USBSerialDevice On Fomu PVT Fails With Windows Host

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Introduction

When working with embedded systems, encountering device recognition issues, especially on different operating systems, is a common challenge. This article delves into a specific problem encountered with the USBSerialDevice on the Fomu PVT (Production Validation Test) board when connected to a Windows host. The error manifests as the COM port being recognized but failing to start, with Windows reporting "This device cannot start." This comprehensive guide aims to provide a detailed analysis of the issue, potential causes, and troubleshooting steps, along with the relevant code snippets and environment details. Understanding and resolving such issues is crucial for developers working on embedded projects, ensuring seamless communication and functionality across platforms.

Problem Description

The core issue revolves around the Fomu PVT board, running a custom loopback program, failing to initialize correctly on a Windows host. The device manager recognizes the COM port associated with the USBSerialDevice, but attempts to start the device result in an error. This behavior suggests a potential mismatch or incompatibility between the device's USB communication protocol and the Windows USB driver stack. To effectively address this, a systematic approach is needed, examining both the hardware configuration and the software implementation.

The problem presents itself despite the device being recognized, indicating that the basic USB enumeration process is successful. However, the subsequent initialization phase fails, suggesting a deeper issue within the communication layer. This could stem from incorrect USB descriptors, driver-related problems, or issues within the firmware itself. To gain a clearer understanding, it's essential to examine the error messages and device manager details closely, alongside the software configuration and hardware setup.

This article will explore the possible reasons behind this failure, providing steps to diagnose and resolve the issue. We will cover aspects ranging from toolchain versions to code implementation, ensuring a holistic approach to troubleshooting. Understanding the intricacies of USB communication and device drivers is paramount in such situations, and this guide aims to equip developers with the necessary knowledge to tackle similar challenges.

Toolchain and Code Overview

To provide context, let's examine the toolchain versions and the code used in this scenario. The toolchain versions are:

  • Yosys 0.54+23
  • Nextpnr-ice40 0.8-43

These versions are crucial for ensuring compatibility and stability during the hardware synthesis and place-and-route processes. Any discrepancies or bugs within these tools could potentially lead to issues in the generated bitstream, affecting the device's functionality. Therefore, verifying the toolchain versions is a fundamental step in the debugging process.

The code provided is an Amaranth-based implementation for a simple loopback program on the Fomu PVT platform. Here's a breakdown of the code:

import os, shutil, subprocess

from amaranth import *
from amaranth.lib import wiring


from amaranth_boards.fomu_pvt import FomuPVTPlatform


from luna.full_devices import USBSerialDevice


class Top(wiring.Component):
def init(self, with_usb=False):
self.with_usb = with_usb


def elaborate(self, platform):
    m = Module()

    input_clock = platform.request('clk48').i

    # Raw Clock Domains
    m.domains.clk48 = cd_clk48 = ClockDomain(local=True)
    m.domains.clk12 = cd_clk12 = ClockDomain(local=True)

    m.submodules += Instance('SB_PLL40_CORE',
                             p_DIVR = 0,
                             p_DIVF = 15,
                             p_DIVQ = 5,
                             p_FILTER_RANGE = 1,
                             p_FEEDBACK_PATH = 'SIMPLE',
                             p_DELAY_ADJUSTMENT_MODE_FEEDBACK = 'FIXED',
                             p_FDA_FEEDBACK = 15,
                             p_DELAY_ADJUSTMENT_MODE_RELATIVE = 'FIXED',
                             p_FDA_RELATIVE = 0,
                             p_SHIFTREG_DIV_MODE = 1,
                             p_PLLOUT_SELECT = 'GENCLK_HALF',
                             p_ENABLE_ICEGATE = 0,
                             # IO
                             i_REFERENCECLK = ClockSignal('clk48'),
                             o_PLLOUTCORE = ClockSignal('clk12'),
                             i_BYPASS = 0,
                             i_RESETB = 1,
                            )

    # Named Clock Domains
    m.domains.sync = ClockDomain(local=True)
    m.domains.usb = ClockDomain(local=True)
    m.domains.usb_io = ClockDomain(local=True)

    m.d.comb += [
        ClockSignal('clk48').eq(input_clock),
        ClockSignal('sync').eq(ClockSignal('clk12')),
        ClockSignal('usb').eq(ClockSignal('clk12')),
        ClockSignal('usb_io').eq(ClockSignal('clk48')),

        ResetSignal('sync').eq(ResetSignal('clk12')),
        ResetSignal('usb').eq(ResetSignal('clk12')),
        ResetSignal('usb_io').eq(ResetSignal('clk48')),
    ]

    platform.add_clock_constraint(cd_clk48.clk, frequency=48e6)
    platform.add_clock_constraint(cd_clk12.clk, frequency=12e6)

