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Stuck with LTC1854 ADC Driver

 
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Owais



Joined: 23 Sep 2010
Posts: 3

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Stuck with LTC1854 ADC Driver
PostPosted: Thu Sep 23, 2010 4:16 am     Reply with quote
I am trying to interface two LTC1854 12bit ADC devices over SPI bus to a PIC18F8680 device. The constraint here is that SPI must be used in hardware mode (not software bit banging) and SPI mode has to be mode 0 or mode 3. I am using ADCs in single ended acquisition

The ADCs seem to be working fine if configured for SPI mode 3, but for channel 0 only. It seems the ADC device is unable to configure itself for acquisition of other channels. Code returned for +5 input on channel 0 gives 0x7DE0 code, ~= 4.91 V, but still garbage result for other channels.

If configured for mode 0,0 the value returned is not correct at all. For +5 input on channel 0 0x1DE0 code is returned ~= 1.16 V

Can someone help after looking at the LTC1854 datasheet.

Code:

//Program to acquire analog data from 2x LTC1854
#include <18F8680.h>

#FUSES NOWDT                    //No Watch Dog Timer
#FUSES WDT128                   //Watch Dog Timer uses 1:128 Postscale
#FUSES HS                       //High speed Osc (> 4mhz for PCM/PCH) (>10mhz for PCD)
#FUSES STVREN                   //Stack full/underflow will cause reset
#FUSES NOLVP                    //No low voltage prgming, B3(PIC16) or B5(PIC18) used for I/O
#FUSES MCLR                     //Master Clear pin enabled

#use delay(clock=20000000)
#use rs232(baud=9600,parity=N,xmit=PIN_C6,rcv=PIN_C7,bits=8)

#define SPI_MODE_0_0  (SPI_L_TO_H | SPI_XMIT_L_TO_H)
#define SPI_MODE_0_1  (SPI_L_TO_H)
#define SPI_MODE_1_0  (SPI_H_TO_L)
#define SPI_MODE_1_1  (SPI_H_TO_L | SPI_XMIT_L_TO_H)

#define TRISTC  0xf94
#define SSPSTAT 0xfc7
#define SSPCON1 0xfc6
#define SSPCON2 0xfc5

#define ADC1START PIN_E5  //75
#define _ADC1RD   PIN_E6  //74
#define ADC2START PIN_E3  //77
#define _ADC2RD   PIN_E4  //76

//Local Functions
void Initialize_ADCs(void);
void ReadLTC1854_1(int8 channel, unsigned int8* dataoutm, unsigned int8* dataoutl);
void ReadLTC1854_2(int8 channel, unsigned int8* dataoutm, unsigned int8* dataoutl);

unsigned int8 ChannelSelect[8]=
{
//for single ended acquisition
//SGL  ODD  SELECT1   SELECT0  x  x  NAP  Sleep
0b10000000,//80
0b11000000,//c0
0b10010000,//90
0b11010000,//d0
0b10100000,//a0
0b11100000,//e0
0b10110000,//b0
0b11110000 //f0
};

void main()
{
   int8 i;
   unsigned int16  adc2smagnitude;//unsigned magnitude
   unsigned int8 adcvaluem,adcvaluel;//conversion result
   int1 adc2ssign;//sign bit
   float adcresult;//floating point value for user printed as string

   setup_adc_ports(NO_ANALOGS|VSS_VDD);
   setup_adc(ADC_OFF);
   setup_psp(PSP_DISABLED);
   setup_spi(SPI_MASTER|SPI_MODE_0_0|SPI_CLK_DIV_4|SPI_SAMPLE_AT_END);
   setup_wdt(WDT_OFF);
   setup_timer_0(RTCC_INTERNAL);
   setup_timer_1(T1_DISABLED);
   setup_timer_2(T2_DISABLED,0,1);
   setup_comparator(NC_NC_NC_NC);
   setup_vref(FALSE);
   //Setup_Oscillator parameter not selected from Intr Oscillator Config tab

   printf("Testing SPI Port for ADC (LTC1854). Uart setting 9600-8-N-1.\n");     
   Initialize_ADCs();   
   
   printf("TRISC = %X\n", *TRISTC);
   printf("SSPSTAT = %X\n", *SSPSTAT);
   printf("SSPCON1 = %X\n", *SSPCON1);
   printf("SSPCON2 = %X\n", *SSPCON2);   
   
  while(TRUE)
  {
   //Acquiring ADC 1 channels
   printf("\nAcquiring ADC 1 channels.\n");
   for(i=0;i<8;i++)
   {     
     ReadLTC1854_1(i, &adcvaluem, &adcvaluel);//Using third diagram in figure 7 in datasheet
     adc2smagnitude = (adcvaluem & 0x7F)<<8; //sign not needed
     adc2smagnitude += adcvaluel;     
     adc2smagnitude = adc2smagnitude >> 4;//4 unused nibbles for 12 bit device
     adc2ssign = bit_test(adcvaluem,7);
     
