ADS131M06: Programming SPI via Python

Hi Christian,

I'm not sure if I have fully understood your question. I guess the "message" you are talking is actually a frame which starts at the falling edge of /CS and ends at the rising edge of /CS. You want to know how much data in a frame which are sent by the ADC, also you want to know where your conversion data start in a frame or which bits are the conversion data, please correct me if my understanding is incorrect.

Can you please show me the timing you have captured including /CS, SCLK, DIN and DOUT? Also, please let me know your word size which is configurable by programming the WLENGTH[1:0] bits in the MODE register, and also if you have enabled input CRC. Thanks.

Best regards,

Dale

Hey Dale,

you are understanding quite right. I'm looking for the construction of a Dataframe, so i can get the analog data out the spi "messages". I'm not sure if i'm testing correctly with my little program. I did not manipulate any of the registers (i only try to read them) and so it's like factory setting. As far as i know it excludes CRC and the word size should be 24 bit long?

Heres a view of the timing diagram i measured with my Oszilloscope:

Furthermore i'm unsure about if i have to do a startup kinda thing. I read about reading the registers of the analog data 2 times to clear them after a measurement pause or a startup. Because i'm programming in python with the library "SPIdev" i thought that reading the registers 2 time means, i read two messages by sending NULL and then send a rreg command to read than again. My program for this looks like this:

#imports
import time
import spidev
import RPi.GPIO as GPIO

# clk is a mini class to generate a Master-Clock signal over a PWM Pin
import clk as clk

#includes all Commands for the ADS131M06
#and also a function which gets two hex numbers and gives back the "Message" for send
from ADS131_Command import *

adcC = ADS131_Command()

#settings
GPIO.setmode(GPIO.BCM)
GPIO.setup(17, GPIO.IN, pull_up_down=GPIO.PUD_UP)
GPIO.add_event_detect(17,GPIO.FALLING)

spi = spidev.SpiDev()

spi.open(0,0) 
spi.mode = 0b01
spi.max_speed_hz = 2048000
spi.lsbfirst = False

def send(msg):
    while True:
        if GPIO.event_detected(17):
            #print("send some Data")
            spi.writebytes(msg)
            break
    
def receive(n):
    while True:
        if GPIO.event_detected(17):
            #print("receive some Data")
            respbin= 0
            resp = spi.readbytes(n)
            
            for i in resp:
                respbin = respbin<<8
                respbin = respbin+i
                    
            print(bin(respbin))
            break
        
if __name__ == '__main__':
    clk.clock_en(True)
    send([0b1010, 0b0001])
    receive(24)
    spi.close()
    clk.clock_en(False)

I got a hint about programming the "first-time-use"-Frame, but i did not try to get it run. At the moment the complexity is still overwhelming.

Thanks for your help in advance.

Hi Christian,

Thank you for providing more information.

Yes, the word size is 24-bit and the CRC on the ADC input is disabled by default, however the output CRC always appears at the end of the output frame and it can not be disabled. Your microcontroller can ignore the data if the output CRC is not used. In a frame, you should be able to see a status register data at the beginning if you did not read or write register in the previous frame, followed by 4-ch conversion data plus a word for CRC, so you need total 192 SCLKs (24+24+24+24+24+24+24+24) in a frame. Right now, you are only sending 4 bytes=32 SCLKs in a frame. 

Lastly, I guess you were talking about "collecting data for the first time or after a pause in data collection". You can use one of two methods which are introduced in the section 8.5.1.9.1 in the datasheet.to clear FIFO and realign the ADC output with your host controller. You can use /SYNC/RSTn pin if your controller is controlling this pin or you can read data twice instead of using /SYNC/RSTn pin.

Best regards,

Dale