Update on xum1541 development

Earlier this year, I announced the xum1541 project. This is a microcontroller board that connects a C64 floppy drive via USB to a PC. It is intended to run at a high enough speed to support copying protected disks. Here’s an update on the project’s recent progress.

When I first started this work, I examined the xu1541 adapter developed by Till Harbaum in 2007. It used an AVR microcontroller and a software USB stack. It was a neat project but had a few limitations. The goal for the xu1541 was to be as cheap as possible and use only through-hole parts. Since it used software USB, the microcontroller spent a lot of time bit-banging the USB port and so could not transfer data as fast as the 1541 could (especially with a parallel cable). Also, it required JTAG support to program the microcontroller the first time, something not all users would have. Still, it was a very neat project and is now available for purchase.

I started over with the AT90USB microcontroller. This device has a hardware USB engine that can run at full speed while the main CPU core is running your firmware. It also comes with a bootloader pre-programmed at the factory so users can install the firmware simply by plugging it into a USB port. There is a very nice open-source interface layer for this USB hardware by Dean Camera called LUFA. There are also many pre-built development kits so adding the IEC connectors is all the soldering that is needed.

The first version of the xum1541 was backwards-compatible with the xu1541. You could use it with the stock OpenCBM software from CVS. However, it had some limitations that made this approach a dead-end. The xu1541 works entirely via USB control transfers, which are not intended for high throughput. The AT90USB does not support double-buffering on the control endpoint. Even with a hardware USB engine, the control transfers hit a limit. There was no way I could get the latency down into the 25 microsecond/byte range needed to match the nibbler protocol. However, I did see a good speed increase over the xu1541 simply due to the hardware USB engine.

Thus, I decided to change to using two bulk endpoints, similar to the mass storage IO model that is implemented in USB flash drives. The AT90USB supports double-buffering for two endpoints of 64 bytes each in this configuration. This means that the hardware USB engine will clock data out the bus while the CPU is filling the other buffer. Then the pages are flipped and the process continues. With this approach, I could decrease the latency for nibbler support and get a performance boost for regular transfers as well.

This took a while to implement since it involved rewriting both the firmware and the host plugin component of OpenCBM to work together. Even though I made no effort to optimize the code, the results are already impressive.

There are several beta testers who have built their own copy of the hardware and are testing this version. Once we have ironed out any remaining bugs, I will release the pinouts and first version of the code. One notable feature that will be missing for a little while is the nibbler support. It will require more tuning to work reliably. However, it can be supported simply with a software upgrade so there’s no reason to delay the xum1541 release once the basic feature set is stable, which should be soon. It’s already useful as a fast option for transferring unprotected floppy images. I have no plans to produce a commercial version of this product, but I expect someone will take the design and build a cost-reduced model with a nice enclosure.

Thanks for all the words of support and patience as this hobby project nears completion.