    # USB module
    usb_pads = platform.request('usb')
    m.submodules.usb_serial = usb_serial = USBSerialDevice(bus=usb_pads, idVendor=0x1209, idProduct=0x70b1)

    m.d.comb += [
        usb_serial.connect.eq(1),

        usb_serial.tx.payload  .eq(usb_serial.rx.payload),
        usb_serial.tx.valid    .eq(usb_serial.rx.valid),
        usb_serial.tx.first    .eq(usb_serial.rx.first),
        usb_serial.tx.last     .eq(usb_serial.rx.last),
        usb_serial.rx.ready    .eq(usb_serial.tx.ready),
    ]

    return m



def add_dfu_suffix(fn):
fn_base, _ = os.path.splitext(fn)
fn_dfu = fn_base + '.dfu'
shutil.copyfile(fn, fn_dfu)
subprocess.check_call(['dfu-suffix', '--pid', '1209', '--vid', '70b1', '--add', fn_dfu])


def main():
platform = FomuPVTPlatform()


platform.build(Top(with_usb=True), build_dir='bin')

add_dfu_suffix('bin/top.bin')

return 0



if name == 'main':
exit(main())

The code defines a Top module that utilizes the USBSerialDevice from the luna.full_devices library. It sets up clock domains, instantiates a PLL, and connects the USB serial interface for a loopback functionality. The idVendor and idProduct are set to 0x1209 and 0x70b1, respectively. The add_dfu_suffix function appends a DFU (Device Firmware Upgrade) suffix to the generated binary, which is crucial for flashing the firmware onto the Fomu PVT board.

This code structure is fairly standard for Amaranth-based hardware designs, providing a solid foundation for the USB serial communication. However, issues may arise from incorrect clock configurations, USB descriptor problems, or driver-related issues. Each aspect needs careful examination to pinpoint the root cause of the problem.

Analyzing the Error: "This device cannot start"

The error message "This device cannot start" in Windows Device Manager is a generic error that indicates a failure during the device initialization phase. This failure can stem from a variety of issues, including driver problems, resource conflicts, or hardware malfunctions. To effectively troubleshoot this error, it's essential to delve deeper into the specifics of the device and the system's interaction with it.

When a USB device is connected to a Windows host, the operating system attempts to identify the device based on its USB descriptors. These descriptors contain crucial information such as the vendor ID, product ID, device class, and other configuration details. If Windows cannot properly read or interpret these descriptors, it may fail to load the appropriate drivers, leading to the "This device cannot start" error. Therefore, a primary focus of investigation should be on the accuracy and completeness of the USB descriptors defined in the firmware.

Another potential cause is a driver conflict. Windows may have multiple drivers that could potentially serve the device, and if the system selects an incompatible or outdated driver, the device may fail to initialize. Checking for driver updates or attempting to manually install a specific driver version can sometimes resolve this issue. Furthermore, the Windows Event Viewer can provide valuable insights into the specific errors encountered during device initialization, offering clues about the underlying cause.

Hardware issues, while less common, should also be considered. A faulty USB port, a damaged cable, or a hardware problem on the Fomu PVT board itself could lead to initialization failures. Testing the device on different USB ports and with different cables can help rule out these possibilities. Additionally, examining the board for any visible damage or loose connections is a prudent step in the troubleshooting process.

Potential Causes and Troubleshooting Steps

To effectively troubleshoot the "This device cannot start" error, a systematic approach is necessary. Here are several potential causes and corresponding troubleshooting steps:

  1. Driver Issues: The most common cause for this error is a driver-related problem.

    • Troubleshooting: Try uninstalling the device from Device Manager, including deleting the driver software, and then reconnecting the device. Windows will attempt to reinstall the drivers. You can also try manually installing drivers, either from the manufacturer or a generic CDC-ACM driver. Driver conflicts can often be resolved by manually selecting the appropriate driver.

  2. USB Descriptors: Incorrect or incomplete USB descriptors can prevent Windows from properly identifying and initializing the device.

    • Troubleshooting: Verify that the idVendor and idProduct in the code match the intended values and that the descriptors are correctly formatted. Use a USB analyzer tool (e.g., USBlyzer, Wireshark) to capture the USB enumeration process and examine the descriptors being sent by the device. Ensure that the descriptors are valid and comply with the USB specifications. Incorrectly formatted descriptors are a common cause of initialization failures.

