri Mar 18, 2011 2:08 pm     Reply with quote

Quote:
I'm receiving data as large as FFFFFA in the DOUT pin, which is essentially negative voltage reference

I don't think so. It's a small negative value:
Quote:
The ADS1271 outputs 24 bits of data in two’s complement format.
geolover



Joined: 08 Feb 2011
Posts: 18

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Issues with SDI pin outputting data
PostPosted: Sat Mar 19, 2011 11:43 am     Reply with quote

Thank you, was being stupid this time....

However, there is an issue related to the SDI pin RF7, it seems RF7 is kept at output state even if I had manually set the tris to input followed by setup_spi() function. Now it will generate 3 continous pulse....
temtronic



Joined: 01 Jul 2010
Posts: 9247
Location: Greensville,Ontario

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PostPosted: Sat Mar 19, 2011 12:44 pm     Reply with quote

I've downloaded the datasheet and it says it's a read only device, so why are you writing to it ?
below code cut from your OP..


spi_write(h);
delay_us(100);
spi_write(i);
delay_us(100);
spi_write(t);
delay_us(100);
spi_write(colon);
// spi_write(0x3a);
delay_us(100);
output_low(PIN_A7);
spi_write(ad_samples1);//write 8-bit data into spi
delay_us(100);
spi_write(ad_samples2);
delay_us(100);
spi_write(ad_samples3);
delay_us(100);
geolover



Joined: 08 Feb 2011
Posts: 18

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PostPosted: Sat Mar 19, 2011 2:20 pm     Reply with quote

temtronic wrote:
I've downloaded the datasheet and it says it's a read only device, so why are you writing to it ?
below code cut from your OP..


spi_write(h);
delay_us(100);
spi_write(i);
delay_us(100);
spi_write(t);
delay_us(100);
spi_write(colon);
// spi_write(0x3a);
delay_us(100);
output_low(PIN_A7);
spi_write(ad_samples1);//write 8-bit data into spi
delay_us(100);
spi_write(ad_samples2);
delay_us(100);
spi_write(ad_samples3);
delay_us(100);

Apologies, my mistake for not clarifying it.

The spi_write() command send samples received from ADC back to a 18F4550 device then feed into the PC through a USB.
temtronic



Joined: 01 Jul 2010
Posts: 9247
Location: Greensville,Ontario

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PostPosted: Sat Mar 19, 2011 4:25 pm     Reply with quote

Whew , sure glad you cleared that up ! Thought I might have found your problem, but guess that isn't it.....
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