Retromaster’s Electronics Projects

…related to old computers and other assorted stuff…

Cumulus Further Progress

Posted by retromaster on November 22, 2010

Cumulus doesn’t work quite reliably yet, but here is a screenshot of it booting into Oric DOS.

I’ve implemented the seek/step/restore commands, together with the read sector and read address commands. This seems to be the bare minimum for the Microdisc ROM to boot the OS. Unfortunately, there is a random error with the Oric Bus that causes miserable crashes (the screen filling up with garbage, etc.). When it does not happen, I can get the Oric to boot the OS. Most of the time, it does not work, though.

Getting to this stage took a fair bit of debugging and some important modifications. My original CPLD implementation contained an almost exact replica of the original Microdisc schematics. I found out that this did not work very well with the CPLD in some cases. For example, the WD179X contain an edge-trigger /WE input that controls writes to internal registers. This, however, is a problem with an XC95 CPLD, because signal transitions in the /WE line (gated clock into register flip-flops) cause random glitches. My guess is that this behaviour is due to the much higher speed of the CPLD compared to the 74-series logic chips used in the original Microdisc design. The solution here is to use a single, system-wide clock (the CPU bus clock) into all flip-flop clock inputs and use the /WE line only as an enable control. Similar issues arose also in the interface between the MCU and the CPLD, and the solution was the same.

Anyway, getting back to the currently unsolved problem I mentioned above… I modified the original Microdisc ROM to create a test ROM that will hopefully help me debug the issue. It uses the original routines in the Microdisc ROM to continuously read sectors from emulated floppy into RAM and it compares them against copies stored in the ROM. This test ROM usually works fine for several sectors, but eventually, and invariably, I see the same bus glitches occurring with this setup as well. Hopefully, I’ll soon solve this issue and I’ll have a first working prototype of Cumulus in my hands.

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