Connect RasberryPi with TI ADS1298 through SPI with Python3

Question

I am trying to connect to TI chip ADS1298 (datasheet) through SPI communication using a Rasbperry Pi 3b+ and Python3.

The configuration must be as written. Also I am using a reverse bits function because, as far as I know, the order RPi transfers bits and ADS1298 receives them is the contrary. I am only interested in CHANNEL2.

I am using the ADS1298ECG FE-PDK, which includes the official software, and using it the ECG is correctly acquired so board and connections must be OK.

I've created subrutines to communicate, and when it's necessary they are executed. However, I couldn't even read ID register, adressed at 0x00 and 1 byte long. The lines for this should be:

STAR_COMM() #defined by me
GPIO.output(CS_gpio, False)
ADS.writebytes([RB(0x20)]) #2: read register; 0: adress
ADS.writebytes([RB(0x00)]) #n registres -> n-1 (1 register ->0)
ID=ADS.readbytes(1)
GPIO.output(CS_gpio,True)

And I should get 0x92(hexadecimal) (or 146 decimal), but it is not the case, and the number read changes. I assume that my code is wrong, but I can't find the error.

My code for communication is as follows:

import spidev
import RPi.GPIO as GPIO
import time
cc = True

def START_COMM():
    """
    Necessary steps to establish communication with ADS1298 through SPI
    """
    global ADSReset_gpio, CS_gpio, DRDY_gpio, ADS, cc
    GPIO.setmode(GPIO.BCM)
    ADSReset_gpio = 6
    CS_gpio = 8
    DRDY_gpio = 12
    GPIO.setup(ADSReset_gpio, GPIO.OUT)
    GPIO.output(ADSReset_gpio, True)
    GPIO.setup(CS_gpio, GPIO.OUT)
    GPIO.output(CS_gpio, True)
    GPIO.setup(DRDY_gpio, GPIO.IN, pull_up_down = GPIO.PUD_UP)
    
    
    ADS = spidev.SpiDev()
    ADS.open(0,0)
    ADS.mode = 0b01
    ADS.max_speed_hz = 2000000 #2MHz, for SPI comm
    
    if cc:
        GPIO.output(ADSReset_gpio, False)
        time.sleep(0.2)
        GPIO.output(ADSReset_gpio, True)
        time.sleep(1)
        cc = False
        init_conf()
    return ADS     

def init_conf():
    """
    Set up of ADS1298 chip.
    """
    GPIO.output(CS_gpio, False)
    #WAKE UP
    ADS.writebytes([RB(0x02)])
    #STOP READ DATA CONTINUOUS MODE
    ADS.writebytes([RB(0x11)])
    time.sleep(0.2)
    
    #WRITE REGISTERS STARTITNG AT 1
    ADS.writebytes([RB(0x41)])
    ADS.writebytes([RB(0x02)]) #WRITE 3 REGISTERS
    ADS.writebytes([RB(0x86)]) #CONFIG 1: 86h = 10000110 (1: HR mode, 0: DAISY-CHAIN mode,
#                                0: OSCILLATOR CLOCK OUTPUT disabled, 00, 110: fMOD/1024 (500 SPS IN HR MODE)
    ADS.writebytes([RB(0x00)]) #CONFIG 2: 00h = 00000000 (0, 0: CHOPPING FREQ VARIES, 0: TEST SIGANLS DIVEN EXTERNALLY.
#                                0, 0:CALIBRATION SIGNAL AMPLITUDE 1X -(VREFp-VREFn)/2400
#                                00: CALIBRATION SIGNAL FREQ pulsed at fCLK/2^21)
    ADS.writebytes([RB(0xC0)]) #CONFIG 3: C0h = 11000000 (1: ENABLE INTERNAL REFERENCE BUFFER, 1, 0: VREF = 2.4V (1: VREF = 5V),
#                                0: RLD MEASUREMENT OPEN, 0: RLDREF SIGNAL FED EXTERNALLY, 0: RLD BUFFER POWERED DOWN,
#                                0: RLD SENSE DISBALED, 0: RLD POWERED DOWN)
    
    #WRITE EGISTERS AT 5
    ADS.writebytes([RB(0x45)])
    ADS.writebytes([RB(0x07)]) #WRITE 8 REGISTERS
    
    for i in range(8):
        #HERE THE CHANNELS ARE SET
        if i==1:# or i==2: ##python starts at 0
            ADS.writebytes([RB(0x00)]) # 60h = 01100000 (0: NORMAL OPERATION, 110: GAIN 12, 0,
#                                                000: NORMAL ELECTRODE INPUT)
        else:
            ADS.writebytes([RB(0x81)]) # 81h = 10000001 (1: POWERED DOWN CHANNEL, 000: GAIN BY DEFAULT(6),
#                                                0, 001: INPUT SHORTED)
    
    #START/ RESTART
    ADS.writebytes([RB(0x08)])
    #ENABLE READ DATA CONTINUOUS MODE
    ADS.writebytes([RB(0x10)])
    GPIO.output(CS_gpio, True)
    
def RB(byte):
    """
    This function reverse the order of the bits in a byte. i.e. 00000001 -> 10000000
    
    Parameters
    ----------
    byte: byte whose bits to be reversed
    """
    byte = ((byte & 0xF0) >> 4) | ((byte & 0x0F) << 4)
    byte = ((byte & 0xCC) >> 2) | ((byte & 0x33) << 2)
    byte = ((byte & 0xAA) >> 1) | ((byte & 0x55) << 1)
    return byte

def read_spi():
    """
    Receive data from ADS1298 and select those bytes corresponding to required channels (2 & 3)
    """
    try:
        if GPIO.input(DRDY_gpio) == True:
            GPIO.output(CS_gpio, False)
            raw =  ADS.readbytes(27)
            ch2 = int.from_bytes(raw[6:8], byteorder = 'big')
            GPIO.output(CS_gpio, True)
            return ch2
    except:
        pass

def END_COMM(ADS):
    """
    Close communication
    
    Parameters
    ----------
    ADS: spi object.
    """
    ADS.close()
    GPIO.cleanup()

Answer 1

I had the same issue, the registers such as ID, CONFIG3 and any others were changing in time. Recently I found a solution. In my specific case, the CS pin of ADS1298 was connected to GPIO.BCM=8 (CE0) and the line of code to open spi was :

spi.open(bus,cs) \
spi.open(0,0)

After that, the registers that I tried to read changed. Then I just changed the connection of CS pin to GPIO.BCM(7) and now I can read all the registers with no changes in the code at all.