SPI setup bugs

[jrosenstein]

This was a helpful starting point, but it only works after fixing a couple of important SPI setup problems.

1. Instead of wiringPiSPISetup, it needs to use wiringPiSPISetupMode, with SPI_MODE=1
2. It suffers from this WiringPi-Python bug, and needs the same workaround of creating a new string inside the call to wiringPiSPIDataRW: https://github.com/WiringPi/WiringPi-Python/issues/25

[GrahamOB]

This code could be a gem. Anyone fancy getting it working? If I had the expertise I would pick it up immediately.

[jrosenstein]

I don't have write access to this, but I forked and uploaded my version of pyads1256.py here:
https://github.com/jrosenstein/PyADS1256

[ul-gh]

I have just now tried https://github.com/jrosenstein/PyADS1256 on Raspbian Jessie (Linux 4.4.21-v7+) with the "Waveshare Precision" ADC hardware.

I can confirm, this works.

jrosenstein: Thank you very much!

Now, the abovementioned hardware is of course no good for real 24-bit precision, but should my current project, a simple water flow calorimeter (Wärmebilanzkalorimeter) ever evolve into a precision instrument, (50% likelyhodd, my guess), I will see if I can make public a proper analog frontend design.

[ul-gh]

Test example attached.

ADS1256_example.py.txt

[GrahamOB]

Thank you very much for getting this working. I do have one small question regarding the conversion of the raw output to volts. Why is the result divided by 167? Thanks again for your help.

[ul-gh]

Hi Graham,

the full scale reading of course depends on the hardware as well as the setting of the ADS1256 internal programmable gain amplifier. If you run above example without any changes to the default values, the setting is PGA=1. If Vref is connected to 2.5V as with the "Waveshare" hardware, and if you look on page three of the ADS1256 datasheet, the full-scale input voltage is then 2*Vref/PGA = 5V. I.e. 5V input corresponds to 23-Bit full-scale (neglecting any offset and gain errors, and correction: output is in 24-Bit two's complement format, see table 16 on page 23 of the ADS1256 datasheet). From this you can check your output values.

I don't have the harware at hand, but I can check if this is correct with my board tomorrow.

[ul-gh]

Hi guys, I forked the original project just now. This is a complete rework - I worked through the Ti ADS1255/ ADS1256 datasheet from beginning through end. HTH.

[ul-gh]

https://github.com/ul-gh/PyADS1256

[renderit]

@ul-gh Thanks for this but when I try the attached test above with the code in your repo, I get error:

Traceback (most recent call last):
  File "test.py", line 7, in <module>
    idNumber = ads.ReadID()
  File "/home/pi/Downloads/High-Precision-AD-DA-Board-Code/Raspberry/PyADS1256/pyads1256.py", line 518, in ReadID
    myid = self.ReadReg(self.REG_STATUS, 1)
TypeError: ReadReg() takes exactly 2 arguments (3 given)

I am using, this High precision AD/DA Board with Raspberry Pi 3 which has a ADS1256 on it..
Also, is @jrosenstein repo code exactly same as @ul-gh code?

[ul-gh]

Hi,

thanks for your message! I fixed above error just now. and commited to:
https://github.com/ul-gh/PyADS1256/.

Will upload some example files tomorrow. Best wishes.

[renderit]

@ul-gh Example files would be awesome! Will look forward to that!! Quick question, when i run test.py I am seeing output like:

ADS1256 reported ID value: 3

Press CTRL-C to interrupt..
AIN_0 value: 3009986
AIN_1 value: 1334144

Is it possible to convert those readings into voltage? like in the example C code on the website? I want to see output like This C example:

ID=
Ok, ASD1256 Chip ID = 0x3
0=131349,  1250121 ( 0.748 575 V) 
1=132D81,  1256833 ( 0.752 594 V) 
2=1D62D4,  1925844 ( 1.153 200 V) 
3=1D64DB,  1926363 ( 1.153 510 V) 
4=1D648C,  1926284 ( 1.153 463 V) 
5=1D6562,  1926498 ( 1.153 591 V) 
6=1D6534,  1926452 ( 1.153 564 V) 
7=1D637D,  1926013 ( 1.153 301 V) 

Honestly, I'm sure what those reading in the brackets imply. Would anybody know what exactly they mean? Is it "575 volts"?

[ul-gh]

Hi Jim, just to let you know: I said "tomorrow", but I decided to do a complete rewrite of the class definition for a true object-based interface, using Python properties to access the ADC settings and results. Since that changes the whole API, I do not want to encourage you to use the current state code. Sorry, this will take a few days. I am already using the ADC using the code I uploaded, and yes, of course you can read all eight input pins (nine even, if you count the AINCOM). Give me until the end of next week - I promise you a working example with the new extended code!

[ul-gh]

BTW, yes, the output of the manufacturer example code in the parentheses of course means "volts", which is more exactly the voltage difference between the respective AIN pin number and the AINCOM pin.

However, the manufacturer code has some serious bugs. You would have to read the datasheet thoroughly and fix those bugs if you wanted to change any of the settings. Again, I promise you an example next week.

[renderit]

@ul-gh Excellent! Looking forward to that!!