Troubleshooting Hello World Example On Pulpissimo Architecture A UART Debugging Guide

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This article delves into troubleshooting a common issue encountered when working with the Pulpissimo architecture on a ZCU104 board: the absence of "Hello World" output on the JTAG terminal despite successful bitstream generation, binary creation, and debugging capabilities. We'll explore potential causes, diagnostic steps, and solutions to help you get your Pulpissimo system up and running.

Understanding the Problem

The core of the issue lies in the UART (Universal Asynchronous Receiver/Transmitter) communication. While the bitstream is generated successfully, indicating a proper hardware configuration, and debugging via OpenOCD and GDB works flawlessly, suggesting the RISCY core is operational, the lack of output on the JTAG terminal points towards a problem in the data transmission path specifically related to the UART. Let's dissect the problem further.

Key Symptoms

  • Successful bitstream generation from examples like those found in the pulp-runtime-examples repository.
  • Functional connection to the debug unit via OpenOCD.
  • Successful connection with GDB in a separate terminal, allowing binary execution.
  • No output (specifically, the expected "Hello World" message) on the JTAG terminal.
  • Suspected issue with the UART port.
  • Confirmation that the RISCY core is running as debugging is possible.

Potential Causes

Several factors could contribute to this issue. We'll categorize them for clarity:

  1. UART Configuration: Incorrect UART configuration within the Pulpissimo system or the software running on the RISCY core is a primary suspect. This includes baud rate, data bits, parity, and stop bits.
  2. Hardware Connection: While the bitstream generation implies a proper connection, it's crucial to verify the physical connections related to the UART, especially if external transceivers are used.
  3. JTAG Terminal Setup: The JTAG terminal software itself might have incorrect settings, preventing it from properly interpreting the data stream from the UART.
  4. Software Bug: A subtle bug in the "Hello World" program or the underlying UART driver could prevent the message from being transmitted.
  5. Clocking Issues: An incorrect clock configuration for the UART peripheral within the Pulpissimo system can disrupt communication.

Troubleshooting Steps

Let's break down the troubleshooting process into a series of steps, starting with the most common and easily verifiable causes.

1. Verify JTAG Terminal Settings

The first and simplest step is to ensure that your JTAG terminal software is configured correctly. Common settings to check include:

  • Baud Rate: This is the most critical setting. The baud rate of the JTAG terminal must match the baud rate configured in the software running on the RISCY core. Common baud rates are 9600, 115200, and 1000000. Inspect your "Hello World" program and the Pulpissimo UART driver configuration to determine the correct baud rate.
  • Data Bits: Typically set to 8 bits.
  • Parity: Usually set to None.
  • Stop Bits: Typically set to 1 stop bit.
  • Flow Control: Should be set to None unless your system specifically uses hardware or software flow control.

If these settings are incorrect, you won't see any output, or you might see garbled characters. Correcting these settings is often the quickest solution.

2. Examine the "Hello World" Code

The "Hello World" program itself needs careful examination. Ensure that the program is indeed sending the output to the correct UART peripheral and that the UART driver is initialized correctly. Look for these key aspects:

  • UART Initialization: Verify that the UART is properly initialized with the correct baud rate, data bits, parity, and stop bits. This initialization typically involves writing to specific registers within the UART peripheral.
  • Output Function: The function used to send the "Hello World" message must be correctly implemented. It should transmit each character of the string through the UART.
  • Buffer Overflow: Check for potential buffer overflows in the output function. An overflow can corrupt the data being sent or even crash the program.
  • Interrupt Handling: If the UART driver uses interrupts, ensure that the interrupt handler is correctly configured and that interrupts are enabled.

Use GDB to step through the code and verify that the output function is being called and that data is being written to the UART transmit buffer. Inspect the UART registers using GDB to confirm the configuration.

