Stories with Tag Peripheral

• Yes, the Apple II MouseCard IRQ Is Synced to the VBL

  permalink

  Posted: 2025-05-08 12:19:55

  Verbose tl;dr

  The Apple II MouseCard's interrupt requests (IRQs) are indeed synchronized with the vertical blanking interval (VBL). Through oscilloscope analysis and examining the MouseCard's firmware, the author confirmed that the card cleverly uses the VBL signal to time its counter, ensuring consistent IRQ generation every 1/60th of a second. This synchronization prevents screen tearing and jerky mouse movement, as updates are coordinated with the display refresh. Despite prior speculation and documentation suggesting otherwise, the investigation conclusively demonstrates the VBL-synced nature of the MouseCard's IRQ.

  This blog post by Colin Holgate meticulously investigates and confirms a long-held assumption about the Apple II MouseCard: its interrupt request (IRQ) signal is indeed synchronized with the vertical blanking interval (VBL) of the Apple II's display. The VBL is the period during which the electron beam in a CRT monitor returns to the top of the screen after refreshing the image, a time when the system can perform updates without causing visual artifacts.

  Holgate begins by outlining the common belief that the MouseCard IRQ is tied to the VBL and explaining why this synchronization is beneficial. It allows the mouse driver software to update the cursor position during the VBL, preventing flickering or tearing that might occur if the update happened mid-refresh. He then notes that while this synchronization was generally accepted, definitive documentation proving it was lacking.

  To investigate this, Holgate employs a logic analyzer to capture and analyze the timing signals of both the MouseCard IRQ and the Apple II's VBL. He details his experimental setup, which involved connecting the logic analyzer probes to the appropriate pins on the MouseCard and using a simple program on the Apple II to trigger the mouse IRQ.

  The resulting captured data, visualized in the blog post as a logic analyzer trace, clearly demonstrates the temporal relationship between the two signals. The MouseCard IRQ is shown to consistently occur within the VBL window. Holgate explains how the trace reveals that the IRQ is not only within the VBL but occurs at a specific and consistent point within it. He elaborates on the significance of this precise timing, highlighting that it further strengthens the evidence of intentional synchronization.

  Finally, after presenting and analyzing the empirical data, Holgate concludes that the long-held belief is indeed correct: the Apple II MouseCard IRQ is demonstrably synchronized with the VBL. He underscores the importance of this finding for developers working with Apple II emulation or those interested in the low-level hardware operation of this vintage system. The post effectively provides conclusive evidence for a previously undocumented but important aspect of the Apple II MouseCard's functionality.

  • Apple II
  • MouseCard
  • IRQ
  • VBL
  • Interrupt
  • Hardware
  • Retrocomputing
  • vintage computing
  • Apple
  • Peripheral
  • input device
  • Timing
  • synchronization

  Summary of Comments ( 11 )
  https://news.ycombinator.com/item?id=43925414

  Verbose tl;dr

  HN commenters discuss the intricacies of the Apple II MouseCard's interrupt handling, particularly its synchronization with the vertical blanking interval (VBL). Some express admiration for the clever engineering required to achieve stable mouse input within the constraints of the Apple II's hardware. One commenter recounts experiences with similar timing challenges on the Atari 8-bit and C64, emphasizing the difficulty of accurate timing without dedicated hardware support. Others delve into the specifics of the MouseCard's design, mentioning the use of a shift register and the challenges of debouncing button presses. The overall tone is one of appreciation for the ingenuity required to implement seemingly simple features on older hardware.

  The Hacker News post titled "Yes, the Apple II MouseCard IRQ Is Synced to the VBL" has generated a modest discussion with several insightful comments.

  One commenter highlights the cleverness of syncing the MouseCard interrupt request (IRQ) with the vertical blanking interval (VBL). They explain that this synchronization prevents visual artifacts or "snow" that could occur if the interrupt happened during active screen drawing. This method effectively utilizes the VBL's inherent downtime for interrupt handling, maximizing the efficiency of the system.

  Another commenter dives deeper into the Apple II's hardware limitations, explaining the rationale behind the chosen IRQ method. They point out that the 6502 processor lacked sophisticated interrupt handling capabilities, making a simple, synchronized approach like tying the IRQ to the VBL the most practical solution. This constraint forced elegant design choices that leveraged existing system timings.

  A further comment contrasts the Apple II's method with other systems that used periodic interrupts for mouse input. While acknowledging the potential for slight inaccuracies in the Apple II's approach, they emphasize the simplicity and effectiveness of the VBL-synced IRQ within the context of the Apple II's hardware.

  Another commenter shares a personal anecdote about their experience with the Apple II MouseCard, highlighting the surprisingly good performance despite the seemingly rudimentary interrupt mechanism. They suggest that this positive user experience is a testament to the effectiveness of the VBL synchronization.

  The overall sentiment in the comments is one of appreciation for the ingenuity and practicality of the Apple II's design choices, particularly in light of the hardware constraints of the era. The discussion provides a fascinating glimpse into the challenges and solutions of early computer engineering.

