A Z80 CPU module supporting speeds up to 20Mhz - but with the required wait states and clock control to ensure compatibility with existing software and RC2014 modules.

What is it?

This module is designed to replace your existing RC2014 CPU and Clock Modules and give you the full power of a Z80 running at 20Mhz, yet still be broadly compatible with original software and your other RC2014 modules.

For a RC2014 configured to run a full MSX configuration, you typically need to run your CPU at just 3.5Mhz, as software written for the platform at the time, would assume your system was clocked at this speed. If you attempt to run programs at a higher cpu clock speed, you will probably get video corruption, I/O problems -- it just wont work!.

But these days, its possible to buy a brand new Zilog Z80 chip, rated at 20Mhz (Z84C0020PEG). For me in the mid 80s, that would have been an unimaginable speed!

We achieve compatibility by applying a combination of hardware wait states (pausing the CPU for a bit) and automatically slowing the clock down to the 3.5Mhz for short periods of time when the CPU is interacting with your other modules.

A 3 way slider, soldered on the front of the module, gives you total control of the CPU speed. Using this switch, you can at any time switch into 1 of 3 modes:

1. Full 20Mhz clock speed, with 1 wait state when accessing memory, and slows the clock to 3.5Mhz for 31 clock cycles when it accesses IO devices (SIO/2, PPI, V9958 etc).
2. Full 20Mhz clock speed as mode 1, but with 3 wait states for accessing memory.
3. Standard MSX speed 3.5Mhz CPU, M1 Wait state for accessing memory.

Despite the extra wait states and clock slow down, I have found a typical speed improvement of between 4 and 5 times faster - even for software the does lots of interactions with the V9958, you can still see a very large improvement.

Key features:

• Support Z80 running at 20Mhz (16Mhz oscillator also supplied if you have issue with your specific kit)
• 3 way slider to select fast, medium or standard speeds
• Blinky Leds to indicate turbo mode and clock slow down states
• PLD to control wait state counts and clock slow down - allowing for experimentation if you want to try other speeds configurations

What is the 'Yellow MSX Series of Boards'?

These are a series of boards that I am developing to achieve MSX compatibility for RC2014 systems.

The idea is that you can build each board one at a time, test it and play with it under RomWBW - and then once you have the set - load up some MSX/MSX2+ games!

The V9958 board is the first in that series.

More details can be found on my hackaday project

What's included in this kit

The full kits includes everything you need (PCB, capacitors, IC sockets, connectors, and the ICs). The 2 PLD chips are flashed with logic to control the wait and clock slow down logic.

Also included is a 20Mhz and an alternative 16Mhz oscillator.

Operation Reliability

I have tested running at 20Mhz on my specific back-plane and set of modules without any problems. But due to differences in combination and configuration of RC2014 kits - you may find with your specific situation, there are signal degradation issues at 20Mhz that may cause non-reliable operation. With the supplied 16Mhz alternative oscillator, you should be able to achieve good operation, but at a slightly lower speed.

> 2024-07-21: Please note, a recent incompatibility of the Turbo CPU with the latest build of RomWBW has been identified. The issue appears with the serial SIO/2 driver stopping working shortly after booting. See Turbo CPU readme notes for workaround.

Within a Yellow MSX configuration, the Turbo CPU module continues to works correctly.

Bill of Materials

What else do I need to make this work?

You need a RC2014 build. This can be configured with MSX modules or other modules.

I have tested on a conventional RC2014 build and with the MSX Modules.

This modules replaces your RC2014 CPU and Clock Modules.

Disclaimer

Please note that this is a kit, produced by a non-professional (me) for hackers, DIYers' and retro lovers, to tinker with. I will do my best to answer any support questions you may have.