  3. Power Issues: Insufficient power supply to the USB device can cause it to fail during initialization.

    • Troubleshooting: Try connecting the Fomu PVT to a powered USB hub or a different USB port that provides more power. Insufficient power can lead to erratic behavior and initialization failures, especially during the initial enumeration phase.

  4. Clock Configuration: Incorrect clock settings can lead to communication failures.

    • Troubleshooting: Double-check the clock constraints and PLL configuration in the Amaranth code. Ensure that the 48MHz and 12MHz clocks are correctly generated and stable. Incorrect clock configurations can disrupt the timing-sensitive USB communication, leading to errors.

  5. Firmware Issues: Bugs in the firmware can cause the device to fail during initialization.

    • Troubleshooting: Review the firmware code, particularly the USB initialization and communication routines. Add debugging output to monitor the device's state during initialization. Firmware issues can manifest in various ways, from incorrect descriptor handling to communication protocol errors.

  6. Hardware Issues: Although less common, hardware problems can cause initialization failures.

    • Troubleshooting: Try connecting the Fomu PVT to different USB ports and using different USB cables. Inspect the board for any visible damage or loose connections. Hardware faults can range from simple connection problems to more complex issues within the device's circuitry.

  7. Operating System: Sometimes, the issue might be specific to the operating system.

    • Troubleshooting: Try connecting the device to a different computer running a different operating system (e.g., Linux) to see if the issue persists. This helps isolate whether the problem is specific to the Windows environment.

By systematically addressing each of these potential causes, it becomes possible to narrow down the source of the problem and implement the appropriate solution.

Detailed Steps for USB Descriptor Verification

Given that USB descriptors are a critical component in device initialization, let's delve into the detailed steps for verifying their correctness. This involves both inspecting the code and using USB analysis tools.

  1. Code Inspection: Begin by examining the Amaranth code related to USB descriptors. In the provided code, the USBSerialDevice likely encapsulates the USB descriptor definitions. Ensure that the idVendor and idProduct are correctly set to 0x1209 and 0x70b1, respectively. However, this is just the initial step; the actual descriptors used by the device are often more complex and defined within the luna.full_devices library or related modules.

    You will need to inspect the source code of luna.full_devices.USBSerialDevice to understand how the descriptors are generated. Look for structures that define the device descriptor, configuration descriptor, interface descriptor, and endpoint descriptors. Ensure that these descriptors conform to the USB specifications and are appropriate for a CDC-ACM (Communication Device Class Abstract Control Model) device, which is commonly used for USB serial communication.

  2. USB Analyzer Tools: Use a USB analyzer tool to capture the USB enumeration process when the Fomu PVT is connected to the Windows host. Tools like USBlyzer or Wireshark (with USBPcap) can capture the raw USB traffic and decode the descriptors being sent by the device. This provides a real-time view of what the device is advertising to the host.

    • Capturing USB Traffic: Start the USB analyzer tool and begin capturing traffic before connecting the Fomu PVT. Connect the device and observe the captured traffic. The initial packets will contain the device descriptor requests and responses.
    • Decoding Descriptors: The USB analyzer tool will typically decode the descriptors, allowing you to view their contents in a human-readable format. Verify that the vendor ID, product ID, device class, and other relevant fields match the expected values. Look for any anomalies or errors in the descriptor structure.
    • Identifying Issues: Pay close attention to the configuration descriptor, interface descriptor, and endpoint descriptors. Ensure that the endpoints are correctly defined for bulk transfers (for data) and interrupt transfers (for control signals). If any descriptor is malformed or contains invalid values, it can prevent the device from initializing correctly.

  3. Comparison with USB Specifications: Compare the captured descriptors with the USB specifications for CDC-ACM devices. The specifications define the required fields and their valid values. Any deviation from the specifications can lead to compatibility issues. Refer to the official USB documentation for the CDC-ACM class for detailed requirements.

  4. Troubleshooting Descriptor Issues: If you identify issues with the descriptors:

    • Correct the Code: Modify the descriptor definitions in the Amaranth code to fix any errors. This might involve adjusting field values, adding missing descriptors, or correcting the descriptor structure.
    • Rebuild Firmware: After making changes, rebuild the firmware and flash it onto the Fomu PVT board.
    • Re-capture and Verify: Recapture the USB traffic with the analyzer tool to ensure that the corrected descriptors are being sent by the device.

By meticulously verifying the USB descriptors, you can rule out a significant potential cause of the "This device cannot start" error. This process requires a combination of code inspection, USB traffic analysis, and adherence to USB specifications, ensuring a thorough investigation.

