The final board was supposed to be Rev. 4. The order was placed the day China went on Covid-19 lockdown and what should have been a week turned in to a month. This provided some time to reflect on the current design and see if it was possible to squeeze any additional functionality out of the already limited chip count... so by the time the Rev. 4 board showed up there was a Rev. 5 board ready to ship!
The original design uses 128k RAM divided in to two banks: user and video. The bank is selected by a bit in the instruction, allowing fast switching between these two memory banks. The video memory is the only bank read by the GPU and must always be selected during the GPU RAM cycle. However, the other bank could be further selected and there are up to 3-bits available from the E register to do this.
It was possible to implement this change by swapping a quad 2-input OR gate (74F32) with a quad 2:1 mux (74F157). A lot of things had to get moved around and the board went through a three-week long revision. The layout didn't change much, but almost all the ECU traces had to be rerouted. A jumper (shown below) was added to select between the original 128k and the new 512k memory chip.
Another change was the power supply. Unfortunately, the high voltage buck converter didn't make it through the final round of Rev. 4 testing. There is over 3 watts being generated in just one cubic inch of space and things were overheating. The temperature is stable if air can flow over the area, but it starts to get too hot once the enclosure is sealed up.
The alternative was a linear regulator, specifically the LDO variety. There are 1.5A versions available that can operate within 0.4V of the input and accommodate a 6V power supply. Like a lot of power supplies, the one currently under test outputs 5% over the rated voltage. There's about 366m ohms between the supply and the regulator dropping this to about 5.75V. The remaining drop across the regulator results in 1.125W being dissipated as heat, almost 1/3rd of the buck regulator design.
The thermal properties of the board are much improved. The minimal board was assembled and tested up to 50C. Not only was it stable, it could run this hot at 35MHz with no decoupling capacitors fitted! This is a first and it appears the changes in the design and PCB layout have all been positive.