3. Inspect the UART Driver Configuration

The UART driver is the software layer that interfaces between the "Hello World" program and the UART hardware. Examine the driver code for the following:

  • Register Addresses: Ensure that the driver is using the correct memory-mapped addresses for the UART registers. A typo in the address can lead to the driver writing to the wrong location.
  • Register Values: Verify that the driver is writing the correct values to the UART registers to configure the baud rate, data bits, parity, and stop bits.
  • FIFO Configuration: If the UART has a FIFO (First-In, First-Out) buffer, ensure that it's correctly configured. An improperly configured FIFO can lead to data loss.
  • Interrupts: If the driver uses interrupts, check the interrupt configuration and handler.

4. Verify Hardware Connections

Although bitstream generation suggests a correct hardware setup, it's crucial to double-check the physical connections related to the UART, especially if you're using external transceivers or level shifters.

  • UART TX and RX Lines: Ensure that the transmit (TX) line from the Pulpissimo system is connected to the receive (RX) line of the JTAG terminal adapter, and vice versa.
  • Ground Connection: A proper ground connection between the Pulpissimo system and the JTAG terminal adapter is essential.
  • Voltage Levels: Verify that the voltage levels are compatible between the Pulpissimo system and the JTAG terminal adapter. Level shifters might be required if there's a voltage mismatch.
  • Signal Integrity: Check the signal integrity of the UART lines. Long or poorly shielded cables can introduce noise and signal degradation.

Use a multimeter or oscilloscope to verify the signal levels and check for any shorts or opens in the connections.

5. Check Clock Configuration

The UART peripheral requires a clock signal to operate. If the clock is not configured correctly, the UART won't be able to transmit data. Check the following:

  • Clock Source: Identify the clock source for the UART peripheral. This might be a dedicated oscillator or a clock derived from the system clock.
  • Clock Frequency: Ensure that the clock frequency is within the operating range of the UART peripheral. An incorrect clock frequency will result in incorrect baud rate generation.
  • Clock Enable: Verify that the clock to the UART peripheral is enabled. In many embedded systems, peripherals have to be explicitly enabled to conserve power.

Consult the Pulpissimo documentation and the ZCU104 board schematics to determine the correct clock configuration for the UART.

6. Use a Logic Analyzer

A logic analyzer is an invaluable tool for debugging UART communication. It allows you to capture the signals on the UART TX and RX lines and analyze the data being transmitted. With a logic analyzer, you can:

  • Verify Baud Rate: Measure the actual baud rate on the UART lines and compare it to the configured baud rate.
  • Inspect Data Packets: Examine the data packets being transmitted to ensure that they contain the correct "Hello World" message.
  • Identify Timing Issues: Detect timing issues, such as incorrect start and stop bits, or timing skew.
  • Troubleshoot Hardware Problems: Pinpoint hardware problems, such as signal degradation or incorrect voltage levels.

7. Simplify the Setup

To isolate the issue, try simplifying the setup as much as possible. For example:

  • Minimal "Hello World" Program: Use a very simple "Hello World" program that only sends the message once. This eliminates potential issues related to loops or complex logic.
  • Direct Connection: If you're using external transceivers, try connecting the UART lines directly (if voltage levels allow) to eliminate potential issues with the transceivers.
  • Different JTAG Terminal: Try a different JTAG terminal software to rule out issues with the terminal itself.

8. Consult Documentation and Community Forums

The Pulpissimo documentation and community forums are valuable resources for troubleshooting. Check the documentation for information on UART configuration, driver details, and common issues. Search the forums for similar problems and solutions.

Conclusion

Troubleshooting UART communication issues, especially in embedded systems like Pulpissimo, requires a systematic approach. By carefully examining the JTAG terminal settings, the "Hello World" code, the UART driver configuration, hardware connections, and clock settings, you can identify the root cause of the problem and get your system communicating correctly. Remember to use tools like logic analyzers and consult documentation and community resources to aid in your debugging efforts. With persistence and a methodical approach, you'll be able to overcome these challenges and unlock the full potential of your Pulpissimo system. The key is to isolate each component and verify its functionality step by step until the issue is found. Good luck, and may your "Hello World" message soon grace your terminal!