M62 – Z80 SBC Card – Rev 3


Details of this revision:

  • Backwards compatible with Rev 1 and 2.
  • Almost everything is surface mount, this reduces the size.
  • We now have a Compact Flash socket included on the board.
  • There is an optional isolated USB-UART interface based on the CH340G (I did notice I made a mistake in the silkscreen)
  • We have 512KB each of ROM and RAM (Paged through Bank 0, more on that later)
  • You can select an external clock for the Z80, the 4MHz remains connected to the bus and the on-board UART

Sections in more detail

Compact Flash:

This revision now has a Compact Flash socket, it is accessed in 8-bit IDE mode. There is a jumper selection on the board to allow us to select the base address of the CF from between $08 and $38 in $08 increments.

I have been working on FAT16 support for CF for the BIOS and M62-OS, so far I can read files in the root directory/folder, see here for more information on that.

ROM/RAM structure:

The ROM and RAM structure is pretty much the same as the other revisions on reset.

Bank 0 is mapped to the first page of ROM, and Bank 1 to the first page of RAM (This is a fixed mapping). Each page is 16K in size.

With this revision, we have a write-only memory configuration register at I/O port $00 that assigns the mapping of Bank 0.

Bit 0 selects whether we are using the ROM or the RAM, setting this bit selects the RAM while clearing it (default) selects the ROM>

Bits 1-5 configure the A14-A18 lines, which basically select which 16K page of the ROM/RAM we are using.

Note: Bank 1 is always mapped to RAM page 0 regardless of Bank 0 configuration, this is because the stack is stored at the top of bank 1 and removing access to it would cause the system to crash.

Upon reset, this register will default to $00.


It’s been a while since the boards arrived and I’ve not written any updates unfortunately.

Here is is all populated!

The Isolation circuitry for the UART didn’t perform as expected, so it was bypassed as seen in this photo.

The rest of the board works designed. The full 512KB of RAM is accessible and the full FlashROM (up to 512KB) is available too.

Upon testing the CF card worked exactly as it did on the prototype CF interface card.