We are going back to breadboarding for designing what will be the base for the new Steckschwein computer core. By “core” we mean CPU, RAM, ROM and the glue logic which will be accommodated in the ATF1508 CPLD. In order to communicate with the outside word, we also count the 16C550 UART as part of the core.

Design goals

Our main design goals are:

• Integrate SPI into the CPLD -> DONE
  Using a hardware based SPI implementation similiar to Daryl Rictor’s SPI65, but tightly integrated into the CPLD will be an efficient use of the CPLD resources and will provide a much more performant SPI bus as opposed to the current semi-bit-banged solution. This way, it will be much more performant to add more SPI based components such as USB host or networking (see below).
  Update: Done! We decided to use Andre Fachat’s SPI implementation from his MicroPET. The main advantage over rolling our own is - it’s already there. Another main advantage over other existing Implementation is that the MicroPET one is pretty small, which is important when CPLD resources are at a premium.
• Implement a priorising vectorising interrupt controller
  This will improve interrupt handling by assigning a dedicated ISR routine per interrupt source instead of one system ISR.
  Update: We are already scratching the limit of the ATF1508. The PLCC84 version we use in our prototype can not handle more pins, so we are unable to add the inputs needed for all the interrupt sources we need. While this is unfortunate, we have not scrapped the idea of an interrupt controller. We might move to the TQFP-100 version at some point, which obviously has more pins.
• DUART instead of UART
  Replacing our trusty 16C550 with a 16C2752 DUART will provide two serial interfaces without increasing the chip count.
  With two serial ports, we will be able to upload programs on one serial port, while observing the output on the other one. Also, a separate upload interface will allow us to explore UART interrupts on the other.

Other changes/optimizations

Other things that will be optimized are:

Since we introduced our new banking logic to access 512k RAM, our glue logic is being accommodated by a Xilinx XC9572 (without XL) CPLD. This component has long been deprecated when we started using it. We chose it because it was what we had available. The upgrade path would be the successor family XC95..XL. Those have been rather expensive lately, and finally AMD/Xilinx axed their whole CPLD line in 2024, leaving us at a dead end. Where do we go from here? The market for CPLDs is not exactly growing. Only a few manufacturers are still actively producing them.

We talked to Martin Wolf from Golem.de about our Project, how it came to be, and of course about our version of Pacman.

Listen to our Episode (in German) here!

After we moved this website to Codeberg Pages, we now finished moving our Repositories to www.codeberg.org.

The resources page has been updated with the new URLs.

Our repositories on GitHub will remain available as push mirrors, but may vanish at some point.

Codeberg is a non-profit, community-led effort that provides Git hosting and other services for free and open source projects.

At this year’s installment of the Vintage Computer Festival Europa we talked about how the Steckschwein came to be in its current incarnation. Marko presented his feature complete implementation of Pacman - the only one for 6502 based systems that replicates the look and the behaviour (including bugs) of the original arcade almost completely.

In case you could not attend our talk or even VCFe, you still can download the Steckschwein slides and the Pacman slides.

This year’s installment of the Vintage Computer Festival Europa is - again - a special one for us: VCFe 24.0 will be focused around “50 years of 6502”!

So be prepared for lots of 6502 related and also unrelated exhibitions and talks!

We will of course be there, too, presenting Marko’s Pacman implementation - the only one for 6502 based systems that replicates the look and the behaviour (including bugs) of the original arcade almost completely.