Apologies for the long silence on the blog; I’ve had work and health issues that ate up a considerable amount of time during the first half of the year. Fortunately that’s all over and I am returning to my many projects.
First, I’d like to introduce the JRC-1, or Josh’s Retro Computer 1. This is a direct successor to COLE-2, with the aim of actually bringing the design to completion. Along those lines I ditched some of the more complex features of COLE-2, including built-in video and the game/joystick ports. Instead, it has an expansion bus with three slots that can be used to add features down the road.
Unlike its predecessor, JRC-1 is a 5V design. I had previously moved to 3.3V in order to gain access to larger RAMs, but it added complexity, as some parts of the design still needed to be 5V.
At the moment JRC-1 is still in the early planning stage. Once I am confident my design can be implemented I will begin work on the schematic and PC board. I am planning to skip the home prototyping stage for this project, and instead go directly to a manufactured PCB.
As part of planning out JRC-1 I have been testing some design ideas by using my original COLE-1 board. However, that board is a bit problematic: it has power instability issues, and the footprint for the 6850 is wrong and required an ugly hack.
Since having a good test bed for my design ideas is important, I decided to spend a few hours fixing up the COLE-1 design a bit:
- The 6850 now has the correct footprint
- Added a large electrolytic capacitor to help smooth out the power supply
- Replaced all of the discrete logic with a single 22V10D GAL. This also allowed me to tighten up the address decoding to make all but the bottom 1K of the ROM available.
- Swapped out the CPU for a 65C816, because it was very easy to do so.
- Removed the dedicated 1.8432 MHz crystal for the 6850. Instead, the CPU and the 6850 now share a 3.6864 MHz crystal.
- Reduced the serial port speed to 57.6k bps, due to the use of a double-speed clock crystal.
- Made the board layout more compact, and made all ICs align vertically.
- Added some silkscreen to label the pins on the expansion headers.
In the interest of time I used the FreeRouting autorouter to route the board. It actually did a pretty decent job, and I only needed to make a few tweaks to some via locations. I may give it a try for JRC-1 down the road just to see how well it works on a larger design.
The final result of this work is officially the COLE-1+, as the feature set is almost identical to COLE-1. Boards are already on order from JLCPCB; I should have them in about a week. Hopefully I didn’t mess up anything this time around!
(Nope! It seems I left off a pullup resistor for the 6850’s IRQ line. Sigh. Fortunately this will be a very simple and relatively clean fix during assembly.)