Raspberry Pi Interface with ADS1256

I am attempting to integrate an ADS1256 (link to TI: ADS1256 ) with my Raspberry Pi. I purchased this board to do the hardware interfacing and would like to use MATLAB to do the rest. I am executing the following code:

mypi = raspi;
enableSPI(mypi);
myADS1256 = spidev(mypi,'CE0',1,2000000);

to read/write to some registers, however I am having a difficult time read/writing to registers successfully every time. I believe is because I am not 100% certain of the SPI Bus speed (fCLKIN = 7.68MHz is the clock of the ADS1256). I have tried multiple bus speed with some not working at all and others working only half the time. Any help would be appreciated.

The following command I am using to read the status:

status = writeRead(myADS1256,[bin2dec('00010000'), bin2dec('00000000')]); %read status

The following command I am using to read the data rate:

dataRate = writeRead(myADS1256,[bin2dec('00010011'),bin2dec('00000000')]); % read data rate

The following command I am using to write a new data rate:

setDataRate = writeRead(myADS1256,[bin2dec('01010011'), bin2dec('00000000'), bin2dec('00100011')]); % setting data rate to 10SPS ('00100011')

and I re-read the data rate to confirm the register was written to.

[Francis B.]

After looking at the specification sheet for the ADS1256, the bus speed needs to be set to 500000 or 1000000 Hz. This based on being fclkin = 7.68 MHz and SCLK must be less than fclkin/4. It is still unknown why I am only receiving intermittent responses to requesting status and data rate.

Answer 1

[Francis B.]

I am successful if I send a wake command after every writeRead... A wake command looks like:

 status = writeRead(myADS1256,0);

So my startup code looks like:

status = writeRead(myADS1256,[bin2dec('00010000'), 0]); %read status
status = writeRead(myADS1256,0);
dataRate = writeRead(myADS1256,[bin2dec('00010011'), 0]); % read data rate
dataRate = writeRead(myADS1256,0); % original data rate should equal 240 or '11110000'
if dataRate == 240
  fprintf('ADS1256 Data Rate: 30000 SPS\n');
end
setStatus = writeRead(myADS1256,[bin2dec('01010000'), 0,  bin2dec('00110100')]); % setting auto calibration on
pause(0.4); % Self calibration takes approximately 300 ms for 10 SPS
setStatus = writeRead(myADS1256,0);
statusNew = writeRead(myADS1256,[bin2dec('00010000'),0]); %read status
statusNew = writeRead(myADS1256,0);
if statusNew == 52
  fprintf('ADS1256 Auto Calibrated\n');
end
setDataRate = writeRead(myADS1256,[bin2dec('01010011'), 0, bin2dec('00100011')]); % setting data rate to 10SPS ('00100011')
setDataRate = writeRead(myADS1256,0);
dataRateNew = writeRead(myADS1256,[bin2dec('00010011'),0]); % read data rate
dataRateNew = writeRead(myADS1256,0);
if dataRateNew == 35
  fprintf('ADS1256 Data Rate: 10 SPS\n');
end
setADCON = writeRead(myADS1256,[bin2dec('01010010'), 0,  bin2dec('00000000')]); % setting ADCON register,
setADCON = writeRead(myADS1256,0);
ADCONNew = writeRead(myADS1256,[bin2dec('00010010'),0]); %read ADCON register
ADCONNew = writeRead(myADS1256,0);
if ADCONNew == 0
  fprintf('ADS1256 A/D Control Register Set\n');
end
setStatus = writeRead(myADS1256,[bin2dec('01010000'), 0,  bin2dec('00110000')]); % setting auto calibration off
pause(0.4); % Self calibration takes approximately 300 ms for 10 SPS
setStatus = writeRead(myADS1256,0);
statusNew = writeRead(myADS1256,[bin2dec('00010000'),0]); %read status
statusNew = writeRead(myADS1256,0);
if statusNew == 48
  fprintf('ADS1256 Auto Calibration Off\n');
end

To actually read data in its a bit more tricky:

x = 0;
while x == 0
  volts = zeros(8,1);
  for i = 1:8
      nibble_0 = '1000'; % sets negative to be common ground
      switch i
          case 1
              nibble_1 = '0000'; % ain #0
          case 2
              nibble_1 = '0001'; % ain #1
          case 3
              nibble_1 = '0010'; % ain #2
          case 4
              nibble_1 = '0011'; % ain #3
          case 5 
              nibble_1 = '0100'; % ain #4
          case 6
              nibble_1 = '0101'; % ain #5
          case 7
              nibble_1 = '0110'; % ain #6
          case 8
              nibble_1 = '0111'; % ain #7
      end
      muxSelect = [nibble_1,nibble_0];
      setMux = writeRead(myADS1256,[bin2dec('01010001'), 0,  bin2dec(muxSelect)]); % selecting appropriate mux
      sync = writeRead(myADS1256,bin2dec('11111100')); % syncing a/d conversion
      pause(1/(7.68*1000000*4)); % pause
      wakeup = writeRead(myADS1256,0); % wake command
      readDataOnceCommand = writeRead(myADS1256,bin2dec('00000001')); %read data once
      readDataOnce(1:3) = writeRead(myADS1256,[0,0,0]);
      binReadDataOnce_1 = dec2bin(readDataOnce(1));
      while length(binReadDataOnce_1) < 8
          binReadDataOnce_1 = ['0', binReadDataOnce_1]; % padding with zeros
      end
      binReadDataOnce_2 = dec2bin(readDataOnce(2));
      while length(binReadDataOnce_2) < 8
          binReadDataOnce_2 = ['0', binReadDataOnce_2]; % padding with zeros
      end
      binReadDataOnce_3 = dec2bin(readDataOnce(3));
      while length(binReadDataOnce_3) < 8
          binReadDataOnce_3 = ['0', binReadDataOnce_3]; % padding with zeros
      end
      binaryValue = [binReadDataOnce_1,binReadDataOnce_2,binReadDataOnce_3];
      if strcmp(binaryValue(1),'1') % negative value lookup
          negativeCount = bin2dec(binaryValue);
          counts = negativeCount-2^24;
      else
          counts = bin2dec(binaryValue);
      end
      volts(i) = counts*2*2.5/(1*(2^23 - 1)); % LSB == 2*VREF/(PGA*(2^23 − 1)).
      DRDYValue = readDigitalPin(mypi,17);
      while DRDYValue % wait until DRDY is false
          DRDYValue = readDigitalPin(mypi,17);
      end
  end
  ain = [volts(2:8); volts(1)] % prints to workspace in the correct order (since this reads previous readWrite command
end

[Walter Roberson]

By the way, your sections such as

      while length(binReadDataOnce_2) < 8
          binReadDataOnce_2 = ['0', binReadDataOnce_2]; % padding with zeros
      end

can be avoided by changing

        binReadDataOnce_1 = dec2bin(readDataOnce(1));

to

        binReadDataOnce_1 = dec2bin(readDataOnce(1), 8);

[Francis B.]

Thanks Walter!