Spidev problems in xfer2()

Can you check the return codes? Maybe the spi bus is not opening and then your xfer2 hangs.

Clem

Clem,

I am a newbe so forgive a basic question:  How do I check the return codes?

I tried this:

!

    import sys

    import spidev

    print('Starting test code')

    print('initializing the SPI bus instance')

    spi_bus = spidev.Spider()

    print('opening the spi bus')

    exit_code = spi_bus.open(0, 0)

    print ("exit code from open is", exit_code )

    print('getting the adc output ')

    adc_output = spi_bus.xfer2(1, 240, 0 )

    print('adc read complete')

!

When I run it I get:

    pi@raspberrypi:~/Software $ sudo python3 spi_testcode.py

    Starting test code

    initializing the SPI bus instance

    opening the spi bus

    exit code from open is None

    getting the adc output

    pi@raspberrypi:~/Software $

The exit code of the open() was 'None'.  In python I believe that means there was no exit code.   Is there another way to check?

import spidev
import time
spi = spidev.SpiDev()
spi.open(0,0)
while True:
   resp = spi.xfer2([0x00])
   print resp[0]
   time.sleep(1)

I googled this from https://www.raspberrypi.org/forums/viewtopic.php?f=44&t=56669

See how after the open, A truth statement will not execute unles Sspi.open works? I have not tried this, but it expresses the intent. Reading furtherer they say a permission problem. You may not be running this with root permission. On the raspberry Pi you cannot as a plain user affect the hardware. Linux tries to allow only root or users with root permissions(sudo) to do hardware things.

Clem

Clem,

  I don't see how the 'while True' could be affected by the previous open() statement.  'True' is a python keyword that is always 'true'

However,  you still helped tremendously!!!  I tried the code in your reply and things started running more or less as you would expect... I got a print statement every second.   That tells me it was getting through the xfer2() code.    Hmmmmmm..... a clue!!!

I looked closely at the code you suggested and noticed it was only passing one hex byte in brackets to xfer2():  xfer2([0x00])

My code was passing 3 decimal bytes without brackets:

xfer2(1,240,0)

After playing with different combinations I discovered the brackets made the difference.  The following seems to work:

xfer2([1,240,0])

So.... xfer2() is looking for a list and I was passing 3 separate variables.

Now the embarrassing thing:   The code examples on the web that I was copying had the brackets!!   I visually compared my code to the examples several time but never noticed that difference..... :-(.

I caused the problem but it is too bad the spidev code did not flag the error better

Problem solved.... now I am able to move forward with the build.

Thanks!!

Glad to help you realize the situation. Please help me get points by marking this answer as correct

Clem

PS here is why:

Anything in a bracket in Python is a list of items. Without the list means more parameters which the wrapper cannot expect. The bracketed list is just one parameter.