Hello world with ADS1298

Let's make first simple communication with ADS1298 (delta-sigma ADC). To confirm communication with this ADC, we will ask for its ID.

  Hardware

To talk to ADS1298 we will use SPI and bcm2835 library. SPI is an intuitive solution here. ADS1298 is capable of 20MHz SPI communication, anyway due to Raspberry Pi problems with SPI speeds higher than 16MHz we will stay on 8MHz (7.8125MHz exactly).

To provide this basic communication connectivity we need an SPI (hardware) connections with separately defined /CS signal. This is due to inconsistency in providing spacing between end of CLK and rising end of /CS.  We will use RESET signal as well.

Be aware that to establish communication with ADS1298, there is a need of both, simultaneously: CLK (in range freqency  from 1.945 525MHz to 2.415,459MHz) and SCLK (SPI clock: max 20MHz when powered 2.7 V ≤ DVDD ≤ 3.6 V and 15MHz when 1.65 V ≤ DVDD ≤ 2 V). This may be counter intuitive, but from experience: it is a must to achieve communication.

Software/Firmware

The work is done on RaspberryPi. Lets open Geany and place code from below there. Save the file as 'ads.c'. Compile.
Compiling in Geany may need to tweak compile line. Fortunately it is possible to setup once and have it always. Go to 'Build' menu, and select 'Set Build Commands' and in table next to Build option add -lbcm2835 (check how to in other article http://michals-diy-electronics.blogspot.com/2018/04/rule-pins-of-raspberry-pi-3.html )

/*
 * ads.c
 *
 * Code to perform communication with ADS1298
 * this time, /CS pin control done "by hand" to rule the delay between end of SCLK and
 * /CS rise (end of frame)
 *
 */

#define RESETPIN RPI_BPLUS_GPIO_J8_16 //RESET pin
#define CS_PIN  RPI_BPLUS_GPIO_J8_24 //SPI CS pin
#define CS_DELAY 3  //how many microseconds of delay

#include <stdio.h>
#include <bcm2835.h>

enum spi_command {
  //system commands
  WAKEUP = 0x02,
  STANDBY = 0x04,
  RESET = 0x06,
  START = 0x08,
  STOP = 0x0a,
  //read commands
  RDATAC = 0x10,
  SDATAC = 0x11,
  RDATA = 0x12,
  //register commands
  RREG = 0x20,
  WREG = 0x40
};



enum reg {
  // device settings
  ID = 0x00,

  // global settings
  CONFIG1 = 0x01,
  CONFIG2 = 0x02,
  CONFIG3 = 0x03,
  LOFF = 0x04,

  // channel specific settings
  CHnSET = 0x04,
  CH1SET = CHnSET + 1,
  CH2SET = CHnSET + 2,
  CH3SET = CHnSET + 3,
  CH4SET = CHnSET + 4,
  CH5SET = CHnSET + 5,
  CH6SET = CHnSET + 6,
  CH7SET = CHnSET + 7,
  CH8SET = CHnSET + 8,
  RLD_SENSP = 0x0d,
  RLD_SENSN = 0x0e,
  LOFF_SENSP = 0x0f,
  LOFF_SENSN = 0x10,
  LOFF_FLIP = 0x11,

  // lead off status
  LOFF_STATP = 0x12,
  LOFF_STATN = 0x13,

  // other
  GPIO = 0x14,
  PACE = 0x15,
  RESP = 0x16,
  CONFIG4 = 0x17,
  WCT1 = 0x18,
  WCT2 = 0x19
};

void cs_select(void){  //CS low (remember about negative logic here)
 bcm2835_gpio_clr(CS_PIN);
}


void cs_deselect(void){  //CS high (negative logic here)
 bcm2835_gpio_set(CS_PIN);
} 




int main(int argc, char **argv){

 int idVal = 0;
 int maxChan = 0;
 char buf[3] = {0,0,0};


 if(!bcm2835_init()){
  printf("Oops, bcm2835 not initialised correctly");
  return 1;
 }

 if(!bcm2835_spi_begin()){ //_spi_begin returns 1 if spi_begin does its job with success
  printf("SPI not initialised correctly (Maybe you are not running as root?)");
  return 1;
 }

 bcm2835_gpio_fsel(RESETPIN, BCM2835_GPIO_FSEL_OUTP); //RESETPIN as output
 bcm2835_gpio_write(RESETPIN, HIGH); //reset high

 bcm2835_spi_setBitOrder(BCM2835_SPI_BIT_ORDER_MSBFIRST); //bit order (non-active in bcm2835 ver1.55 - always MSBFIRST)

