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Owais
Joined: 23 Sep 2010 Posts: 3
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Stuck with LTC1854 ADC Driver |
Posted: Thu Sep 23, 2010 4:16 am |
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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
}
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PCM programmer
Joined: 06 Sep 2003 Posts: 21708
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Posted: Thu Sep 23, 2010 5:20 pm |
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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);
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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);
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There is a lot more code that can be removed (for this test). |
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Owais
Joined: 23 Sep 2010 Posts: 3
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Posted: Mon Sep 27, 2010 3:55 am |
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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
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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 ? |
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Ttelmah
Joined: 11 Mar 2010 Posts: 19383
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Posted: Mon Sep 27, 2010 4:36 am |
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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;
}
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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
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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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