Some quick notes on debugging the new revision of the decode card:

• plugging card in prevents reset from making any progress. With the logic analyser it's clear that rst_trig is being triggered every clock cycle. This is because there's a bug in the logic for controlling the control memory daughter …

I had thought the MMU was pretty solid, since I gave it a good test after its v1.1 rework, and didn't find any issues. In testing the control unit, though, I've noticed some weirdness. Mapping addresses didn't work consistently. More often than not, about half the bits were incorrect …

Notes:

• low-temperature solder paste does not flow nearly as well as original version. Maybe because it's not thermally stable, and I'm not storing it at a constant temperature?

• short between VCC and GND after assembly and soldering. Removed 48 of 60 capacitors before finding the culprit on the 49th.

• bug …

Since the first version of the μcode address generator had a few issues, I re-worked it to fix them, and while I was at it I let a bit of feature-creep happen. The 1.1 revision of the board has quite a lot more functionality than the 1.0 one …

I combined the PC and immediate units into one board since the immediate unit has three outputs (data, B bus, and PC), and combining the imm and PC units onto one board saved 16 lines on the backplane for the imm->PC connection. Technically, there's a LUI 'unit' here too …

It's been almost 18 months since I had any time to work on this (life got in the way), but I've started assembling and testing the last batch of boards I designed, and working on the next lot.

The μcode address generator was the simplest piece of the control unit …

I finally decided to stop tweaking the design of the pieces of the control unit that have been mostly complete for almost a year now, and just send them to be manufactured. There are four new boards here:

• a new lower backplane
• a new tester board to allow an Arduino …

I finally got a bit of time to fix the bugs in the memory and MMU boards, and to solder up the memory board (waiting for some chips for the MMU still).

The rev 1.1 design of the memory board is almost the same as the 1.0, except …

I finally put together the MMU, the fourth of the seven PCBs that arrived in my August batch. A couple of things were obvious right off the bat:

1. I'd somehow screwed up the silkscreening, and all the text labels for the current MMU mode on the left of the board …

A big batch of PCBs arrived last week from China (7 different boards!), and I decided to have a go at the reset board first. This is a straightforward conversion of my breadboard prototype, so I thought it would be quick and easy to assemble and test, and I could …

I finally got time to assemble the ALU PCB, after having been away for 6 weeks, waiting for components to arrive, then dealing with the start of the lockdown.

Overall, I'm pretty happy with the result. I had a few problems:

• I ordered the wrong size of component for all …

The ALU PCB design is finished, finally. It's similar in design to the breadboard implementation, but with a few changes:

• a full shifter in place of the fixed right-shift-by-one element on the breadboard
• support for addc and subb instructions, in case I want to add them later
• no dedicated zero …

The register board is way more complicated than my first PCB, so I'm a little reluctant to solder everything on it until I've given it a bit of testing, so I just assembled the control logic and two of the 13 non-zero registers. And it works! I'm very pleased and …

My register boards arrived from JLPCB! Overall, pretty happy with them, but the quality is definitely lower than PCBWay. Three of the five boards have defects in the solder mask (none of which look fatal, though the bare copper you can see in the second photo above might be a …