UART1 Hardware Flow Control Abnormality on Nvidia Jetson Orin Nano

Issue Overview

Users are experiencing unexpected behavior with UART1 hardware flow control on the Nvidia Jetson Orin Nano development board. Specifically, when UART1_RTS and UART1_CTS are disconnected and hardware flow control is enabled in minicom, data transfer still occurs successfully. This behavior raises questions about the proper functioning of the RTS/CTS flow control mechanism.

The issue occurs during UART1 function testing, where:

• UART1_TXD is connected to UART1_RXD (loopback configuration)
• UART1_RTS and UART1_CTS are left disconnected
• Minicom is used for testing with hardware flow control enabled
• The test is performed on a custom board running Jetpack version 5.1.2

Possible Causes

1. Normal RTS/CTS Behavior: The observed behavior might be expected, as RTS/CTS flow control typically only engages when the RX buffer is full.

2. Configuration Mismatch: There could be a mismatch between the hardware configuration and the software settings, causing the flow control to be ineffective.

3. Driver Issues: The UART driver might not be properly implementing the hardware flow control, leading to unexpected behavior.

4. Hardware Malfunction: There could be a problem with the UART hardware on the custom board, preventing proper flow control operation.

5. Buffer Size: The size of the receive buffer might be large enough to handle the test data without triggering flow control.

Troubleshooting Steps, Solutions & Fixes

1. Verify Hardware Connections:

   • Ensure that the UART1_TXD to UART1_RXD loopback connection is correct.
   • Test with UART1_RTS and UART1_CTS connected to verify if the behavior changes.

2. Check UART Configuration:

   • Use the stty command to verify and configure UART settings:

     stty -F /dev/ttyTHS0 115200 crtscts
     

   • This sets the baud rate to 115200 and enables hardware flow control.

3. Perform Loopback Test:

   • Use cat and echo commands to verify UART functionality:

     echo "Test Data" > /dev/ttyTHS0
     cat /dev/ttyTHS0
     

   • This should display the sent data if the loopback is working correctly.

4. Test with Large Data Transfer:

   • Create a large file and attempt to transfer it through the UART:

     dd if=/dev/urandom of=testfile bs=1M count=10
     cat testfile > /dev/ttyTHS0
     

   • Monitor if flow control engages during large data transfers.

5. Analyze UART Traffic:

   • Use a logic analyzer or oscilloscope to monitor the RTS/CTS lines during data transfer.
   • This can help determine if the flow control signals are being asserted correctly.

6. Update Jetpack and Drivers:

   • Ensure you are using the latest Jetpack version and UART drivers available for the Orin Nano.

7. Consult Nvidia Documentation:

   • Review the Nvidia Jetson Orin Nano technical documentation for any known issues or specific configurations required for hardware flow control.

8. Test with Different UART Devices:

   • If possible, test the UART with an external device that is known to use hardware flow control correctly.
   • This can help isolate whether the issue is specific to the loopback configuration or a more general problem.

9. Adjust Buffer Size:

   • If the issue persists, try adjusting the UART buffer size to force flow control to engage:

     stty -F /dev/ttyTHS0 115200 crtscts -icanon min 1 time 0
     

   • This sets a minimal canonical mode, which may cause flow control to engage more readily.

10. Contact Nvidia Support:

    • If the issue remains unresolved after trying these steps, consider reaching out to Nvidia support for further assistance, especially if you’re using a custom board.

Remember that hardware flow control may only be necessary for high-speed data transfers or when connecting to devices with limited buffer capacity. For many applications, software flow control or no flow control may be sufficient.