Investigating Driver-Related Problems

If the USB descriptors are correct, the next area to investigate is driver-related issues. Windows uses drivers to communicate with USB devices, and problems with these drivers can lead to initialization failures. Here are detailed steps to diagnose and resolve driver-related issues:

  1. Device Manager Inspection: Open Device Manager and locate the Fomu PVT device. If it's displaying the "This device cannot start" error, it may have a yellow exclamation mark next to it. Right-click the device and select "Properties" to view the device status. The "Device status" section may provide more specific error codes or messages.

    • Error Codes: Pay attention to error codes such as Code 10 (device cannot start), Code 31 (device is not working properly), or Code 43 (Windows has stopped this device because it has reported problems). These codes can provide clues about the nature of the driver issue.
    • Driver Details: In the "Driver" tab, you can view the driver details, including the driver files and their versions. Note the driver files and their locations, as this information can be useful for further troubleshooting.

  2. Uninstall and Reinstall Drivers: A common troubleshooting step is to uninstall the device and its drivers and then allow Windows to reinstall them.

    • Uninstall Device: In Device Manager, right-click the Fomu PVT device and select "Uninstall device". Check the box that says "Delete the driver software for this device" if it's available. This ensures that the driver files are removed from the system.
    • Reconnect Device: Disconnect the Fomu PVT and then reconnect it. Windows will attempt to reinstall the drivers. Observe whether the device initializes correctly this time.

  3. Manual Driver Installation: If Windows fails to install the drivers automatically, you may need to manually install them.

    • Download Drivers: Obtain the appropriate drivers for the Fomu PVT. This might involve downloading drivers from the Fomu project's website or using generic CDC-ACM drivers. Generic drivers can often be found in the Windows driver store or through online resources.
    • Update Driver: In Device Manager, right-click the Fomu PVT device and select "Update driver". Choose the option to "Browse my computer for drivers" and navigate to the directory containing the driver files. Follow the on-screen instructions to install the driver.

  4. Driver Conflicts: Driver conflicts can occur if multiple drivers are competing for the same device. This is particularly common with USB serial devices.

    • Identify Conflicts: Look for multiple entries for the Fomu PVT in Device Manager or conflicts with other serial devices. Disable or uninstall any conflicting drivers.
    • Select Specific Driver: In the "Update driver" dialog, you can choose to "Let me pick from a list of available drivers on my computer". This allows you to select a specific driver for the device, which can resolve conflicts.

  5. Windows Event Viewer: The Windows Event Viewer logs system events, including driver-related errors. Reviewing the Event Viewer logs can provide valuable insights into the cause of the driver issue.

    • Open Event Viewer: Open Event Viewer (search for "Event Viewer" in the Start menu).
    • Check Logs: Navigate to "Windows Logs" -> "System" and filter the logs for events related to device installations or driver errors. Look for events with warnings or errors that occurred around the time the Fomu PVT was connected.
    • Analyze Events: Examine the details of the events to identify specific error messages or codes. These messages can provide clues about the nature of the driver issue and potential solutions.

By systematically investigating driver-related problems, you can often resolve the "This device cannot start" error. This process involves using Device Manager, manual driver installation, conflict resolution, and reviewing the Windows Event Viewer logs for detailed error information.

Conclusion

Troubleshooting the “USBSerialDevice on Fomu PVT fails with Windows host” issue requires a systematic and comprehensive approach. This article has detailed the steps involved in diagnosing and resolving the "This device cannot start" error, covering key areas such as toolchain verification, code analysis, USB descriptor verification, and driver-related problem investigation. By understanding the intricacies of USB communication, firmware implementation, and driver interactions, developers can effectively tackle such challenges.

From verifying toolchain versions to inspecting the Amaranth code for clock configurations and USB descriptor definitions, each step plays a crucial role in identifying the root cause. The use of USB analyzer tools, such as USBlyzer or Wireshark, provides valuable insights into the USB enumeration process, allowing for a detailed examination of the descriptors being sent by the device. Furthermore, understanding how to navigate Device Manager, manually install drivers, and review the Windows Event Viewer logs is essential for resolving driver-related issues.

The key takeaway is that persistence and a methodical approach are paramount. By carefully considering each potential cause and implementing the corresponding troubleshooting steps, you can effectively resolve the "This device cannot start" error and ensure seamless communication between the Fomu PVT and the Windows host. This guide serves as a comprehensive resource for developers facing similar challenges, providing the knowledge and tools necessary to navigate the complexities of embedded systems development.

In conclusion, while the "This device cannot start" error can be frustrating, a systematic approach, coupled with a deep understanding of the underlying hardware and software, will ultimately lead to a resolution. This article aims to empower developers with the necessary skills to troubleshoot and resolve similar issues, ensuring the successful deployment of embedded projects.