Some assistance needed with u3 and SPI

[michK]

Hi,

I am trying to read temperature with the MAX6675 with a U3 LJ and SPI.

I followed this example and seem to have the PSI communication set up correctly because I can see the expected chip select, clock, and return signal on an oscilloscope, however, I am not sure how to set up the d.spi() command to read back only the required bits.

Here is my script:

import os
import time
import u3

d = u3.U3() #Open first found U3 over USB
d.configIO(FIOAnalog = 0x0F) #For this example FIO4-7 need to be digital IO

# Press ctrl-C to exit the program
try:
    while(True):
        results = d.spi([0x4<<2, 0])
        print(results)
        time.sleep(0.1)
except KeyboardInterrupt:         #this allows the program to quit when you press "ctrl C"
    print("ctrl C received, shutting down program")
    d.close()                      #close the device   

Here is a screenshot of the scope output: https://u.pcloud.link/publink/show?code=XZEsxIVZhKq97lWxxtBJFH1mVRERBLQWpWIV

Manually digitizing bits D14–D3 gives the binary sequence 000001101000, which in decimal is 104, and interpolating this as a 12 bit number between temperatures of 0°C and 1023.75°C as per the datasheet gives a reasonable 26°C result for the conditions I took the data.

Is there a way to set up the d.spi() command to return the binary sequence as mentioned above?

[sjarman28]

The spi function returns a dictionary as follows:

LabJackPython/src/u3.py

Line 1326 in f81a3c4

return {'NumSPIBytesTransferred': result[7], 'SPIBytes': result[8:]}

If you do not care to look at the number of bytes transferred, you can just get a list of the bytes returned by accessing the SPIBytes entry:

resultBytes = d.spi([0x4<<2, 0])['SPIBytes']

or

results = d.spi([0x4<<2, 0])
resultBytes = results['SPIBytes']

If you are trying to translate the bytes to an integer value, you could use int.from_bytes:

resultInt = int.from_bytes(resultBytes, byteorder='big')

If you have additional support questions, please contact LabJack support at support@labjack.com.