 //Set CS pin polarity to low (LOW when CS active)
 //due to incosistency in time between end of SCLK and raising edge of /CS
 //this /CS pin will be handled manually now
 //see https://groups.google.com/forum/#!topic/bcm2835/wsMqsNqiquo (google group of bcm2835, thread titled: SPI co-operation with ADS1298. Incosistent reading.)
 //bcm2835_spi_setChipSelectPolarity(BCM2835_SPI_CS0, 0); //original line

 //Set which CS pin to use for next transfers
 //bcm2835_spi_chipSelect(BCM2835_SPI_CS0); //commented out because of: see previous lines
 bcm2835_spi_chipSelect(BCM2835_SPI_CS_NONE); //do NOT set any /CS automatically

 bcm2835_gpio_fsel(CS_PIN, BCM2835_GPIO_FSEL_OUTP); //CS is an output
 cs_select();

 //Set SPI clock speed
 // BCM2835_SPI_CLOCK_DIVIDER_128   = 128,     ///< 128 = 512ns = = 1.953125MHz
 // BCM2835_SPI_CLOCK_DIVIDER_64    = 64,      ///< 64 = 256ns = 3.90625MHz
 // BCM2835_SPI_CLOCK_DIVIDER_32    = 32,      ///< 32 = 128ns = 7.8125MHz
 // BCM2835_SPI_CLOCK_DIVIDER_16    = 16,      ///< 16 = 64ns = 15.625MHz
 bcm2835_spi_setClockDivider(BCM2835_SPI_CLOCK_DIVIDER_64); //< 64 = 256ns = 3.90625MHz  8 SCLK = 2.048us

 //Set SPI data mode
 // BCM2835_SPI_MODE1 = 1,  // CPOL = 0, CPHA = 1, Clock idle low, data is clocked in on falling edge, output data (change) on rising edge
 bcm2835_spi_setDataMode(BCM2835_SPI_MODE1); //(SPI_MODE_# | SPI_CS_HIGH)=Chip Select active high, (SPI_MODE_# | SPI_NO_CS)=1 device per bus no Chip Select

 //reset the device
 bcm2835_gpio_write(RESETPIN, LOW);
 delay(1);
 bcm2835_gpio_write(RESETPIN, HIGH);
 delay(150);//wait x ms (0.128s required)   

 buf[0]=(RREG | ID);
 buf[1]=0x00;
 buf[2]=0x00;

 //determine model number and number of channels available
 //register read command: send two bytes to ADS1298: 001r rrrr and 000n nnnn,
 //where r rrrr - starting register, n nnnn - (number of register to read - 1) ALL IN ONE BURST (no CS change)

 printf("Before spi_transfern:\n");
 printf("Hex value of buf[0] = %x \n", buf[0]);
 printf("Hex value of buf[1] = %x \n", buf[1]);
 printf("Hex value of buf[2] = %x \n", buf[2]); 

 cs_select();
 bcm2835_spi_transfer(SDATAC);
 bcm2835_delayMicroseconds(CS_DELAY);
 cs_deselect();
 bcm2835_delayMicroseconds(5); //minimum of 2 SCLK (0.5us for 3.9MHz)
 cs_select();
 bcm2835_spi_transfern(buf, 3);
 bcm2835_delayMicroseconds(CS_DELAY);
 cs_deselect();

 printf("After spi_transfern:\n");
 printf("Hex value of buf[0] = %x \n", buf[0]);
 printf("Hex value of buf[1] = %x \n", buf[1]);
 printf("Hex value of buf[2] = %x \n", buf[2]);

 switch (buf[2] & 0b00011111){ //least significant bits say about channels
  case 0b10000: //16
   maxChan = 4; //asd1294
   break;
  case 0b10001: //17
   maxChan = 6; //ads1296
   break;
  case 0b10010: //18
   maxChan = 8; //ads1298
   break;
  case 0b11110: //30
   maxChan = 8; //ads1299
   break;
  default:
   maxChan = 0;
  }  

 idVal=buf[2];
 printf("idVal: %x \n", idVal); //hex92 (or dec146) expected from ads1298
 printf("Device Type (ID Control Register): ");
 printf("%d\n", idVal);
 printf("Channels: ");
 printf("%d\n\n", maxChan);

 delay(20);

 cs_select();
 bcm2835_spi_transfer(SDATAC);
 bcm2835_delayMicroseconds(CS_DELAY);
 cs_deselect();
 //Return SPI pins to default inputs state
 bcm2835_spi_end(); 
 return 0;
}

Let's run it by sudo ./ads This shows the window shown below. Seeing this proves we established basic communication with ADS1298. In next arrticle I will try to show how to fetch some data from this ADC.