     if(adc2ssign==1)
       adcresult = -5.0f; // span is -5 to +5 volts, MSB = 1 means -5v weight here
     else
       adcresult = 0;
     
     adcresult += adc2smagnitude*0.00244140625;// magnitude *  10v / 2^12 (12 bit device)
               
      printf("ADC1-Channel %d, MSB = %X, LSB = %X, Result in Volts = %f\n",i,adcvaluem,adcvaluel,adcresult);
      delay_ms(10);//test to settle adc pins
   }
   
   delay_ms(400);
   printf("\n");   
   
   //Acquiring ADC 2 channels
   printf("\nAcquiring ADC 2 channels.\n"); 
   for(i=0;i<8;i++)
   {     
     ReadLTC1854_2(i, &adcvaluem, &adcvaluel);
     adc2smagnitude = (adcvaluem & 0x7F)<<8; //sign not needed
     adc2smagnitude += adcvaluel;     
     adc2smagnitude = adc2smagnitude >> 4;//4 unused nibbles for 12 bit device
     adc2ssign = bit_test(adcvaluem,7);
     
     if(adc2ssign==1)
       adcresult = -5.0f; // span is -5 to +5 volts, MSB = 1 means -5v weight here
     else
       adcresult = 0;
     
     adcresult += adc2smagnitude*0.00244140625;// magnitude *  10v / 2^12 (12 bit device)
               
      printf("ADC2-Channel %d, MSB = %X, LSB = %X, Result in Volts = %f\n",i,adcvaluem,adcvaluel,adcresult);
      delay_ms(10);//test to settle adc pins
   }
   
   delay_ms(400);
   printf("\n");
   
  }//while

}//main

void Initialize_ADCs(void)
{
output_low(ADC1START);//ADC1 conversion start signal, active high
output_low(ADC2START);//ADC2 conversion start signal, active high
output_high(_ADC1RD);//ADC 1 RD or CS signal, active low
output_high(_ADC2RD);//ADC 2 RD or CS signal, active low
}

//Read ADC 1
//channel = 0-7

void ReadLTC1854_1(int8 channel, int8* dataoutm, int8* dataoutl)
{
printf("Sampling Channel Command %X: ", ChannelSelect[channel]);
output_low(_ADC1RD);//enable chip
spi_write(ChannelSelect[channel]);//read command
delay_us(1);//hold
spi_write(ChannelSelect[channel]);//read command
output_high(_ADC1RD);//disable chip
delay_us(5);//hold

output_high(ADC1START);
delay_us(2);//hold
output_low(ADC1START);
delay_us(2);//wait for conversion

output_low(_ADC1RD);//enable chip
*dataoutm = spi_read(0);
*dataoutl = spi_read(0);
output_high(_ADC1RD);//disable chip
}


//Read ADC 2
//channel = 0-7

void ReadLTC1854_2(int8 channel, int8* dataoutm, int8* dataoutl)
{
printf("Sampling Channel Command %X: ", ChannelSelect[channel]);
output_low(_ADC2RD);//enable chip
spi_write(ChannelSelect[channel]);//read command
delay_us(1);//hold
spi_write(ChannelSelect[channel]);//read command
output_high(_ADC2RD);//disable chip
delay_us(5);//hold

output_high(ADC2START);
delay_us(2);//hold
output_low(ADC2START);
delay_us(2);//wait for conversion

output_low(_ADC2RD);//enable chip
*dataoutm = spi_read(0);
*dataoutl = spi_read(0);
output_high(_ADC2RD);//disable chip
}
PCM programmer



Joined: 06 Sep 2003
Posts: 21708

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PostPosted: Thu Sep 23, 2010 5:20 pm     Reply with quote
Quote:

The ADCs seem to be working fine if configured for SPI mode 3, but for
channel 0 only. It seems the ADC device is unable to configure itself for
acquisition of other channels. Code returned for +5 input on channel 0
gives 0x7DE0 code, ~= 4.91 V, but still garbage result for other channels.

The text description of the SPI communication in the data sheet sounds
like Mode 3 to me, so your experience matches what I read. The data
transfer diagrams don't show mode 3. They don't match the text
description. I don't know what the intention of their tech writers is.

My advice is to cut down your program so it only tests channel 1 (for
example). If possible, get rid of at least 75% of your code. Then I think
it would be simple enough to trouble-shoot.

