There were a couple changes to make rev3 .. the audio chip just did not seem happy on the main system bus; all the signals are good, just the chip can't seem to get the data bus values. I suspect the waitstates are just not long enough for the old chip - a 1983 chip working with somethign 20 years its junior. Being sick of making costly revisions I've tied the eZ80 spare GPIO to the data bus on the audio chip, and now all is well.. excepting that GPIO is nolonger spare when the audio chipselect is active. A real shame not having a completely spare GPIO, but on the off chance it gets needed.. add some chipselect handling in your GPIO consumer, or don't drive audio at the same time :) (Or use the joystick GPIOs, or I2C, or SPI, or the cartridge expansion slot or stm32 spare GPIO or… lots of options. Anyway.)
I've also added the option to go to 512K RAM chip (128KB, 256KB and 512KB supported now – same pinout more or less); there are some extra pads added for setting a few choices or even adding a USB bus.
But all told, I think this is a 'good' gate; I've not tested everything (need to write a keyboard protocol driver to really test that, and make sure the full colour DAC still working), but none of that has changed in some revisions.
So. what is next?
Lots of roads to go down in the software side – making an actual cart game or two? getting BASIC interpreter working? formalizing the BIOS and memory map? Adding a SD card reader shield? A larger actual cart .. maybe an EPROM instead of EEPROM (seems they're getting harder to get..)
Some sick ideas are .. now that 512KB of RAM is in place, could perhaps port over uclinux; imagine a Linux cartridge for a Z80 system? Minds blown!
Or NuttX port..
Whose to say?