Cumulus 6502 Bus Working

Cumulus Test ROM running on Oric

Cumulus LA Debugging Setup

Over the last week I worked on debugging the Cumulus 6502 interface. One of the shots above shows a small test program (coloured bars in hi-res graphics mode) I wrote for testing. The program resides on the 28C256 EPROM on the Cumulus board and is executed by the Oric on reset.

The problem with the Oric bus turned out to be due to the 74HCT221-based circuitry on the Cumulus board. This circuitry generates a couple of signals from the 6502 O2 signal coming from the Oric. Those signals are in turn used for controlling some Oric bus-related signals such as 28C256 output enable. The pulses generated by the 221 were too long, actually about twice the length specified in the datasheet.

I managed to solve the problem by manually adjusting the component values until I got proper the pulse widths. The problematic circuitry actually comes straight from the original Oric Microdisc schematics, with one difference: The original circuit uses an LS variant instead of the HCT that I used. According to the datasheets, there does not seem to be a difference, but I may have missed something.

There are still some problems though. Sometimes the Oric does not properly reset, most likely due to Cumulus affecting the reset sequence. Sometimes there is some wave interference on the video output. Other times, the video output looks as if the Oric never switches to the high-res mode, remaining in text mode (I get coloured bars, but longer). I’ll probably conceive a shorter program to reproduce these issues and debug using my current setup with the logic analyzer.

An Older Project Revisited: A2601

A2601 Rev B Homebrew PCB After Etching Top Side

Having made quite a bit of progress with UFE, I’ve decided it’s time to take a little break from it while I wait and see how recent developments in the manufacturing front pan out (sorry, cannot give details on that yet). So, I am now revisiting a never-before-released project of mine that is almost five years old now.

The project is an ATARI 2600 clone on FPGA, including 6502 and TIA cores in VHDL and dedicated PCB layout, all developed by me. The design fits in a 100K gate Xilinx Spartan 3E, and works quite well in the simulator. The PCB design contains the FPGA, resistor-ladder DACs for video and audio, custom joystick input ports and flash memory for storing ROMs. Few years ago I even built a prototype. It worked well, but unfortunately it did not survive, since my PCB-making techniques were not as advanced back then…

So, what’s new this time around? Basically, a reworking of the PCB layout that fits in a smaller case and contains a single input port for a Genesis/MegaDrive gamepad instead of custom-wired pads. I’ve ended up choosing MegaDrive pads because they are commonly available, very easy to drive and they contain the necessary auxiliary buttons (start/select etc.). I’ve already started manufacturing the board and the photo above shows the work in progress. Hopefully, all will go well and next time I’ll be posting working photos of the board…