For an example of cutting it down, get rid of all this:
Quote:

setup_adc_ports(NO_ANALOGS|VSS_VDD);
setup_adc(ADC_OFF);
setup_psp(PSP_DISABLED);
setup_spi(SPI_MASTER|SPI_MODE_0_0|SPI_CLK_DIV_4|SPI_SAMPLE_AT_END);
setup_wdt(WDT_OFF);
setup_timer_0(RTCC_INTERNAL);
setup_timer_1(T1_DISABLED);
setup_timer_2(T2_DISABLED,0,1);
setup_comparator(NC_NC_NC_NC);
setup_vref(FALSE);

Replace it with this. It uses Mode 3 and it also slows the clock way down.
Code:

setup_spi(SPI_MASTER | SPI_MODE_1_1 | SPI_CLK_DIV_16);

There is a lot more code that can be removed (for this test).
Owais



Joined: 23 Sep 2010
Posts: 3

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PostPosted: Mon Sep 27, 2010 3:55 am     Reply with quote
I tried the suggestion and reduced code to: (beginning code is same)

Code:

void main()
{ 
  unsigned int16  adc2smagnitude;//unsigned magnitude
  unsigned int8 adcvaluem,adcvaluel;//conversion result
  int1 adc2ssign;//sign bit
  float adcresult;//floating point value for user printed as string
 
  setup_spi(SPI_MASTER | SPI_MODE_1_1 | SPI_CLK_DIV_64);
 
  output_high(_ADC1RD);//enable chip
 
  while(TRUE)
  {       
   printf("Command word for Channel 1 (single ended) = %X: ", ChannelSelect[1]);
   output_low(_ADC1RD);//enable chip
   spi_write(ChannelSelect[1]);//read command
   delay_us(1);//hold
   spi_write(0);//read command, dont care by LTC this byte
   output_high(_ADC1RD);//disable chip
   delay_us(5);//hold
   
   output_high(ADC1START);
   delay_us(2);//hold
   output_low(ADC1START);
   delay_us(2);//wait for conversion
     
   output_low(_ADC1RD);//enable chip
   adcvaluem = spi_read(0);
   adcvaluel = spi_read(0);
   output_high(_ADC1RD);//disable chip
   
   adc2smagnitude = (adcvaluem & 0x7F)<<8; //sign not needed
   adc2smagnitude += adcvaluel;     
   adc2smagnitude = adc2smagnitude >> 4;//4 unused nibbles for 12 bit device
   adc2ssign = bit_test(adcvaluem,7);
   
   if(adc2ssign==1)
     adcresult = -5.0f; // span is -5 to +5 volts, MSB = 1 means -5V weight here
   else
     adcresult = 0;
   
   adcresult += adc2smagnitude*0.00244140625;// magnitude *  10v / 2^12 (12 bit device)
             
   printf("MSB = %X, LSB = %X, Result in Volts = %f\n",adcvaluem,adcvaluel,adcresult);     
   delay_ms(400);   
  }//while
}//main


But still only channel no 0 is sampled and i cant get the ADC to be configured for sampling other channels. I have used two different LTC1854 devices and both shouldn't be damaged. Also the controller (on a small header PCB) is functional as i can drive some serial memories just fine on another board with it.

any other suggestions anyone ?
Ttelmah



Joined: 11 Mar 2010
Posts: 19552

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PostPosted: Mon Sep 27, 2010 4:36 am     Reply with quote
First, I'd encapsulate the 16bit transfers. So something like:
Code:

union combiner {
   int8 b[2]
   int16 w;
}

int16 talk_to_adc(union combiner command) {
   union combiner result;
   //Trigger last configured conversion
   output_high(ADC1START);
   delay_cycles(1);//Only 40nSec required high
   output_low(ADC1START);
   delay_us(5);//wait for conversion - note tconv, is 5uSec worst case
   
   output_low(_ADC1RD);//enable chip
   result.b[1] = spi_read(command.b[1]);
   result.b[0] = spi_read(command.b[0]);
   output_high(_ADC1RD);//disable chip
   return result.w;
}

Then the single command, reads the last configured conversion, and sets the new configuration.
Then your test loop becomes:

Code:

  int16 value;
  while(TRUE) {       
   printf("Command word for Channel 1 (single ended) = %X: ", ChannelSelect[1]);
   value=talk_to_adc(ChannelSelect[1]*256L); //Send 16bit command, get
   //16bit value from last command.

   //Then for testing, don't fiddle around converting to volts, but print
   //the value returned
   printf("%ld",value);     
   delay_ms(400);   
  }//while
}//main

Then I'd also try channel 2. Given that the 'odd' channels are effectively the -ve inputs in differential mode, it is possible the problem is switching to single ended operation, rather than with retrieving the other channels - worth testing.
I'm using the internal 'cast' abilities of C here, to convert a 16bit value, into a union to access the bytes, then to treat the MSB of this as the sign for output.

Best Wishes
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