SPI Register readout fails (ADS 1256)

Olivier Gallant

Good afternoon,

I've been working for my thesis on a project to do measurements on PV systems. However I am pretty inexperienced in SPI communication and programming in general (it's really not our department). I've been struggeling alot to get a simple readout of for example the status register of the ADC (to verify if the SPI communication and PCB are working properly).

I'm trying to do the readout trough the supported python library and trough the spi-gpio-driver-master.

"
import onionSpi
import time

spi = onionSpi.OnionSpi(1, 32766)
spi.sck = 7
spi.mosi = 8
spi.miso = 9
spi.cs = 6
spi.speed = 100000
spi.mode = 0b01
spi.lsbfirst = False
spi.checkDevice()
spi.registerDevice()
spi.setupDevice()
spi.setVerbosity(1)

ret = spi.write([0b00010000,0b00000001]) #read command to readout register 0x00
data = spi.readBytes(0x00, 1)
print (data)
"

As I expect the omega transfers the command bytes to the ADC trough the MOSI line.
Afterwards I would expect to receive the status register from the ADC altough this does not seem to happen.

What am I missing out?

Attached files:

Datasheets ADS1256: http://www.ti.com/lit/gpn/ads1256

Greetings,

Olivier

Maximilian Gerhardt

Found a mistake by reading the datasheet: The "read register command" is of the form [page 34]

[0001 <reg. number> ]  [0000 nnnn]
       first byte      second byte

Where as n is the number of registers you want to read minus one. So if you want to read one register, n = 0.

Also have a look at the read/write register functions from an Arduino ADS1256: https://github.com/adienakhmad/ADS1256/blob/master/ADS1256.cpp#L36

According to that, maybe do

import time

RREG = 0x10 #constant
ret = spi.write([RREG | 0x00 , 0x00]) #read command to readout register 0x00
time.sleep(0.001) #sleep for 50*tCLKIN, since I don't know the master clock freq for your ADS board; 1ms should be enough..
#now read register
data = spi.readBytes(0x00, 1)
# arduino codes sleeps after reading registers, too.
print("STATUS REG: " + hex(data[0]))

If that doesn't help you can attach a logic analyzer to see what the ADC receives/sends and can you compare that with e.g. what signals an arduino with a ads1256 lib produces from the link above.

Olivier Gallant

@Maximilian-Gerhardt
Thanks alot for the fast response. Will try this out asap. I'll keep it posted! Cheers