Strange behavior from SPI signals, loopback gets garbage

Issue Overview

Users of the Nvidia Jetson Orin Nano are experiencing issues with SPI communication, specifically during loopback tests. The symptoms include unexpected voltage levels on the SPI signals and incorrect data being read back.

• Symptoms:

  • CS signal averages about 2.5 volts instead of staying low.
  • CLK signal averages around 1 volt, which is slightly low.
  • MOSI signal averages less than 0.1 volts when sending 0xFF, indicating it is not reaching the expected high state.
  • MISO signal remains at zero, as expected since it is open and pulled down by the SoC.

• Context:
  The issue arises during a loopback test where the user connects MOSI and MISO to verify data transmission. The user is running a custom program using the Linux spidev driver and has configured the necessary pins using jetson-io.py.

• Hardware/Software Specifications:

  • Device: Nvidia Jetson Orin Nano
  • Driver: Linux spidev (/dev/spidev0.0)
  • Test frequency: 1 kHz

• Frequency:
  The problem appears consistently during testing.

• Impact:
  This issue affects the ability to properly communicate with SPI devices, leading to unreliable data transmission and potentially hindering project development.

Possible Causes

1. Hardware Incompatibilities or Defects:

   • If the board or connected devices are faulty, they may not function correctly, causing signal integrity issues.

2. Software Bugs or Conflicts:

   • Bugs in the spidev driver or conflicts with other software could lead to incorrect handling of SPI signals.

3. Configuration Errors:

   • Incorrect settings in jetson-io.py or in the SPI configuration code may result in improper signal behavior.

4. Driver Issues:

   • The version of the spidev driver might have bugs that affect performance, particularly under specific configurations.

5. Environmental Factors:

   • Power supply issues or electrical noise could affect signal integrity, especially if components are not properly grounded.

6. User Errors or Misconfigurations:

   • Mistakes in wiring or configuration parameters (like mode settings) could lead to unexpected results.

Troubleshooting Steps, Solutions & Fixes

1. Verify Configuration:

   • Ensure that jetson-io.py is correctly configured for the intended SPI pins (19, 21, 23, 24, 26).
   • Double-check that the correct mode and bit settings are applied in your SPI configuration code.

2. Use an Oscilloscope:

   • While a multimeter is currently being used, an oscilloscope would provide more accurate readings of signal behavior over time.

3. Check Signal Levels with Commands:

   • Run the following commands to check the actual signal levels at specific memory addresses:

     sudo busybox devmem 0x0243d028 # SCLK
     sudo busybox devmem 0x0243d018 # MISO
     sudo busybox devmem 0x0243d040 # MOSI
     sudo busybox devmem 0x0243d008 # CS0
     sudo busybox devmem 0x0243d038 # CS1
     

4. Isolate Hardware Issues:

   • Test with different hardware configurations (e.g., using another Jetson Orin Nano or different SPI devices) to rule out hardware defects.

5. Update Drivers and Firmware:

   • Ensure that all drivers and firmware are up to date. Check for any updates related to JetPack or specific drivers for the Orin Nano.

6. Review Code for Errors:

   • Examine the provided test code for potential errors:

     int configure_spi(int fd) {
         // Configuration code...
     }
     

   • Ensure that all ioctl calls succeed and handle errors appropriately.

7. Test with Simple Loopback Code:

   • Simplify your loopback test code to ensure that basic functionality works before adding complexity.

8. Consult Documentation and Community Forums:

   • Refer to Nvidia’s official documentation for any additional configuration steps specific to your setup.
   • Engage with community forums for insights from other users who may have faced similar issues.

9. Best Practices for Future Prevention:

   • Always validate hardware connections before running tests.
   • Document configurations and changes made during troubleshooting for future reference.

By following these steps, users can systematically diagnose and potentially resolve issues related to SPI communication on the Nvidia Jetson Orin Nano Dev board.