• A note on the USB-to-PS/2 mouse adapter that came with Microsoft mouse devices

  permalink

  Posted: 2025-03-28 00:16:29

  Verbose tl;dr

  Microsoft's older USB mice often included a small USB-to-PS/2 adapter. This adapter wasn't just a passive wiring converter; it contained active circuitry that translated USB signals into PS/2 signals. This allowed the mouse to function on computers with only PS/2 ports, and importantly, enabled support for the "Wake-on-Mouse" feature in some systems, which required a PS/2 connection. The adapter effectively made the USB mouse appear as a PS/2 device to the computer's BIOS, enabling this functionality even on motherboards lacking USB wake support. Therefore, discarding the seemingly insignificant adapter meant losing the potential for wake-on-mouse capabilities.

  Raymond Chen, in his blog post "A note on the USB-to-PS/2 mouse adapter that came with Microsoft mouse devices," delves into the intricacies of the seemingly simple green USB-to-PS/2 adapters bundled with Microsoft mice. He elucidates that these adapters are not merely passive converters rerouting signals between different connectors. Instead, they contain active circuitry, effectively transforming the USB mouse into a PS/2 mouse at a hardware level. This clever design allows the mouse to function on systems that may not have USB support, particularly older computers still relying on the PS/2 interface.

  Chen further explains the inherent challenges of such a conversion. The USB and PS/2 protocols are fundamentally different, not just in their physical connections, but also in their communication methods. USB is a bi-directional, polled protocol where the host computer actively requests data from the device. PS/2, on the other hand, is interrupt-driven, meaning the mouse sends data to the computer only when there is a change in its state, such as movement or button clicks. The adapter bridges this gap by actively monitoring the USB communication from the mouse and generating the appropriate PS/2 interrupts for the legacy system. This active translation is why the adapter requires a small amount of power, drawn from the keyboard's PS/2 port.

  The post highlights the adapter's intelligence by explaining its ability to identify the type of port it is plugged into. If connected to a USB port, it remains inactive, allowing the mouse to function normally as a USB device. However, when plugged into a PS/2 port, the adapter's internal circuitry activates, performing the necessary protocol conversion. This automatic detection simplifies usage for the end user, requiring no manual switching or configuration.

  Finally, Chen underscores the dedication and engineering ingenuity behind this seemingly minor accessory. The adapter embodies a thoughtful solution to maintain compatibility with older systems while allowing users to enjoy the benefits of modern USB mice. It's a testament to providing a seamless user experience even during technological transitions.

  • USB
  • PS/2
  • Mouse
  • Adapter
  • Microsoft
  • Hardware
  • Peripheral
  • Legacy Devices
  • compatibility
  • input devices
  • Device Drivers
  • Technical
  • Blog Post

  Summary of Comments ( 94 )
  https://news.ycombinator.com/item?id=43499823

  Verbose tl;dr

  Hacker News users discuss the intricacies of the Microsoft USB-to-PS/2 adapter, focusing on its active conversion of USB signals to PS/2 rather than simple pin mapping. Several commenters praise the adapter's sophistication, highlighting its ability to handle higher polling rates than standard PS/2 and even emulate multiple PS/2 devices from a single USB port. Some express surprise at learning this detail, having previously assumed passive conversion. Others reminisce about similar PS/2 to serial port adapters, while some debate the technical challenges and cleverness of the implementation. The discussion touches on the historical context of transitioning between these technologies, the complexities of bidirectional communication, and the surprising amount of intelligence packed into this seemingly simple adapter.

  The Hacker News post discussing the Microsoft USB-to-PS/2 mouse adapter generated several comments exploring various aspects of the technology and its history.

  One commenter shared their experience of needing such an adapter in the past due to BIOS incompatibility with USB mice during the boot process. They noted the utility of these adapters in allowing users to navigate BIOS settings with a newer USB mouse on older hardware. This highlighted the transitional period where USB was becoming dominant but legacy PS/2 ports were still necessary for full system functionality.

  Another commenter delved into the technical details of the adapter, explaining that it wasn't simply a passive wiring adapter. They pointed out the active electronics within the adapter that translated the USB signals into PS/2 signals, effectively emulating a PS/2 mouse. This emphasized the complexity behind the seemingly simple adapter.

  Expanding on the active nature of the adapter, a further comment described the clever technique employed by these adapters to identify themselves to the computer. They described how the adapter would briefly short the data and clock lines, mimicking the behavior of a PS/2 mouse during initialization. This allowed the computer to correctly identify the connected device, despite it being a USB mouse connected through the adapter.

  Several commenters reflected on the transition period between PS/2 and USB, reminiscing about the days when both ports were common on computers. They discussed the gradual phasing out of PS/2 in favor of the more versatile USB standard.

  One commenter corrected a common misconception, clarifying that the green PS/2 port was not necessarily better or faster than the purple keyboard port. They explained that both ports were functionally identical and the color coding was simply a convention for easier identification.

  The discussion also touched upon the challenges of using USB keyboards with older BIOS systems. While USB mice were often supported via adapters, using USB keyboards with older BIOS versions could be more problematic due to limitations in the BIOS itself.

  Finally, a commenter expressed surprise at the longevity of the PS/2 standard, noting that it persisted for a considerable time even after USB became prevalent. They mused on the reasons behind this extended lifespan, considering factors such as cost and existing hardware compatibility.

  In summary, the comments on the Hacker News post provided a range of perspectives on the USB-to-PS/2 mouse adapter, from personal anecdotes about its use to technical explanations of its functionality and reflections on the broader technological transition from PS/2 to USB.